publications

2024

1. A Generalized Model of Sea Surface Slopes and its Application to Sun Glint Correction on HY-1C/COCTS Imagery

   Hong Gao, Ninghui Li, Tinglu Zhang, and 3 more authors

   IEEE Transactions on Geoscience and Remote Sensing, 2024

   Abs DOI

   A generalized probability density function (PDF) is introduced to enhance sea surface slope modeling for remote sensing applications. This new PDF, which incorporates the anisotropy index to better capture the direction and tilt of surface waves relative to the classical Cox & Munk model proposed 70 years ago, is then applied to sun glint correction in satellite imagery. Sixteen different mean square slope (MSS) models are reviewed to establish both the strengths and limitations of the classical model and the stability and adaptability of the anisotropy index. The new sea surface model applies to a wider range of sea surface states, including those in coastal environments, and provides a stable quantitative description of sea surface topography. Application of the generalized PDF to sun glint correction in satellite imagery demonstrates its overall accuracy and improved efficacy compared to the Cox & Munk model, particularly in maintaining the integrity of sea surface and cloud features in complex weather environments. This initial study provides a promising approach to improve the accuracy and reliability of sun glint correction in remote sensing of water surfaces, with applications to improving both historical and future satellite-based climate data records.

2. Relationships Between the Wintertime Atmospheric Blocking and Cold Surges Over Eurasia Revealed by Clustering

   Zifan Yang, Wenyu Huang, Jonathon S. Wright, and 2 more authors

   Journal of Geophysical Research: Atmospheres, 2024

   Abs DOI

   A unified clustering framework based on pattern correlation is used to identify eight blocking regimes over Eurasia and surrounding oceans during the winter months (December–March) of 1948–2021. The regimes are labeled based on their centers of action, which are located over the West Atlantic, Greenland, the East Atlantic, Scandinavia, the Ural Mountains and Siberia, the Okhotsk Sea, the Bering Strait, and the North Pacific. It should be noted that the classification is almost insensitive to the time period but mainly depends on the percentage of different blocking events. The spatial distributions of cold surges differ substantially among these eight regimes. Due mainly to the cold advection downstream of the centers of blocking activities, cold surges can be observed over parts of the Eurasian continent. Possible relationships between the eight blocking regimes and large‐scale modes of climate variability, including the Arctic Oscillation, the North Atlantic Oscillation, and the El Niño‐Southern Oscillation, are explored. Not only contemporary connections but also the predictive value of large‐scale modes is discussed. Atmospheric blocking, an important circulation system in middle and high latitudes, has significant local and upstream/downstream impacts on weather and short‐term climate. Since blockings with diverse occurrence locations affect different regions, it is necessary to classify them by a unified framework. In this study, a clustering method based on pattern correlations is adopted to identify wintertime blocking regimes over Eurasia and surrounding oceans to establish their connections with the corresponding spatial distribution of cold surges. The clustering result is almost insensitive to the time period, which mainly depends on the percentage of blocking events located at different regions. We also examine the large‐scale climate background for each regime to provide a reference for the cold surge forecast over the Eurasian continent. Eight wintertime blocking regimes over Eurasia and surrounding oceans are revealed by a clustering method Blocking activities induce cold surges over parts of the Eurasian continent due to downstream cold advection The cluster regimes are almost insensitive to the time period Eight wintertime blocking regimes over Eurasia and surrounding oceans are revealed by a clustering method Blocking activities induce cold surges over parts of the Eurasian continent due to downstream cold advection The cluster regimes are almost insensitive to the time period

3. Cloud Detection and Sea Surface Temperature Retrieval by HY-1C COCTS Observations

   Ninghui Li, Lei Guan, and Jonathon S Wright

   IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, 2024

   Abs DOI

   Sea surface temperature (SST) is a vital oceanic parameter that significantly influences air–sea heat flux and momentum exchange. SST datasets are crucial for identifying and describing both short-term and long-term climate perturbations in the ocean. This article focuses on cloud detection and SST retrievals in the Western Pacific Ocean, using observations obtained by the Chinese Ocean Color and Temperature Scanner (COCTS) onboard the Haiyang-1C satellite. To distinguish between clear-sky and overcast regions, reflectance after sun glint correction and brightness temperature are used as inputs for an alternative decision tree (ADTree). The accuracy of cloud detection is 93.85% for daytime and 91.98% for nighttime, respectively. Application of the cloud detection algorithm improves the accuracy and data availability (spatiotemporal coverage) of SST retrievals. We implement a nonlinear algorithm to retrieve the SST and validate these retrieved values against buoy measurements of SST. Comparisons are conducted for measurements within ±1 h and 0.01° × 0.01° of the retrieval. During the day, the bias and standard deviation (SD) are −0.01 °C and 0.63 °C, respectively, while at night, they stand at −0.08 °C and 0.71 °C, respectively. Furthermore, the intercomparison between the SST products derived from the moderate-resolution imaging spectroradiometer (MODIS) onboard Terra and the results are conducted. During the day, the bias and SD are 0.03 °C and 0.42 °C, respectively, whereas at night, they are 0.25 °C and 0.76 °C, respectively. This article improves the accuracy and applicability of the SST retrieved from the COCTS thermal infrared channels.

4. Diagnosing the Quasi-Equilibrium Response of ENSO Variability under a Range of CO2 Levels

   Mengke Zhu, Hong-Li Ren, Zizhan Hu, and 2 more authors

   Journal of Climate, 2024

   Abs DOI

   Abstract Several recent studies have highlighted differences in simulated properties of El Niño–Southern Oscillation (ENSO) under transient and equilibrium responses to increasing CO 2 . However, the reasons behind these disparate responses and the extent to which they are robust to different scales of CO 2 forcing remain unclear. In this study, we adopt a climate system model with reduced SST bias in the eastern tropical Pacific and incrementally apply abrupt increases in CO 2 , analyzing outputs after each simulation reaches quasi-equilibrium with the imposed forcing. The results suggest that ENSO activity under quasi-equilibrium first increases and then decreases with increasing CO 2 , peaking in simulations with CO 2 concentrations similar to the present day. Bjerknes–Jin stability analysis indicates that changes in the ENSO growth rate result primarily from changes in the thermocline feedback and thermodynamic damping terms. While thermodynamic damping increases monotonically with increasing CO 2 , the positive thermocline feedback varies within the range of internal variability up to twice the preindustrial value of CO 2 and then weakens sharply with further increases. The mechanisms behind these changes include weaker mean ocean upwelling and weaker dynamical coupling between the atmosphere and subsurface ocean associated with substantial near-surface freshening at higher levels of CO 2 . These changes steepen the thermodynamic barrier to mixing between the surface and subsurface, weakening the east–west temperature gradient in the mean state and suppressing variability in the cold tongue. Analysis of similar model simulations from the Coupled Model Intercomparison Project (CMIP6) archive indicates that changes in the Bjerknes–Jin stability index are robust but do not establish a consensus as to the mechanisms behind them. Significance Statement This study investigates how El Niño–Southern Oscillation changes with increasing CO 2 forcing by using a global model with improved tropical Pacific climate. The simulations roughly correspond to scenarios in which emissions are reduced to maintain CO 2 concentrations at near-constant values over a long period, with levels ranging from about 2/3 to more than 5 times the present-day concentration. Analysis of these simulations suggests that ENSO activity is strongest when CO 2 concentrations are similar to the present-day and becomes substantially weaker when CO 2 is more than double its present-day value. Reduced ENSO activity with further increases in CO 2 is caused by weaker interactions between the atmosphere and the subsurface ocean. Both the amplitude of warm (El Niño) events and the occurrence frequency of warm and cold (La Niña) events decrease as ENSO events become first more difficult to grow and then more difficult to trigger with increasing CO 2 .

5. Effects of Anthropogenic Aerosols on the East Asian Winter Monsoon

   Shenglong Zhang, Jonathon S. Wright, Zengyuan Guo, and 2 more authors

   Journal of Geophysical Research: Atmospheres, 2024

   Abs DOI

   Circulation patterns linked to the East Asian winter monsoon (EAWM) affect precipitation, surface temperature, and air quality extremes over East Asia. These circulation patterns can in turn be influenced by aerosol radiative and microphysical effects through diabatic heating and its impacts on atmospheric vorticity. Using global model simulations, we investigate the effects of anthropogenic aerosol emissions and concentration changes on the intensity and variability of the EAWM. Comparison with reanalysis products indicates that the model captures the mean state of the EAWM well. The experiments indicate that anthropogenic aerosol emissions strengthen the Siberian High but weaken the East Asian jet stream, making the land areas of East Asia colder, drier, and snowier. Aerosols reduce mean surface air temperatures by approximately 1.5° \1.5{}^{\textbackslashcirc}\C, comparable to about half of the difference between strong and weak EAWM episodes in the control simulation. The mechanisms behind these changes are evaluated by analyzing differences in the potential vorticity budget. Anthropogenic aerosol effects on diabatic heating strengthen anomalous subsidence over southern East Asia, establishing an anticyclonic circulation anomaly that suppresses deep convection and precipitation. Aerosol effects on cloud cover and cloud longwave radiative heating weaken stability over the eastern flank of the Tibetan Plateau, intensifying upslope flow along the western side of the anticyclone. Both circulation anomalies contribute to reducing surface air temperatures through regional impacts on thermal advection and the atmospheric radiative balance. The East Asian winter monsoon (EAWM) is a large‐scale circulation system that controls the occurrence of cold air outbreaks and severe winter storms throughout the densely populated land areas of East Asia. By reducing the amount of sunlight reaching the surface, warming the atmosphere, and changing the properties and lifetimes of clouds, aerosols can alter the atmospheric circulation and regional weather conditions. Here, we examine the characteristics of the EAWM in global model simulations with and without aerosol emissions from industry, energy generation, transportation, and other human activities. Our results show that the aerosols produced by these activities make East Asian land areas colder, drier, and snowier during winter. We explain the reasons for these changes by diagnosing how various physical effects of aerosols impact temperature and winds within the East Asian monsoon region. Anthropogenic aerosols make East Asian land areas roughly 1.5° \1.5{}^{\textbackslashcirc}\C colder during winter while reducing precipitation and increasing snowfall Aerosol effects are diagnosed by analyzing how changes in diabatic heating alter the winter monsoon potential vorticity intrusion Colder surface temperatures can be attributed to both aerosol direct effects and circulation responses to cloud‐aerosol interactions Anthropogenic aerosols make East Asian land areas roughly 1.5° \1.5{}^{\textbackslashcirc}\C colder during winter while reducing precipitation and increasing snowfall Aerosol effects are diagnosed by analyzing how changes in diabatic heating alter the winter monsoon potential vorticity intrusion Colder surface temperatures can be attributed to both aerosol direct effects and circulation responses to cloud‐aerosol interactions

6. Wave‐Convection Interactions Amplify Convective Parameterization Biases in the South Pacific Convergence Zone

   Yuanrui Chen, Wenchao Chu, Jonathon S. Wright, and 1 more author

   Journal of Advances in Modeling Earth Systems, 2024

   Abs DOI

   Climate models have long‐standing difficulties simulating the South Pacific Convergence Zone (SPCZ) and its variability. For example, the default Zhang‐McFarlane (ZM) convection scheme in the Community Atmosphere Model version 5 (CAM5) produces too much light precipitation and too little heavy precipitation in the SPCZ, with this bias toward light precipitation even more pronounced in the SPCZ than in the tropics as a whole. Here, we show that implementing a recently developed convection scheme in the CAM5 yields significant improvements in the simulated SPCZ during austral summer and discuss the reasons behind these improvements. In addition to intensifying both mean rainfall and its variability in the SPCZ, the new scheme produces a larger heavy rainfall fraction that is more consistent with observations and state‐of‐the‐art reanalyses. This shift toward heavier, more variable rainfall increases both the magnitude and altitude of diabatic heating associated with convective precipitation, intensifying lower tropospheric convergence and increasing the influence of convection on the upper‐level circulation. Increased diabatic production of potential vorticity in the upper troposphere intensifies the distortion effect exerted by convection on transient Rossby waves that pass through the SPCZ. Weaker distortion effects in simulations using the ZM scheme allow waves to propagate continuously through the region rather than dissipating locally, further reducing updrafts and weakening convection in the SPCZ. Our results outline a dynamical framework for evaluating model representations of tropical–extratropical interactions within the SPCZ and clarify why convective parameterizations that produce “top‐heavy” profiles of deep convective heating better represent the SPCZ and its variability. The South Pacific convergence zone (SPCZ), a band of strong rainfall that stretches diagonally across the South Pacific from northwest to southeast, is difficult for climate models to simulate well. Here, we suggest that much of this difficulty stems from underestimating both how much heavy rainfall is produced in the SPCZ and how high above the surface this rainfall forms. The SPCZ has previously been described as a “graveyard” for weather systems. Our hypothesis casts the SPCZ more as a toll collector and suggests that the vertical location of the collection point is key. Simulated weather systems that produce heavier rainfall as they move through the SPCZ region release energy higher in the atmosphere, providing the SPCZ with the means to maintain itself. A model that releases this energy lower in the atmosphere by producing too much light rain allows many weather systems to bypass the toll, weakening the simulated SPCZ and drawing it equatorward in search of the energy it needs. Biases in simulated precipitation rate affect diabatic heating and the upper‐level response to transient Rossby waves An improved deep convection parameterization reduces biases in the South Pacific Convergence Zone (SPCZ), especially for heavy rainfall More realistic upper‐level heating strengthens feedbacks between waves and convection, blocking propagation of wave energy locally Biases in simulated precipitation rate affect diabatic heating and the upper‐level response to transient Rossby waves An improved deep convection parameterization reduces biases in the South Pacific Convergence Zone (SPCZ), especially for heavy rainfall More realistic upper‐level heating strengthens feedbacks between waves and convection, blocking propagation of wave energy locally

7. A Coupled Quadrupole Mode in the South Pacific

   Yuanrui Chen, and Jonathon S Wright

   Journal of Climate, 2024

   Abs DOI

   Abstract Although links between the atmospheric convergence zone and the local ocean dipole in the South Atlantic are well established, relationships between the South Pacific convergence zone (SPCZ) and the South Pacific quadrupole (SPQ) remain largely unexplored. Based on maximum covariance analysis applied to a 110-yr monthly coupled atmosphere–ocean reanalysis, we describe a coupled quadrupole mode (CQM) that connects the SPCZ and SPQ during austral summer [December–February (DJF)]. The CQM is linked to the “enhanced SPCZ” mode in the atmosphere and the SPQ in the ocean, with the atmospheric signal leading the ocean signal by about 1 month. This coupled mode essentially represents the atmospheric and oceanic responses to a stationary Rossby wave train that propagates from low- to high latitudes before reflecting back toward lower latitudes around 150°E. Coupled atmosphere–ocean feedbacks help to maintain anomalous convective activity in the SPCZ and related circulation anomalies. The stationary waves that organize the CQM are often rooted in anomalous convection over the Maritime Continent and have close connections with the atmospheric wavenumber-4 mode in the midlatitude Southern Hemisphere. Significance Statement In this study, we investigate the relationships between coherent large-scale patterns in the South Pacific Ocean and the overlying atmosphere. These patterns, which we refer to as a coupled quadrupole for their four centers of action, impact both local communities and the global climate by shaping rainfall and temperature anomalies across the “four corners” of the South Pacific: east–west and north–south. We show that this coupled quadrupole arises as the joint atmospheric and oceanic response to a large-scale wave that arcs across the entire South Pacific basin more than 10 km above the surface and that feedbacks from the ocean to the atmosphere help it to last longer.

8. Climatology of the terms and variables of transformed Eulerian-mean (TEM) equations from multiple reanalyses: MERRA-2, JRA-55, ERA-Interim, and CFSR

   Masatomo Fujiwara, Patrick Martineau, Jonathon S. Wright, and 6 more authors

   Atmospheric Chemistry and Physics, 2024

   Abs DOI

   A 30-year (1980–2010) climatology of the major variables and terms of the transformed Eulerian-mean (TEM) momentum and thermodynamic equations is constructed by using four global atmospheric reanalyses: the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2); the Japanese 55-year Reanalysis (JRA-55); the European Centre for Medium-Range Weather Forecasts (ECMWF) interim reanalysis (ERA-Interim); and the Climate Forecast System Reanalysis (CFSR). Both the reanalysis ensemble mean (REM) and the differences in each reanalysis from the REM are investigated in the latitude–pressure domain for December–January–February and for June–July–August. For the REM investigation, two residual vertical velocities (the original one and one evaluated from residual meridional velocity) and two mass streamfunctions (from meridional and vertical velocities) are compared. Longwave (LW) radiative heating and shortwave (SW) radiative heating are also shown and discussed. For the TEM equations, the residual terms are also calculated and investigated for their potential usefulness, as the residual term for the momentum equation should include the effects of parameterized processes such as gravity waves, while that for the thermodynamic equation should indicate the analysis increment. Inter-reanalysis differences are investigated for the mass streamfunction, LW and SW heating, the two major terms of the TEM momentum equation (the Coriolis term and the Eliassen–Palm flux divergence term), and the two major terms of the TEM thermodynamic equation (the vertical temperature advection term and the total diabatic heating term). The spread among reanalysis TEM momentum balance terms is around 10 % in Northern Hemisphere winter and up to 50 % in Southern Hemisphere winter. The largest uncertainties in the thermodynamic equation (about 50 %) are found in the vertical advection, for which the structure is inconsistent with the differences in heating. The results shown in this paper provide basic information on the degree of agreement among recent reanalyses in the stratosphere and upper troposphere in the TEM framework.

2023

1. Aerosol Effects on Clear‐Sky Shortwave Heating in the Asian Monsoon Tropopause Layer

   Jie Gao, Yi Huang, Yiran Peng, and 1 more author

   Journal of Geophysical Research: Atmospheres, 2023

   Abs DOI

   The Asian tropopause aerosol layer (ATAL) has emerged in recent decades with aerosol accumulation near tropopause above Asian Summer Monsoon region. Although ATAL effects on surface and top‐of‐atmosphere (TOA) radiation budgets are well established, the magnitude and variability of ATAL effects on radiative transfer within the tropopause layer remain poorly constrained. Here, we investigate the impacts of various aerosol types and layer structures on clear‐sky shortwave radiative heating in the Asian monsoon tropopause layer using reanalysis products and offline radiative transfer simulations. ATAL effects on shortwave radiative heating based on the Modern‐Era Retrospective Analysis for Research and Applications, version 2 aerosol reanalysis are on the order of 15% of mean clear‐sky radiative heating within the tropopause layer, although discrepancies among recent reanalysis and forecast products suggest that this ratio could be as small as ∼10% or as large as ∼70%. Uncertainties in surface and TOA flux effects are also large, with values spanning one order of magnitude at the TOA. ATAL effects on radiative heating peak between 150 and 80 hPa (360–400 K potential temperature) along the southern flank of the anticyclone. Clear‐sky and all‐sky shortwave heating are at local minima in this vertical range, which is situated between the positive influences of monsoon‐enhanced water vapor and the negative influence of the “ozone valley” in the monsoon lower stratosphere. ATAL effects also extend further toward the west, where diabatic vertical velocities remain upward despite descent in pressure coordinates. Every summer, a layer of polluted air laden with aerosol particles collects above the convective storms of the Asian monsoon as part of a broad upper‐level circulation centered over the Tibetan Plateau. Researchers have developed a working understanding of how the dynamical environment shapes this Asian tropopause aerosol layer (ATAL). The motivating question for this work is: How might the aerosol layer reshape its environment? Aerosols can absorb and scatter sunlight, affecting both the amount of sunlight transmitted through the layer and the magnitude of solar heating within the layer. These effects depend on aerosol species and their vertical distribution within the layer, both of which are highly variable. In this paper, we translate variations and uncertainties in the amount, composition, and vertical distribution of aerosols near the Asian monsoon tropopause into variations and uncertainties in the absorption and scattering of solar radiation by the aerosol layer. We find that aerosols account for a substantial part (10%–70%) of heating by solar radiation near the tropopause in the region of ATAL. The vertical location and horizontal extent of the aerosol effects are distinct from those of other radiative effects. The Asian tropopause aerosol layer produces a 10%–70% direct enhancement of clear‐sky shortwave heating above the summer monsoon Effects are largest where shortwave heating is weakest, with similar magnitudes to water vapor and ozone effects near the monsoon tropopause Discrepancies across recent aerosol analysis and forecast products cause large uncertainties in aerosol forcing of heating and fluxes The Asian tropopause aerosol layer produces a 10%–70% direct enhancement of clear‐sky shortwave heating above the summer monsoon Effects are largest where shortwave heating is weakest, with similar magnitudes to water vapor and ozone effects near the monsoon tropopause Discrepancies across recent aerosol analysis and forecast products cause large uncertainties in aerosol forcing of heating and fluxes

2. Deep Learning Improves GFS Wintertime Precipitation Forecast Over Southeastern China

   Danyi Sun, Wenyu Huang, Zifan Yang, and 5 more authors

   Geophysical Research Letters, 2023

   Abs DOI

   Wintertime precipitation, especially snowstorms, significantly impacts people’s lives. However, the current forecast skill of wintertime precipitation is still low. Based on data augmentation (DA) and deep learning, we propose a DABU‐Net which improves the Global Forecast System wintertime precipitation forecast over southeastern China. We build three independent models for the forecast lead times of 24, 48, and 72 hr, respectively. After using DABU‐Net, the mean Root Mean Squared Errors (RMSEs) of the wintertime precipitation at the three lead times are reduced by 19.08%, 25.00%, and 22.37%, respectively. The threat scores (TS) are all significantly increased at the thresholds of 1, 5, 10, 15, and 20 mm day−1 for the three lead times. During heavy precipitation days, the RMSEs are decreased by 14% and TS are increased by 7% at the lead times within 48 hr. Therefore, combining DA and deep learning has great prospects in precipitation forecasting. In this paper, we propose a deep learning‐based method to improve the forecast performance of Global Forecast System wintertime precipitation over southeastern China. Due to the imbalanced distribution of precipitation data, we use data from the three other seasons as an augmented data set for wintertime precipitation to train the deep neural network. The results show that the method can reduce the Root Mean Squared Error and improve the TS, a metric of precipitation forecast performance, of the precipitation. In particular, TS at the threshold of 20 mm day−1 are increased by 69.23%, 90.00%, and 100.00% at lead times of 24, 48, and 72 hr. The proposed method performs well during heavy precipitation days at lead times within 48 hr. Combining data augmentation with deep learning provides a successful approach to predicting precipitation. A deep learning model based on data augmentation (DA) is proposed to improve the Global Forecast System wintertime precipitation forecast The deep learning model improves the heavy precipitation forecast at lead times within 48 hr DA plays a critical role in the heavy precipitation forecast A deep learning model based on data augmentation (DA) is proposed to improve the Global Forecast System wintertime precipitation forecast The deep learning model improves the heavy precipitation forecast at lead times within 48 hr DA plays a critical role in the heavy precipitation forecast

3. Multi-decadal variability controls short-term stratospheric water vapor trends

   Mengchu Tao, Paul Konopka, Jonathon S. Wright, and 5 more authors

   Communications Earth & Environment, 2023

   Abs DOI

   Stratospheric water vapor increases are expected in response to greenhouse gas-forced climate warming, and these changes act as a positive feedback to surface climate. Previous efforts at inferring trends from the 3–4 decade-long observational stratospheric water vapor record have yielded conflicting results. Here we show that a robust multi-decadal variation of water vapor concentrations exists in most parts of the stratosphere based on satellite observations and atmospheric model simulations, which clearly divides the past 40 years into two wet decades (1986–1997; 2010–2020) and one dry decade (1998–2009). This multi-decadal variation, especially pronounced in the lower to middle stratosphere and in the northern hemisphere, is associated with decadal temperature anomalies (±0.2 K) at the cold point tropopause and a hemispheric asymmetry in changes of the Brewer-Dobson circulation modulating methane oxidation. Multi-decadal variability must be taken into account when evaluating stratospheric water vapor trends over recent decades. Stratospheric water vapour concentrations show robust multidecadal variability which is stronger in the northern than the southern hemisphere at mid-stratosphere levels, according to satellite observations and atmospheric model simulations.

4. Increased precipitation over land due to climate feedback of large-scale bioenergy cultivation

   Zhao Li, Philippe Ciais, Jonathon S. Wright, and 9 more authors

   Nature Communications, 2023

   Abs DOI

   Bioenergy with carbon capture and storage (BECCS) is considered to be a key technology for removing carbon dioxide from the atmosphere. However, large-scale bioenergy crop cultivation results in land cover changes and activates biophysical effects on climate, with earth’s water recycling altered and energy budget re-adjusted. Here, we use a coupled atmosphere-land model with explicit representations of high-transpiration woody (i.e., eucalypt) and low-transpiration herbaceous (i.e., switchgrass) bioenergy crops to investigate the range of impact of large-scale rainfed bioenergy crop cultivation on the global water cycle and atmospheric water recycling. We find that global land precipitation increases under BECCS scenarios, due to enhanced evapotranspiration and inland moisture advection. Despite enhanced evapotranspiration, soil moisture decreases only slightly, due to increased precipitation and reduced runoff. Our results indicate that, at the global scale, the water consumption by bioenergy crop growth would be partially compensated by atmospheric feedbacks. Thus, to support more effective climate mitigation policies, a more comprehensive assessment, including the biophysical effects of bioenergy cultivation, is highly recommended. Increased global land precipitation, due to the atmospheric feedbacks of large-scale bioenergy cultivation, may partially compensate the water consumption by such rainfed bioenergy crops at the global scale.

5. Land–Atmosphere Feedbacks Weaken the Cooling Effect of Soil Organic Matter Property toward Deep Soil on the Eastern Tibetan Plateau

   Jing Sun, Kun Yang, Hui Lu, and 5 more authors

   Journal of Hydrometeorology, 2023

   Abs DOI

   Abstract Soil organic matter (SOM) is enriched on the eastern Tibetan Plateau, but its effects on the hydrothermal state of the coupled land–atmosphere system remain unclear. This study comprehensively investigates these effects during summer from multiple perspectives based on regional climate modeling, land surface modeling, and observations. Using a regional climate model, we show that accounting for SOM effects lowers cold and wet biases in simulations of this region. SOM increases 2-m air temperature, decreases 2-m specific/relative humidity, and reduces precipitation in coupled simulations. Inclusion of SOM also warms the shallow soil while cooling the deep soil, which may help to preserve frozen soil in this region. This cooling effect is captured by both observations and offline land surface simulations, but it is overestimated in the offline simulations due to no feedback from the atmosphere compared to the coupled ones. Including SOM in coupled climate models could therefore not only imrove their representations of atmospheric energy and water cycles, but also help to simulate the past, present, and future evolution of frozen soil with increased confidence and reliability. Note that these findings are from one regional climate model and do not apply to wetlands. Significance Statement The eastern Tibetan Plateau is rich in soil organic matter (SOM), which increases the amount of water the soil can hold while decreasing the rate at which heat moves through it. Although SOM is expected to preserve frozen soil by insulating it from atmospheric warming, researchers have not yet tested the effects of coupled land–atmosphere interactions on this relationship. Using a regional climate model, we show that SOM typically warms and dries the near-surface air, warms the shallow soil, and cools the deep soil by modifying both soil properties and energy exchanges at the land–atmosphere interface. The results suggest that the cooling effect of SOM on deep soil is overestimated when atmospheric feedbacks are excluded.

6. Prediction of ENSO using multivariable deep learning

   Yue Chen, Xiaomeng Huang, Jing-Jia Luo, and 6 more authors

   Atmospheric and Oceanic Science Letters, 2023

   Abs DOI

   A novel multivariable prediction system based on a deep learning (DL) algorithm, i.e., the residual neural network and pure observations, was developed to improve the prediction of the El Niño–Southern Oscillation (ENSO). Optimal predictors are automatically determined using the maximal information for spatial filtering and the Taylor diagram criteria, enabling the best prediction skills at lead times of eight months compared with most operational prediction models. The hindcast skill for the most challenging decade (2011–18) outperforms the multi-model ensemble operational forecasts. At the six-month lead, the correlation (COEF) skill of the DL model reaches 0.82 with a normalized root-mean-square error (RMSE) of 0.58 °C, which is significantly better than the average multi-model performance (COEF = 0.70 and RMSE = 0.73°C). DL prediction can effectively alleviate the long-standing spring predictability barrier problem. The automatically selected optimal precursors can explain well the typical ENSO evolution driven by both tropical dynamics and extratropical impacts. 摘要 本文基于残差神经网络和观测数据构建了一套深度学习多因子预报测模型, 以改进厄尔尼诺-南方涛动(ENSO)的预报. 该模型基于最大信息系数进行因子时空特征提取, 并根据泰勒图的评估标准可自动确定关键预报因子进行预报. 该模型在超前8个月以内的预报性能要优于当前传统的业务预报模式. 2011–2018年间, 该模型的预报性能优于多模式集成预报的结果. 在超前6个月预报时效上, 模型预报相关性可达0.82, 标准化后的均方根误差仅为0.58°C, 多模式集成预报的相关性和标准化后的均方根误差分别为0.70和0.73°C. 该模型春季预报障碍问题有所缓解, 并且自动选取的关键预报因子可用于解释热带和副热带热动力过程对于ENSO变化的影响.

2022

1. Stratospheric Moistening After 2000

   Paul Konopka, Mengchu Tao, Felix Ploeger, and 4 more authors

   Geophysical Research Letters, 2022

   Abs DOI

   The significant climate feedback of stratospheric water vapor (SWV) necessitates quantitative estimates of SWV budget changes. Model simulations driven by the newest European Centre for Medium‐Range Weather Forecast reanalysis ERA5, satellite observations from the Stratospheric Water and OzOne Satellite Homogenized data set, Microwave Limb Sounder, and in situ frost point hygrometer observations from Boulder all show substantial and persistent stratospheric moistening after a sharp drop in water vapor at the turn of the millennium. This moistening occurred mainly during 2000–2006 and SWV abundances then remained high over the last decade. We find strong positive trends in the Northern Hemisphere and weak negative trends over the South Pole, mainly during austral winter. Moistening of the tropical stratosphere after 2000 occurred during late boreal winter/spring, reached values of ∼0.2 ppm/decade, was well correlated with a warming of the cold point tropopause by ∼0.4 K/decade and can only be partially attributed to El Nino‐Southern Oscillation and volcanic eruptions. Water vapor is an effective greenhouse gas. Human‐induced climate change has led to warmer air in the troposphere, which consequently can hold more moisture, thus enhancing the greenhouse effect. The long‐term change in stratospheric water vapor (SWV) is less clear and currently under debate. Using satellite observations, balloon soundings and model simulations, we find an increase of SWV after 2000. This moistening occurred mainly during 2000–2006 and the stratospheric moisture content then remained high over the last decade. The increase of SWV is stronger in the Northern than in the Southern Hemisphere. Over the South Pole, a weak decrease was found. Moistening of the tropical stratosphere occurred mainly during late winter and spring, and was in line with warming of the tropical tropopause, the coldest region that separates the troposphere and stratosphere. Natural causes such as volcanic eruptions cannot completely explain this stratospheric moistening. Stratospheric moistening after 2000 is clearly detectable in ERA5‐driven simulations, satellite and in situ observations Hemispheric asymmetry is found with strong positive trends in the Northern Hemisphere and weak negative trends over the South Pole Moistening of the lower tropical stratosphere is only partially caused by El Nino‐Southern Oscillation and volcanic eruptions Stratospheric moistening after 2000 is clearly detectable in ERA5‐driven simulations, satellite and in situ observations Hemispheric asymmetry is found with strong positive trends in the Northern Hemisphere and weak negative trends over the South Pole Moistening of the lower tropical stratosphere is only partially caused by El Nino‐Southern Oscillation and volcanic eruptions

2. Impacts of Western Disturbances on Wintertime Precipitation Over the Southeastern Tibetan Plateau

   Tianpei Qiu, Wenyu Huang, Jonathon S. Wright, and 2 more authors

   Journal of Geophysical Research: Atmospheres, 2022

   Abs DOI

   In this work, we examine the impacts of wintertime western disturbances (WDs) on precipitation over the southeastern Tibetan Plateau (SETP) during 1979–2017. Most WDs initially formed within the central Himalayas and dissipated over southeastern China and its neighboring seas. All extreme precipitation days over the SETP region during this period were linked to WDs, often when a WD center entered the region about 10° west of the SETP. Significant warm temperature anomalies over the SETP region preceding WD arrival favored moisture accumulation there. During the passage of WDs, both strong updrafts along the eastern edge of the WD and rapid decreases in temperature over the SETP (linked to WD‐associated cold anomalies below 300 hPa) favored the occurrence of wintertime precipitation over the SETP region. WD strength and moist static energy variations over the SETP region were key factors affecting the intensity of WD‐induced precipitation, while the size and latitudinal position of the WD played minor roles. The relatively strong WDs that produced extreme precipitation days over the SETP mainly originated above the eastern Mediterranean Sea, northern Arabian Peninsula, and Iranian Plateau. A distinct transition of the circumglobal teleconnection pattern modulated the generation of strong‐intensity WDs within these major genesis regions, as well as their eastward propagation and the generation of extreme precipitation over the SETP region. All wintertime extreme precipitation days over the southeastern Tibetan Plateau (SETP) occurred in tandem with western disturbances (WDs) Most WDs with extreme precipitation originated in a band extending from the Eastern Mediterranean to the Iranian Plateau The circumglobal teleconnection modulated the effects of WDs on precipitation over the SETP All wintertime extreme precipitation days over the southeastern Tibetan Plateau (SETP) occurred in tandem with western disturbances (WDs) Most WDs with extreme precipitation originated in a band extending from the Eastern Mediterranean to the Iranian Plateau The circumglobal teleconnection modulated the effects of WDs on precipitation over the SETP

3. A Deep Learning‐Based Bias Correction Method for Predicting Ocean Surface Waves in the Northwest Pacific Ocean

   Danyi Sun, Wenyu Huang, Yong Luo, and 4 more authors

   Geophysical Research Letters, 2022

   Abs DOI

   Ocean waves, especially extreme waves, are vital for air‐sea interaction and shipping. However, current wave models still have significant biases. Based on a numerical wave model and a deep learning model, a BU‐Net by adding batch normalization layers to a U‐Net, we accurately predict the significant wave height (SWH) of the Northwest Pacific Ocean. For each day in 2017–2021, we conducted a 3‐day hindcast experiment using WAVEWATCH3 (WW3) to obtain the SWH forecasts at lead times of 24, 48, and 72 hr, forced by GFS real‐time forecast surface winds. After using BU‐Net, the mean Root Mean Squared Errors (RMSEs) of the SWH forecast from WW3 at lead times of 24, 48, and 72 hr are reduced by 40%, 38%, and 30%, respectively. During typhoon passages, the drop percentages of RMSEs all exceed 20% for three lead times. Therefore, combining numerical models and deep learning is very promising in wave forecasting. Data‐driven deep learning is widely used in geoscience. A new deep learning bias correction method, BU‐Net, is proposed to correct the significant wave height forecast over the Northwest Pacific Ocean. It is demonstrated that BU‐Net improves the forecast performance over four seasons and performs well under extreme weather conditions such as typhoons. This new deep learning bias correction method provides a successful approach that can also be applied to the routine forecast of other atmospheric and ocean phenomena in future. We provide a predicting scheme for significant wave height by combining a numerical wave model and a deep learning model The BU‐Net bias correction model significantly improves the forecast performance of WW3 The predicting scheme improves the forecast performance during typhoon passages We provide a predicting scheme for significant wave height by combining a numerical wave model and a deep learning model The BU‐Net bias correction model significantly improves the forecast performance of WW3 The predicting scheme improves the forecast performance during typhoon passages

2021

1. Long-Term Variability of Relationships between Potential Large-Scale Drivers and Summer Precipitation in North China in the CERA-20C Reanalysis

   Lan Dai, and Jonathon S. Wright

   Atmosphere, 2021

   Abs DOI

   Although much progress has been made in identifying the large-scale drivers of recent summer precipitation variability in North China, the evolution of these drivers over longer time scales remains unclear. We investigate multidecadal and interannual variability in North China summer precipitation in the 110-year Coupled ECMWF Reanalysis of the Twentieth Century (CERA-20C), considering changes in regional moisture and surface energy budgets along with nine circulation indices linked to anomalous precipitation in this region. The CERA-20C record is separated into three distinct periods according to the running climatology of summer precipitation: 1901–1944 (neutral), 1945–1979 (wet), and 1980–2010 (dry). CERA-20C reproduces expected relationships between large-scale drivers and regional summer precipitation anomalies well during 1980–2010, but these relationships generally do not extend to earlier periods. For example, a strong relationship with the Eurasian teleconnection pattern only emerges in the late 1970s, while correlations with the El Niño-Southern Oscillation and the Pacific–Japan pattern change sign in the mid-twentieth century. We evaluate two possible reasons for this nonstationarity: (1) the underlying atmospheric model may require strong data assimilation constraints to capture large-scale circulation influences on North China, or (2) large-scale drivers inferred from recent records may be less general than expected. Our analysis indicates that both factors contribute to the identified nonstationarity in CERA-20C, with implications for the reliability of seasonal forecasts and climate projections based on current models.

2. Upward transport into and within the Asian monsoon anticyclone as inferred from StratoClim trace gas observations

   Marc von Hobe, Felix Ploeger, Paul Konopka, and 11 more authors

   Atmospheric Chemistry and Physics, 2021

   Abs DOI

   Every year during the Asian summer monsoon season from about mid-June to early September, a stable anticyclonic circulation system forms over the Himalayas. This Asian summer monsoon (ASM) anticyclone has been shown to promote transport of air into the stratosphere from the Asian troposphere, which contains large amounts of anthropogenic pollutants. Essential details of Asian monsoon transport, such as the exact timescales of vertical transport, the role of convection in cross-tropopause exchange, and the main location and level of export from the confined anticyclone to the stratosphere are still not fully resolved. Recent airborne observations from campaigns near the ASM anticyclone edge and centre in 2016 and 2017, respectively, show a steady decrease in carbon monoxide (CO) and increase in ozone (O3) with height starting from tropospheric values of around 100 ppb CO and 30–50 ppb O3 at about 365 K potential temperature. CO mixing ratios reach stratospheric background values below ∼25 ppb at about 420 K and do not show a significant vertical gradient at higher levels, while ozone continues to increase throughout the altitude range of the aircraft measurements. Nitrous oxide (N2O) remains at or only marginally below its 2017 tropospheric mixing ratio of 333 ppb up to about 400 K, which is above the local tropopause. A decline in N2O mixing ratios that indicates a significant contribution of stratospheric air is only visible above this level. Based on our observations, we draw the following picture of vertical transport and confinement in the ASM anticyclone: rapid convective uplift transports air to near 16 km in altitude, corresponding to potential temperatures up to about 370 K. Although this main convective outflow layer extends above the level of zero radiative heating (LZRH), our observations of CO concentration show little to no evidence of convection actually penetrating the tropopause. Rather, further ascent occurs more slowly, consistent with isentropic vertical velocities of 0.7–1.5 K d-1. For the key tracers (CO, O3, and N2O) in our study, none of which are subject to microphysical processes, neither the lapse rate tropopause (LRT) around 380 K nor the cold point tropopause (CPT) around 390 K marks a strong discontinuity in their profiles. Up to about 20 to 35 K above the LRT, isolation of air inside the ASM anticyclone prevents significant in-mixing of stratospheric air (throughout this text, the term in-mixing refers specifically to mixing processes that introduce stratospheric air into the predominantly tropospheric inner anticyclone). The observed changes in CO and O3 likely result from in situ chemical processing. Above about 420 K, mixing processes become more significant and the air inside the anticyclone is exported vertically and horizontally into the surrounding stratosphere.

3. Econometrics of the environmental Kuznets curve: Testing advancement to carbon intensity-oriented sustainability for eight economic zones in China

   Chenxi Lu, Sergey Venevsky, Xiaoliang Shi, and 3 more authors

   Journal of Cleaner Production, 2021

   Abs DOI

   Threats posed by global climate change have heightened the urgency for economies to transition to sustainability. However, quantitative metrics that measure sustainability status remain under development, hampered in part by the difficulty of identifying clear relationships between economic growth and sustainability. The Environmental Kuznets Curve hypothesis provides a framework for describing sustainability status relative to socioeconomic development. In this study, the Environmental Kuznets Curve hypothesis was adopted to investigate statistical relationships between the carbon intensity of human well-being (as an indicator of sustainability) and economic development in eight economic zones of China during 1997–2015. The results provide new evidence that seven of eight Chinese economic zones began advances to sustainability (defined here as downward turning points marked by inverted “N” shapes in the Environmental Kuznets Curve) between 2012 and 2015. The lone exception was the Northwestern economic zone, in which an approach to sustainability had not yet occurred by 2015. This study thus supports the contention that environmental policies and technologies have contributed to improving sustainability in terms of carbon intensity. The results suggest two strategic options for further increasing sustainability in China: 1) “first help the weakest”; and 2) “first help the latest to sustainability”.

2020

1. Differences in tropical high clouds among reanalyses: origins and radiative impacts

   Jonathon S. Wright, Xiaoyi Sun, Paul Konopka, and 6 more authors

   Atmospheric Chemistry and Physics, 2020

   Abs DOI

   We examine differences among reanalysis high-cloud products in the tropics, assess the impacts of these differences on radiation budgets at the top of the atmosphere and within the tropical upper troposphere and lower stratosphere (UTLS), and discuss their possible origins in the context of the reanalysis models. We focus on the ERA5 (fifth-generation European Centre for Medium-range Weather Forecasts – ECMWF – reanalysis), ERA-Interim (ECMWF Interim Reanalysis), JRA-55 (Japanese 55-year Reanalysis), MERRA-2 (Modern-Era Retrospective Analysis for Research and Applications, Version 2), and CFSR/CFSv2 (Climate Forecast System Reanalysis/Climate Forecast System Version 2) reanalyses. As a general rule, JRA-55 produces the smallest tropical high-cloud fractions and cloud water contents among the reanalyses, while MERRA-2 produces the largest. Accordingly, long-wave cloud radiative effects are relatively weak in JRA-55 and relatively strong in MERRA-2. Only MERRA-2 and ERA5 among the reanalyses produce tropical-mean values of outgoing long-wave radiation (OLR) close to those observed, but ERA5 tends to underestimate cloud effects, while MERRA-2 tends to overestimate variability. ERA5 also produces distributions of long-wave, short-wave, and total cloud radiative effects at the top of the atmosphere that are very consistent with those observed. The other reanalyses all exhibit substantial biases in at least one of these metrics, although compensation between the long-wave and short-wave effects helps to constrain biases in the total cloud radiative effect for most reanalyses. The vertical distribution of cloud water content emerges as a key difference between ERA-Interim and other reanalyses. Whereas ERA-Interim shows a monotonic decrease of cloud water content with increasing height, the other reanalyses all produce distinct anvil layers. The latter is in better agreement with observations and yields very different profiles of radiative heating in the UTLS. For example, whereas the altitude of the level of zero net radiative heating tends to be lower in convective regions than in the rest of the tropics in ERA-Interim, the opposite is true for the other four reanalyses. Differences in cloud water content also help to explain systematic differences in radiative heating in the tropical lower stratosphere among the reanalyses. We discuss several ways in which aspects of the cloud and convection schemes impact the tropical environment. Discrepancies in the vertical profiles of temperature and specific humidity in convective regions are particularly noteworthy, as these variables are directly constrained by data assimilation, are widely used, and feed back to convective behaviour through their relationships with thermodynamic stability.

2. Contributions of Indonesian Throughflow to eastern Indian Ocean surface variability during ENSO events

   Xiaolin Jin, and Jonathon S. Wright

   Atmospheric Science Letters, 2020

   Abs DOI

   The contribution of ocean transport to the transition between the Indian Ocean Dipole mode (IOD) and the Indian Ocean Basin mode (IOB) during El Niño–Southern Oscillation (ENSO) years is investigated using reanalysis products. Composite analysis suggests that the IOD–IOB transition is robust among ENSO years, with variations in the eastern Indian Ocean mixed layer playing a key role. Although this transition has typically been attributed to changes in surface heat flux in the eastern Indian Ocean, our results suggest that approximately one third of the mixed layer temperature tendency during the transition results from anomalous ocean heat transport. Enhanced surface heat flux anomalies and oceanic advection starting from October during El Niño developing years combine to warm the eastern Indian Ocean, promoting the decay of the positive IOD cold tongue and the emergence of a positive IOB pattern. The contribution of ocean advection to the IOD—IOB transition is dominated by the Indonesian Throughflow (ITF), which accounts for around 70–80% of total ocean advection contribution in the southeastern Indian Ocean (20–25% of the total warming tendency). These variations in ITF heat transport arise in large part from local wind anomalies. Southeasterly wind anomalies along the northeastern edge of an anomalous Indian Ocean anticyclone associated with El Niño intensify surface‐layer heat transport into the Indian Ocean in the ITF outflow region. This change in surface transport contrasts with total ITF transport anomalies during El Niño years, as total transport is dominated by negative subsurface transport anomalies. We investigate the transition between dipole‐ and basin‐mode sea surface temperature anomalies in the tropical Indian Ocean during El Niño‐Southern Oscillation years. Results based on two different ocean reanalyses indicate that anomalous near‐surface ocean advection from the Indonesian Throughflow outflow region plays an important complementary role to surface heat flux anomalies, contributing approximately one third of the mixed layer temperature tendency in the eastern Indian Ocean during this transition.

3. Favorable Circulation Patterns and Moisture Sources for Wintertime Extreme Precipitation Events Over the Balkhash‐Junggar Region

   Xinsheng He, Wenyu Huang, Zifan Yang, and 8 more authors

   Journal of Geophysical Research: Atmospheres, 2020

   Abs DOI

   The Balkhash‐Junggar (B‐J) region is a typical dryland region with strong sensitivity to climate change. The snowstorms of the year 2009 caused financial damage worth of 2,516 million RMB. This study examines the dynamical features and the moisture sources associated with wintertime extreme precipitation over the B‐J region. The analyses are based primarily on the data set from ERA‐Interim during DJF 1979–2017. Both Lagrangian and Eulerian approaches are used to examine the moisture sources. An upper‐tropospheric Rossby wave train that extends from the North Atlantic Ocean to the B‐J region is found to play a leading role in the formation of 127 wintertime extreme precipitation events in the B‐J region. This Rossby wave train deepens a cyclonic anomaly in the lower troposphere over the B‐J region, which favors the development of strong southwesterly moisture transport and strong updrafts there. These conditions are favorable for the occurrence of extreme precipitation events over the B‐J region. Lagrangian moisture source analysis indicates that most of the moisture for precipitation during these events comes from terrestrial sources, with central Asia the key moisture source region. The Eulerian moisture budget analysis further shows that most of the moisture for extreme precipitation enters the B‐J region through its western boundary via enhanced southwesterly flow. These dynamical and moisture source analyses establish a set of valuable precursor conditions for predicting wintertime extreme precipitation events over the B‐J region. A Rossby wave train extending from North Atlantic to Balkhash‐Junggar region plays a leading role in triggering extreme precipitation events Moisture sources in central Asia provide nearly half of all moisture for the extreme precipitation events The majority (60.1%) of moisture contributions come from moisture uptake that occurs within 1–4 days before extreme precipitation events

4. Moisture and Energy Budget Perspectives on Summer Drought in North China

   Lan Dai, Jonathon S Wright, and Rong Fu

   Journal of Climate, 2020

   Abs DOI

   Abstract We investigate the physical processes behind summer drought in North China by evaluating moisture and energy budget diagnostics and linking them to anomalous large-scale circulation patterns. Moisture budget analysis reveals that summer drought in North China was caused dynamically by reduced vertical moisture advection due to anomalous subsidence and reduced horizontal moisture advection due to anomalous northeasterly winds. Energy budget analysis shows that reduced latent heating was balanced dynamically by decreased dry static energy (DSE) divergence in the middle-to-upper troposphere. Linking these results to previous work, we suggest that summer drought in North China was predicated on co-occurrence of the positive phases of the Eurasian (EU) and Pacific–Japan (PJ) teleconnection patterns, potentially modulated by the circumglobal teleconnection (CGT). In the typical case, the negative phase of the CGT intensified the positive EU-related upper-level cyclone. Resulting upper-level cooling and positive surface feedback imposed a cold-core surface anticyclone that weakened with height. By contrast, when the positive phase of the CGT occurred in tandem with the positive EU and PJ patterns, the anticyclone had a warm core and intensified with height. The two cases were unified by strong subsidence but exhibited opposite meridional advection anomalies. In the cold-core cases, meridional moisture inflow was reduced but meridional DSE export was enhanced, further limiting precipitation while maintaining negative thermal anomalies. In the warm-core case, which only occurred once, enhanced meridional inflow of water vapor supplied moisture for sporadic precipitation while reduced meridional DSE export helped to maintain strong static stability.

5. Three Regimes of Temperature Distribution Change Over Dry Land, Moist Land, and Oceanic Surfaces

   Suqin Q. Duan, Kirsten L. Findell, and Jonathon S. Wright

   Geophysical Research Letters, 2020

   Abs DOI

   Climate model simulations project different regimes of summertime temperature distribution changes under a quadrupling of CO2 for dry land, moist land, and oceanic surfaces. The entire temperature distribution shifts over dry land surfaces, while moist land surfaces feature an elongated upper tail of the distribution, with extremes increasing more than the corresponding means by ∼20% of the global mean warming. Oceanic surfaces show weaker warming relative to land surfaces, with no significant elongation of the upper tail. Dry land surfaces show little change in turbulent sensible (SH) or latent (LH) fluxes, with new balance reached with compensating adjustments among downwelling and upwelling radiative fluxes. By contrast, moist land surfaces show enhanced partitioning of turbulent flux toward SH, while oceanic surfaces show enhanced partitioning toward LH. Amplified warming of extreme temperatures over moist land surfaces is attributed to suppressed evapotranspiration and larger Bowen ratios. Dry land, moist land, and oceanic surfaces experience different changes in summertime mean and extreme temperatures Moist land surfaces feature larger warming in extreme‐relative‐to‐mean temperatures, while dry surfaces warm more in the mean Amplified warming in extreme temperatures is attributed to suppressed evapotranspiration and larger Bowen ratios Dry land, moist land, and oceanic surfaces experience different changes in summertime mean and extreme temperatures Moist land surfaces feature larger warming in extreme‐relative‐to‐mean temperatures, while dry surfaces warm more in the mean Amplified warming in extreme temperatures is attributed to suppressed evapotranspiration and larger Bowen ratios

2019

1. The efficiency of transport into the stratosphere via the Asian and North American summer monsoon circulations

   Xiaolu Yan, Paul Konopka, Felix Ploeger, and 4 more authors

   Atmospheric Chemistry and Physics, 2019

   Abs DOI

   Transport of pollutants into the stratosphere via the Asian summer monsoon (ASM) or North American summer monsoon (NASM) may affect the atmospheric composition and climate both locally and globally. We identify and study the robust characteristics of transport from the ASM and NASM regions to the stratosphere using the Lagrangian chemistry transport model CLaMS driven by both the ERA-Interim and MERRA-2 reanalyses. In particular, we quantify the relative influences of the ASM and NASM on stratospheric composition and investigate the transport pathways and efficiencies of transport of air masses originating at different altitudes in these two monsoon regions to the stratosphere. We release artificial tracers in several vertical layers from the middle troposphere to the lower stratosphere in both ASM and NASM source regions during July and August 2010–2013 and track their evolution until the following summer. We find that more air mass is transported from the ASM and NASM regions to the tropical stratosphere, and even to the southern hemispheric stratosphere, when the tracers are released clearly below the tropopause (350–360 K) than when they are released close to the tropopause (370–380 K). For tracers released close to the tropopause (370–380 K), transport is primarily into the northern hemispheric lower stratosphere. Results for different vertical layers of air origin reveal two transport pathways from the upper troposphere over the ASM and NASM regions to the tropical pipe: (i) quasi-horizontal transport to the tropics below the tropopause followed by ascent to the stratosphere via tropical upwelling, and (ii) ascent into the stratosphere inside the ASM/NASM followed by quasi-horizontal transport to the tropical lower stratosphere and further to the tropical pipe. Overall, the tropical pathway (i) is faster than the monsoon pathway (ii), particularly in the ascending branch. The abundance of air in the tropical pipe that originates in the ASM upper troposphere (350–360 K) is comparable to the abundance of air ascending directly from the tropics to the tropical pipe 10 months after (the following early summer) the release of the source tracers. The air mass contributions from the ASM to the tropical pipe are about 3 times larger than the corresponding contributions from the NASM. The transport efficiency into the tropical pipe, the air mass fraction inside this destination region normalized by the mass of the domain of origin, is greatest from the ASM region at 370–380 K. Although the contribution from the NASM to the stratosphere is less than that from either the ASM or the tropics, the transport efficiency from the NASM is comparable to that from the tropics.

2. Temperature and tropopause characteristics from reanalyses data in the tropical tropopause layer

   Susann Tegtmeier, James Anstey, Sean Davis, and 10 more authors

   Atmospheric Chemistry and Physics, 2019

   Abs DOI

   The tropical tropopause layer (TTL) is the transition region between the well-mixed convective troposphere and the radiatively controlled stratosphere with air masses showing chemical and dynamical properties of both regions. The representation of the TTL in meteorological reanalysis data sets is important for studying the complex interactions of circulation, convection, trace gases, clouds, and radiation. In this paper, we present the evaluation of climatological and long-term TTL temperature and tropopause characteristics in the reanalysis data sets ERA-Interim, ERA5, JRA-25, JRA-55, MERRA, MERRA-2, NCEP-NCAR (R1), and CFSR. The evaluation has been performed as part of the SPARC (Stratosphere–troposphere Processes and their Role in Climate) Reanalysis Intercomparison Project (S-RIP). The most recent atmospheric reanalysis data sets (ERA-Interim, ERA5, JRA-55, MERRA-2, and CFSR) all provide realistic representations of the major characteristics of the temperature structure within the TTL. There is good agreement between reanalysis estimates of tropical mean temperatures and radio occultation data, with relatively small cold biases for most data sets. Temperatures at the cold point and lapse rate tropopause levels, on the other hand, show warm biases in reanalyses when compared to observations. This tropopause-level warm bias is related to the vertical resolution of the reanalysis data, with the smallest bias found for data sets with the highest vertical resolution around the tropopause. Differences in the cold point temperature maximize over equatorial Africa, related to Kelvin wave activity and associated disturbances in TTL temperatures. Interannual variability in reanalysis temperatures is best constrained in the upper TTL, with larger differences at levels below the cold point. The reanalyses reproduce the temperature responses to major dynamical and radiative signals such as volcanic eruptions and the quasi-biennial oscillation (QBO). Long-term reanalysis trends in temperature in the upper TTL show good agreement with trends derived from adjusted radiosonde data sets indicating significant stratospheric cooling of around -0.5 to -1 K per decade. At 100 hPa and the cold point, most of the reanalyses suggest small but significant cooling trends of -0.3 to -0.6 K per decade that are statistically consistent with trends based on the adjusted radiosonde data sets. Advances of the reanalysis and observational systems over the last decades have led to a clear improvement in the TTL reanalysis products over time. Biases of the temperature profiles and differences in interannual variability clearly decreased in 2006, when densely sampled radio occultation data started being assimilated by the reanalyses. While there is an overall good agreement, different reanalyses offer different advantages in the TTL such as realistic profile and cold point temperature, continuous time series, or a realistic representation of signals of interannual variability. Their use in model simulations and in comparisons with climate model output should be tailored to their specific strengths and weaknesses.

3. From ERA-Interim to ERA5: the considerable impact of ECMWF’s next-generation reanalysis on Lagrangian transport simulations

   Lars Hoffmann, Gebhard Günther, Dan Li, and 8 more authors

   Atmospheric Chemistry and Physics, 2019

   Abs DOI

   The European Centre for Medium-Range Weather Forecasts’ (ECMWF’s) next-generation reanalysis ERA5 provides many improvements, but it also confronts the community with a “big data” challenge. Data storage requirements for ERA5 increase by a factor of ∼80 compared with the ERA-Interim reanalysis, introduced a decade ago. Considering the significant increase in resources required for working with the new ERA5 data set, it is important to assess its impact on Lagrangian transport simulations. To quantify the differences between transport simulations using ERA5 and ERA-Interim data, we analyzed comprehensive global sets of 10-day forward trajectories for the free troposphere and the stratosphere for the year 2017. The new ERA5 data have a considerable impact on the simulations. Spatial transport deviations between ERA5 and ERA-Interim trajectories are up to an order of magnitude larger than those caused by parameterized diffusion and subgrid-scale wind fluctuations after 1 day and still up to a factor of 2–3 larger after 10 days. Depending on the height range, the spatial differences between the trajectories map into deviations as large as 3 K in temperature, 30 % in specific humidity, 1.8 % in potential temperature, and 50 % in potential vorticity after 1 day. Part of the differences between ERA5 and ERA-Interim is attributed to the better spatial and temporal resolution of the ERA5 reanalysis, which allows for a better representation of convective updrafts, gravity waves, tropical cyclones, and other meso- to synoptic-scale features of the atmosphere. Another important finding is that ERA5 trajectories exhibit significantly improved conservation of potential temperature in the stratosphere, pointing to an improved consistency of ECMWF’s forecast model and observations that leads to smaller data assimilation increments. We conducted a number of downsampling experiments with the ERA5 data, in which we reduced the numbers of meteorological time steps, vertical levels, and horizontal grid points. Significant differences remain present in the transport simulations, if we downsample the ERA5 data to a resolution similar to ERA-Interim. This points to substantial changes of the forecast model, observations, and assimilation system of ERA5 in addition to improved resolution. A comparison of two Lagrangian trajectory models allowed us to assess the readiness of the codes and workflows to handle the comprehensive ERA5 data and to demonstrate the consistency of the simulation results. Our results will help to guide future Lagrangian transport studies attempting to navigate the increased computational complexity and leverage the considerable benefits and improvements of ECMWF’s new ERA5 data set.

4. Surface temperature response to the major volcanic eruptions in multiple reanalysis data sets

   Masatomo Fujiwara, Patrick Martineau, and Jonathon S. Wright

   Atmospheric Chemistry and Physics, 2019

   Abs DOI

   The global response of air temperature at 2 m above the surface to the eruptions of Mount Agung in March 1963, El Chichón in April 1982, and Mount Pinatubo in June 1991 is investigated using 11 global atmospheric reanalysis data sets (JRA-55, JRA-25, MERRA-2, MERRA, ERA-Interim, ERA-40, CFSR, NCEP-NCAR R-1, 20CR version 2c, ERA-20C, and CERA-20C). Multiple linear regression (MLR) is applied to the monthly mean time series of temperature for two periods – 1980–2010 (for 10 reanalyses) and 1958–2001 (for 6 reanalyses) – by considering explanatory factors of seasonal harmonics, linear trends, quasi-biennial oscillation (QBO), solar cycle, tropical sea surface temperature (SST) variations in the Pacific, Indian, and Atlantic Oceans, and Arctic SST variations. Empirical orthogonal function (EOF) analysis is applied to these climatic indices to obtain a set of orthogonal indices to be used for the MLR. The residuals of the MLR are used to define the volcanic signals for the three eruptions separately. First, area-averaged time series of the residuals are investigated and compared with the results from previous studies. Then, the geographical distribution of the response during the peak cooling period after each eruption is investigated. In general, different reanalyses show similar geographical patterns of the response, but with the largest differences in the polar regions. The Pinatubo response shows the largest average cooling in the 60∘ N–60∘ S region among the three eruptions, with a peak cooling of 0.10–0.15 K. The El Chichón response shows slightly larger cooling in the NH than in the Southern Hemisphere (SH), while the Agung response shows larger cooling in the SH. These hemispheric differences are consistent with the distribution of stratospheric aerosol optical depth after these eruptions; however, the peak cooling after these two eruptions is comparable in magnitude to unexplained cooling events in other periods without volcanic influence. Other methods in which the MLR model is used with different sets of indices are also tested, and it is found that careful treatment of tropical SST variability is necessary to evaluate the surface response to volcanic eruptions in observations and reanalyses.

5. Multitimescale variations in modeled stratospheric water vapor derived from three modern reanalysis products

   Mengchu Tao, Paul Konopka, Felix Ploeger, and 5 more authors

   Atmospheric Chemistry and Physics, 2019

   Abs DOI

   Stratospheric water vapor (SWV) plays important roles in the radiation budget and ozone chemistry and is a valuable tracer for understanding stratospheric transport. Meteorological reanalyses provide variables necessary for simulating this transport; however, even recent reanalyses are subject to substantial uncertainties, especially in the stratosphere. It is therefore necessary to evaluate the consistency among SWV distributions simulated using different input reanalysis products. In this study, we evaluate the representation of SWV and its variations on multiple timescales using simulations over the period 1980–2013. Our simulations are based on the Chemical Lagrangian Model of the Stratosphere (CLaMS) driven by horizontal winds and diabatic heating rates from three recent reanalyses: ERA-Interim, JRA-55 and MERRA-2. We present an intercomparison among these model results and observationally based estimates using a multiple linear regression method to study the annual cycle (AC), the quasi-biennial oscillation (QBO), and longer-term variability in monthly zonal-mean H2O mixing ratios forced by variations in the El Niño–Southern Oscillation (ENSO) and the volcanic aerosol burden. We find reasonable consistency among simulations of the distribution and variability in SWV with respect to the AC and QBO. However, the amplitudes of both signals are systematically weaker in the lower and middle stratosphere when CLaMS is driven by MERRA-2 than when it is driven by ERA-Interim or JRA-55. This difference is primarily attributable to relatively slow tropical upwelling in the lower stratosphere in simulations based on MERRA-2. Two possible contributors to the slow tropical upwelling in the lower stratosphere are suggested to be the large long-wave cloud radiative effect and the unique assimilation process in MERRA-2. The impacts of ENSO and volcanic aerosol on H2O entry variability are qualitatively consistent among the three simulations despite differences of 50 %–100 % in the magnitudes. Trends show larger discrepancies among the three simulations. CLaMS driven by ERA-Interim produces a neutral to slightly positive trend in H2O entry values over 1980–2013 (+0.01 ppmv decade-1), while both CLaMS driven by JRA-55 and CLaMS driven by MERRA-2 produce negative trends but with significantly different magnitudes (-0.22 and -0.08 ppmv decade-1, respectively).

6. Managing nitrogen to restore water quality in China

   ChaoQing Yu, Xiao Huang, Han Chen, and 22 more authors

   Nature, 2019

   Abs DOI

   The nitrogen cycle has been radically changed by human activities1. China consumes nearly one third of the world’s nitrogen fertilizers. The excessive application of fertilizers2,3 and increased nitrogen discharge from livestock, domestic and industrial sources have resulted in pervasive water pollution. Quantifying a nitrogen ‘boundary’4 in heterogeneous environments is important for the effective management of local water quality. Here we use a combination of water-quality observations and simulated nitrogen discharge from agricultural and other sources to estimate spatial patterns of nitrogen discharge into water bodies across China from 1955 to 2014. We find that the critical surface-water quality standard (1.0 milligrams of nitrogen per litre) was being exceeded in most provinces by the mid-1980s, and that current rates of anthropogenic nitrogen discharge (14.5 ± 3.1 megatonnes of nitrogen per year) to fresh water are about 2.7 times the estimated ‘safe’ nitrogen discharge threshold (5.2 ± 0.7 megatonnes of nitrogen per year). Current efforts to reduce pollution through wastewater treatment and by improving cropland nitrogen management can partially remedy this situation. Domestic wastewater treatment has helped to reduce net discharge by 0.7 ± 0.1 megatonnes in 2014, but at high monetary and energy costs. Improved cropland nitrogen management could remove another 2.3 ± 0.3 megatonnes of nitrogen per year—about 25 per cent of the excess discharge to fresh water. Successfully restoring a clean water environment in China will further require transformational changes to boost the national nutrient recycling rate from its current average of 36 per cent to about 87 per cent, which is a level typical of traditional Chinese agriculture. Although ambitious, such a high level of nitrogen recycling is technologically achievable at an estimated capital cost of approximately 100 billion US dollars and operating costs of 18–29 billion US dollars per year, and could provide co-benefits such as recycled wastewater for crop irrigation and improved environmental quality and ecosystem services. Estimates of spatial patterns of nitrogen discharge into water bodies across China between 1955 and 2014 show that current discharge rates are almost three times the acceptable threshold, and ways to restore a clean water environment are suggested.

7. Synoptic Conditions and Moisture Sources for Extreme Snowfall Events Over East China

   Zifan Yang, Wenyu Huang, Xinsheng He, and 4 more authors

   Journal of Geophysical Research: Atmospheres, 2019

   Abs DOI

   The favorable circulation pattern and moisture sources for wintertime extreme snowfall events over East China during 1979–2017 are explored. A coupling between a Rossby wave train along the subtropical jet stream (i.e., the circumglobal teleconnection) and central Siberian blocking activities plays a leading role in triggering these events. This coupling leads to the formation of cyclonic anomalies, strong southwesterly winds, large‐scale moisture convergence, and powerful updrafts over East China, all of which prompt the occurrence of precipitation. The central Siberian blocking activities increase the fraction of the precipitation that falls as snowfall through enhancing cold air intrusion into East China. An Eulerian moisture budget analysis reveals that moisture fluxes across the southern and western boundaries of the East China region provide the moisture for the precipitation during the extreme snowfall events, while the increase in the precipitable water helps extend the duration of these events. The geographic distributions of the evaporative moisture sources for both the snowfall and rainfall during these extreme events are estimated using a Lagrangian approach. This analysis reveals that the major moisture sources for snowfall originate from land areas, while those for the rainfall originate from sea areas. In addition, local moisture recycling over East China provides a remarkably higher relative contribution to snowfall than it does to rainfall. The identification of the synoptic conditions and moisture sources not only improves our understanding of the formation of extreme snowfall events over East China but also provides forecast guidance for extreme snow disasters. The coupling of the circumglobal teleconnection and central Siberian blocking activities is critical to East China extreme snowfall events Central Siberian blocking activities favor a considerable part of the precipitation over East China occurring as snowfall Local moisture recycling plays a leading role in providing the evaporative moisture sources for extreme snowfall events

8. Evaluating the Response of Summertime Surface Sulfate to Hydroclimate Variations in the Continental United States: Role of Meteorological Inputs in the GEOS‐Chem Model

   Yuanyu Xie, Yuxuan Wang, Wenhao Dong, and 3 more authors

   Journal of Geophysical Research: Atmospheres, 2019

   Abs DOI

   Understanding the response of sulfate to climate change is crucial given tight couplings between sulfate and the hydrological cycle. As the sources and sinks of sulfate are sensitive to cloud and precipitation processes, the accuracy of model simulations depends on the accuracy of these meteorological inputs. In this study, we evaluate the GEOS‐Chem model in simulating summertime surface sulfate concentrations in the continental United States across different levels of dryness and compare the model performance based on two sets of meteorological fields: Modern Era Retrospective Analysis for Research and Applications (MERRA) and MERRA‐2. Both simulations fail to reproduce observed increases in sulfate during drought, as indicated by negative correlation slopes between surface sulfate concentrations and the standardized precipitation evapotranspiration index (SPEI). This deficiency can be largely attributed to too large a decrease in clouds and hence aqueous phase sulfate production as conditions shift from wet to dry. MERRA‐2‐driven GEOS‐Chem (M2GC) shows improvements in cloud and precipitation fields relative to the MERRA‐driven GEOS‐Chem, hence eliminating approximately half of the bias in the simulated sulfate‐SPEI slope. However, M2GC still underestimates boundary layer cloud fraction, overestimates liquid water content, and overestimates the rates of the decrease in both quantities as conditions become drier. Explicitly correcting these cloud biases in M2GC results in a 60–80% reduction of the bias in the simulated sulfate‐SPEI slope. The strong sensitivity of simulated sulfate to prescribed cloud fields suggests the need for more comprehensive assessment of cloud inputs for sulfate simulations under current and future climate change scenarios. The GEOS‐Chem model does not capture the observed sulfate response to hydroclimate variations during summertime The model deficiency is traced to too large a decrease in clouds and hence aqueous phase sulfate production from wet to dry conditions Simulations with updated cloud fields better reproduce both sulfate concentration and wet deposition flux across different levels of dryness

9. A global database of water vapor isotopes measured with high temporal resolution infrared laser spectroscopy

   Zhongwang Wei, Xuhui Lee, Franziska Aemisegger, and 24 more authors

   Scientific Data, 2019

   Abs DOI

   The isotopic composition of water vapour provides integrated perspectives on the hydrological histories of air masses and has been widely used for tracing physical processes in hydrological and climatic studies. Over the last two decades, the infrared laser spectroscopy technique has been used to measure the isotopic composition of water vapour near the Earth’s surface. Here, we have assembled a global database of high temporal resolution stable water vapour isotope ratios (δ18O and δD) observed using this measurement technique. As of March 2018, the database includes data collected at 35 sites in 15 Köppen climate zones from the years 2004 to 2017. The key variables in each dataset are hourly values of δ18O and δD in atmospheric water vapour. To support interpretation of the isotopologue data, synchronized time series of standard meteorological variables from in situ observations and ERA5 reanalyses are also provided. This database is intended to serve as a centralized platform allowing researchers to share their vapour isotope datasets, thus facilitating investigations that transcend disciplinary and geographic boundaries.

10. Moisture Sources for Wintertime Intense Precipitation Events Over the Three Snowy Subregions of the Tibetan Plateau

    Tianpei Qiu, Wenyu Huang, Jonathon S. Wright, and 7 more authors

    Journal of Geophysical Research: Atmospheres, 2019

    Abs DOI

    Wintertime intense precipitation events often lead to severe snow disasters. In this study, a Lagrangian approach is employed to examine the evaporative moisture sources for wintertime intense precipitation events over the three snowy subregions of the Tibetan Plateau (TP) during 1979–2016, including the western TP (WTP), south central TP (SCTP), and southeastern TP (SETP). More than 80.0% of the moisture for intense precipitation over each subregion originates from terrestrial areas. Although prevailing westerly winds dominate above the TP and its surrounding areas during winter, half of the precipitation over the three subregions is supplied by evaporation from the south (i.e., the Indian Peninsula). Specifically, evaporation from the Indian Peninsula contributes 68.0%, 65.0%, and 45.0% of the moisture for intense precipitation over the WTP, SCTP, and SETP, respectively. The two primary oceanic moisture source regions for intense precipitation are the Arabian Sea and the Bay of Bengal, playing complementary roles in supplying moisture. The relative contributions of the Arabian Sea to intense precipitation over the WTP, SCTP, and SETP are 9.2%, 6.9%, and 1.1%, while those of the Bay of Bengal are 1.1%, 12.1%, and 8.6%. Southerly winds downstream of a cyclonic anomaly over the Indian Peninsula are crucial for the low‐level moisture transport from the south to the Himalayan foothills. Under the combined effects of orographic lifting and favorable large‐scale circulation patterns, moisture ascends further into the three subregions. Changes in the position and intensity of the cyclonic anomaly are particularly crucial to facilitating moisture contributions from the key source regions. The Indian Peninsula plays a leading role in supplying moisture for intense precipitation over the snowy subregions of the Tibetan Plateau A cyclonic anomaly over the Indian Peninsula is crucial for northward lower‐level moisture transport to the Himalayan foothills A cyclonic anomaly embedded in the subtropical jet favors the onset of intense precipitation events over each snowy subregion

11. Impacts of Wintertime Extratropical Cyclones on Temperature and Precipitation Over Northeastern China During 1979–2016

    Daiyu Lin, Wenyu Huang, Zifan Yang, and 4 more authors

    Journal of Geophysical Research: Atmospheres, 2019

    Abs DOI

    This study examines the synoptic impacts of wintertime extratropical cyclones on northeastern China. The 181 analyzed cyclones mainly originate from eastern Mongolia and dissipate over the North Pacific Ocean. Precursor negative geopotential height anomalies for these cyclones emerge over continental areas near the Barents and Kara Seas. Baroclinic instability and midlatitude westerly winds are the dominant factors driving the development and movement of these cyclones from their precursor signals. Owing to the southeastward migration of the cyclones, temperature over northeastern China first increases and then decreases, resulting in a significant increase in the frequency of extreme weather. These cyclones contribute about 70.6% of the extreme precipitation days over northeastern China. Relative moisture contributions from the East China Sea, the Sea of Japan, and East Asia are all anomalously large during cyclones associated with extreme precipitation days, with moisture from these regions entering northeastern China mainly through its southern and western boundaries. Temperature variations over northeastern China play a critical role in the accumulation of moisture for extreme precipitation and the timing of that precipitation. Moisture transport initially contributes to an increase in precipitable water due to warm temperature anomalies. The added precipitable water is then released as extreme precipitation when the atmosphere over northeastern China starts to cool. Precursors for extratropical cyclones over northeastern China occur in continental areas close to the Barents and Kara Seas Extratropical cyclones initially warm then cool northeastern China Extratropical cyclones play a leading role in triggering wintertime extreme precipitation over northeastern China

12. Evapotranspiration Characteristics Distinct to Mangrove Ecosystems Are Revealed by Multiple‐Site Observations and a Modified Two‐Source Model

    Jie Liang, Zhongwang Wei, Xuhui Lee, and 4 more authors

    Water Resources Research, 2019

    Abs DOI

    A quantitative accounting of how mangrove ecosystems respond to tidal perturbations is needed to anticipate changes in these ecosystems when sea level rises. Here we use long‐term field observations and a two‐source ecohydrological model to reveal specialized characteristics of evapotranspiration (ET), soil surface evaporation (E), and canopy transpiration (T) in three subtropical mangrove ecosystems in southeastern China. Average wintertime ET observed in these three mangrove forests (2.6 mm day–1) was consistent with values for semiarid ecosystems, while average summertime ET (6.2 mm day–1) approached that observed in rainforests. By contrast, T fluxes were small year‐round, averaging 1.3 mm day–1 in winter and 2.5 mm day–1 in summer. Combining our results with measurements from three Florida mangroves, observed values of T ranged from 350 to 870 mm year−1, varying primarily with salinity, while T/ET increased exponentially from 30% to 70% with rising leaf area index. Simulations of half‐hourly ET and T using a modified two‐source model were highly correlated with eddy covariance observations of ET (I, index of agreement >0.93 at all three sites) and sap flow gauge‐based estimates of T (I = 0.93 at the Yunxiao site). Variations of T in mangrove ecosystems are distinguished from those in terrestrial forests mainly by the sensitivity of stomatal conductance to leaf temperature, with tidal and salinity effects superimposed. Our modified model accounts for these effects and therefore holds promise for improving our understanding of how mangrove ecosystems may respond to changing stress conditions under global warming and sea level rise. Extension of the two‐source model permits reliable half‐hourly simulations of transpiration fluxes in three tidal mangrove ecosystems Suppression of transpiration under high temperatures is stronger in mangroves than in well‐watered ecosystems The narrow temperature tolerance range and evident tidal effects imply potential further effects of climate change on mangrove transpiration

13. Subtropical High Affects Interdecadal Variability of Tropical Cyclone Genesis in the South China Sea

    Hong Li, Fanghua Xu, Jingru Sun, and 2 more authors

    Journal of Geophysical Research: Atmospheres, 2019

    Abs DOI

    Interdecadal variability of tropical cyclone (TC) genesis in the South China Sea (SCS) during 1982–2015 is investigated using observations and atmospheric reanalysis data. TC genesis primarily occurs in the northern SCS (north of 13 °N) in July–September (summer), while in the southern SCS (south of 13 °N) in October–December (autumn). The TC genesis location is consistent with the climatological distribution of TC genesis potential index. Noticeably, the TC genesis frequency (TCGF) is relatively low in 1982–1993 and 2003–2015 while relatively high in 1994–2002 in summer in the SCS. In autumn, the TCGF shows an abrupt transition from high to low in the early 2000s in the SCS. It is found that such interdecadal change of TCGF is closely related to the east‐westward movement of the subtropical high (SH). When the SH is close to the SCS, large‐scale air subsidence, low‐level divergence, negative vorticity, and high pressure are prominent and inhabit TC genesis in the SCS. On the contrary, when the SH moves away from the SCS, environmental conditions become more favorable for TC genesis. In addition, the localized atmospheric intraseasonal variability can affect TCGF at interdecadal time scales as well. The interdecadal variability of TCGF in the SCS is closely related to the east‐westward movement of WPSH (NASH) in summer (autumn) The east‐westward movement of SH affects TCGF in the SCS by influencing large‐scale environment South China Sea, Tropical Cyclone, Interdecadal Variability, Subtropical High

14. A possible mechanism for the occurrence of wintertime extreme precipitation events over South China

    Wenyu Huang, Zifan Yang, Xinsheng He, and 6 more authors

    Climate Dynamics, 2019

    Abs DOI

    This study examines the formation of circulation patterns favorable to wintertime extreme precipitation events over South China between 1979 and 2013. During these extreme precipitation events, a barotropic wave train having seven centers of action was observed to extend from the Arabian Sea to the west coast of the North American continent with a maximum amplitude at 300 hPa. A center of action located over South China, comprised of cyclonic anomalies, favored powerful updrafts and large-scale moisture convergence over South China. About 77% of wintertime extreme precipitation events in South China were preceded by European blocking events. The formation mechanism for the 141 precipitation events with pre-existing European blocking highs is presented. Rossby wave energy propagation associated with the positive phase of circumglobal teleconnection enabled the European blocking event with a lead time of ∼10 days to generate cyclonic anomalies over South China. Moreover, significant warm anomalies were present over South China before the onset of these blocking-related extreme precipitation events. Increases in atmospheric moisture holding capacity associated with these warm anomalies enabled a buildup of precipitable water via moisture fluxes into South China through the western and southern boundaries. Onset of the extreme precipitation events was then triggered by the intrusion of cold temperature anomalies from the north, which lifted warm moist air upward from the surface and lowered the moisture holding capacity, producing large amounts of precipitation.

15. Links Between the Large‐Scale Circulation and Daily Air Quality Over Central Eastern China During Winter

    Wanru Ge, Yuening Yin, Jonathon S. Wright, and 4 more authors

    Journal of Geophysical Research: Atmospheres, 2019

    Abs DOI

    We examine relationships between day‐to‐day variations in the large‐scale atmospheric circulation and the occurrence, intensity, and duration of haze during several recent winters, beginning with a focused examination of Beijing in 2013–2017 and then evaluating the extent to which the findings applied to other urban areas in China in 2015–2017. Composite analysis for Beijing indicates that haze preferentially occurred when the East Asian winter monsoon (EAWM; based on potential vorticity) and Siberian High (SH; based on sea level pressure) were weak. Daily variations in EAWM and SH are cross‐correlated but have different relationships with conditions that affect haze development. EAWM is more strongly anticorrelated with boundary layer humidity, suggesting a greater influence on particulate growth. SH, by contrast, has stronger relationships with thermodynamic stability and near‐surface wind speed. A linear combination of these two indices provides a simple but effective indicator of heightened pollution risk over much of central eastern China. The particular value of the EAWM index is supported by its applicability to a broader area and its links to air quality in Beijing during the winter of 2017–2018. Clean conditions during November–February coincided with a persistent strong EAWM despite the SH deviating little from its climatology. The EAWM weakened in March, when air quality reverted to conditions typical of the previous four winters. These results confirm and help to clarify links between the large‐scale circulation and air quality at daily to weekly time scales and provide an empirical foundation for evaluating models used to simulate air quality in this region. The Siberian High and East Asian winter monsoon affect the day‐to‐day occurrence, severity, and duration of haze in central eastern China These two circulations each provide some independent constraints on the mechanisms and spatiotemporal extent of haze events A simple combination of circulation indices reliably tracks the risk of severe haze over much of central eastern China

16. Precipitable water and CAPE dependence of rainfall intensities in China

    Wenhao Dong, Yanluan Lin, Jonathon S. Wright, and 3 more authors

    Climate Dynamics, 2019

    Abs DOI

    The influence of temperature on precipitation in China is investigated from two aspects of the atmospheric water cycle: available water vapor and atmospheric instability. Daily observations are used to analyze how rainfall intensities and its spatial distribution in mainland China depend on these two aspects. The results show that rainfall intensities, and especially rainfall extremes, increase exponentially with available water vapor. The efficiency of water vapor conversion to rainfall is higher in northwestern China where water vapor is scarce than in southeastern China where water vapor is plentiful. The results also reveal a power law relationship between rainfall intensity and convective instability. The fraction of convective available potential energy (CAPE) converted to upward velocity is much larger over southeastern China than over the arid northwest. The sensitivities of precipitation to temperature-induced changes in available water vapor and atmospheric convection are thus geographically reciprocal. Specifically, while conversion of water vapor to rainfall is relatively less efficient in southeastern China, conversion of CAPE to upward kinetic energy is more efficient. By contrast, in northwestern China, water vapor is efficiently converted to rainfall but only a small fraction of CAPE is converted to upward motion. The detailed features of these relationships vary by location and season; however, the influences of atmospheric temperature on rainfall intensities and rainfall extremes are predominantly expressed through changes in available water vapor, with changes in convective instability playing a secondary role.

2018

1. Influences of Pacific Climate Variability on Decadal Subsurface Ocean Heat Content Variations in the Indian Ocean

   Xiaolin Jin, Young-Oh Kwon, Caroline C. Ummenhofer, and 5 more authors

   Journal of Climate, 2018

   Abs DOI

   Decadal variabilities in Indian Ocean subsurface ocean heat content (OHC, 50–300m) since the 1950s are examined using ocean reanalyses. This study elaborates on how Pacific variability modulates the Indian Ocean on decadal timescales through both oceanic and atmospheric pathways. High correlations between OHC and thermocline depth variations across the entire Indian Ocean basin suggest that OHC variability is primarily driven by thermocline fluctuations. The spatial pattern of the leading mode of decadal Indian Ocean OHC variability closely matches the regression pattern of OHC on the Interdecadal Pacific Oscillation (IPO), emphasizing the role of the Pacific Ocean in determining Indian Ocean OHC decadal variability. Further analyses identify different mechanisms by which the Pacific influences the eastern and western Indian Ocean. IPO-related anomalies from the Pacific propagate mainly through oceanic pathways in the maritime continent to impact the eastern Indian Ocean. By contrast, in the western Indian Ocean, the IPO induces wind-driven Ekman pumping in the central Indian Ocean via the atmospheric bridge, which in turn modifies conditions in the southwestern Indian Ocean via westward-propagating Rossby waves. To confirm this, linear Rossby wave model is forced with wind stresses and eastern boundary conditions based on reanalyses. This linear model skillfully reproduces observed sea surface height anomalies and highlights both the oceanic connection in the eastern Indian Ocean and the role of wind-driven Ekman pumping in the west. These findings are also reproduced by OGCM hindcast experiments forced by interannual atmospheric boundary conditions applied only over the Pacific and Indian Oceans, respectively.

2. Zonal-mean data set of global atmospheric reanalyses on pressure levels

   Patrick Martineau, Jonathon S. Wright, Nuanliang Zhu, and 1 more author

   Earth System Science Data, 2018

   Abs DOI

   This data set, which is prepared for the Stratosphere–troposphere Processes And their Role in Climate (SPARC) Reanalysis Intercomparison Project (S-RIP), provides several zonal-mean diagnostics computed from reanalysis data on pressure levels. Diagnostics are currently provided for a variety of reanalyses, including ERA-40, ERA-Interim, ERA-20C, NCEP–NCAR, NCEP–DOE, CFSR, 20CR v2 and v2c, JRA-25, JRA-55, JRA-55C, JRA-55AMIP, MERRA, and MERRA-2. The data set will be expanded to include additional reanalyses as they become available. Basic dynamical variables (such as temperature, geopotential height, and three-dimensional winds) are provided in addition to a complete set of terms from the Eulerian-mean and transformed-Eulerian-mean momentum equations. Total diabatic heating and its long-wave and shortwave components are included as availability permits, along with heating rates diagnosed from the basic dynamical variables using the zonal-mean thermodynamic equation. Two versions of the data set are provided, one that uses horizontal and vertical grids provided by the various reanalysis centers and another that uses a common grid (CG) to facilitate comparison among data sets. For the common grid, all diagnostics are interpolated horizontally onto a regular 2.5∘×2.5∘ grid for a subset of pressure levels that are common among all included reanalyses. The dynamical 10.5285/b241a7f536a244749662360bd7839312 and diabatic 10.5285/70146c789eda4296a3c3ab6706931d56 variables are archived and maintained by the Centre for Environmental Data Analysis (CEDA).

3. Distinct Mechanisms of Decadal Subsurface Heat Content Variations in the Eastern and Western Indian Ocean Modulated by Tropical Pacific SST Distinct Mechanisms of Decadal Subsurface Heat Content Variations in the Eastern and Western Indian Ocean Modulated by Tropical Pacific SST

   Xiaolin Jin, Young-Oh Kwon, Caroline C. Ummenhofer, and 3 more authors

   Journal of Climate, 2018

   Abs DOI

   Decadal variability of the subsurface ocean heat content (OHC) in the Indian Ocean is investigated using a coupled climate model experiment, in which observed eastern tropical Pacific sea surface temperature (EPSST) anomalies are specified. This study intends to understand the contributions of external forcing relative to those of internal variability associated with EPSST, as well as the mechanisms by which the Pacific impacts Indian Ocean OHC. Internally generated variations associated with EPSST dominate decadal variations in the subsurface Indian Ocean. Consistent with ocean reanalyses, the coupled model reproduces a pronounced east–west dipole structure in the southern tropical Indian Ocean and discontinuities in westward-propagating signals in the central Indian Ocean around 100°E. This implies distinct mechanisms by which the Pacific impacts the eastern and western Indian Ocean on decadal time scales. Decadal variations of OHC in the eastern Indian Ocean are attributed to 1) western Pacific surface wind anomalies, which trigger oceanic Rossby waves propagating westward through the Indonesian Seas and influence Indonesian Throughflow transport, and 2) zonal wind anomalies over the central tropical Indian Ocean, which trigger eastward-propagating Kelvin waves. Decadal variations of OHC in the western Indian Ocean are linked to conditions in the Pacific via changes in the atmospheric Walker cell, which trigger anomalous wind stress curl and Ekman pumping in the central tropical Indian Ocean. Westward-propagating oceanic Rossby waves extend the influence of this anomalous Ekman pumping to the western Indian Ocean.

4. Assessing the Impacts of Extreme Agricultural Droughts in China Under Climate and Socioeconomic Changes

   Chaoqing Yu, Xiao Huang, Han Chen, and 10 more authors

   Earth’s Future, 2018

   Abs DOI

   Agricultural food production in China is deeply vulnerable to extreme droughts. Although there are many studies to evaluate this issue from different aspects, comprehensive assessments with full consideration of climate change, crop rotations, irrigation effects, and socioeconomic factors in broad scales have not been well addressed. Considering both the probability of drought occurrence and the consequential yield losses, here we propose an integrated approach for assessing past and future agricultural drought risks that relies on multimodel ensemble simulations calibrated for rice, maize, and wheat (RMW) in China. Our results show that irrigation has reduced drought‐related yield losses by 31 ± 2%; the largest reductions in food production were primarily attributable to socioeconomic factors rather than droughts during 1955–2014. Unsustainable water management, especially groundwater management, could potentially cause disastrous consequences in both food production and water supply in extreme events. Our simulations project a rise of 2.5\textbackslashtextasciitilde3.3% in average rice, maize, and wheat productivity before 2050 but decrease thereafter if climate warming continues. The frequency of extreme agricultural droughts in China is projected to increase under all examined Representative Concentration Pathway (RCP). A current 100‐year drought is projected to occur once every 30 years under RCP 2.6, once every 13 years under RCP 4.5, and once every 5 years under RCP 8.5. This increased occurrence of severe droughts would double the rate of drought‐induced yield losses in the largest warming scenario. Policies for future food security should prioritize sustainable intensification and conservation of groundwater, as well as geographically balanced water resource and food production. The development of irrigation in China has alleviated negative drought impacts on food productivity, the presence of irrigation systems explaining 31 ± 2% of avoided yield loss Frequency and intensity of extreme droughts are projected to increase significantly under future climate change, leading to double the drought‐induced yield losses in the largest warming scenarios The sharpest drops in grain production during the past 60 years have been driven primarily by socioeconomic disturbances rather than by large droughts

5. Long-Term Annual Mapping of Four Cities on Different Continents by Applying a Deep Information Learning Method to Landsat Data

   Haobo Lyu, Hui Lu, Lichao Mou, and 8 more authors

   Remote Sensing, 2018

   Abs DOI

   Urbanization is a substantial contributor to anthropogenic environmental change, and often occurs at a rapid pace that demands frequent and accurate monitoring. Time series of satellite imagery collected at fine spatial resolution using stable spectral bands over decades are most desirable for this purpose. In practice, however, temporal spectral variance arising from variations in atmospheric conditions, sensor calibration, cloud cover, and other factors complicates extraction of consistent information on changes in urban land cover. Moreover, the construction and application of effective training samples is time-consuming, especially at continental and global scales. Here, we propose a new framework for satellite-based mapping of urban areas based on transfer learning and deep learning techniques. We apply this method to Landsat observations collected during 1984–2016 and extract annual records of urban areas in four cities in the temperate zone (Beijing, New York, Melbourne, and Munich). The method is trained using observations of Beijing collected in 1999, and then used to map urban areas in all target cities for the entire 1984–2016 period. The method addresses two central challenges in long term detection of urban change: temporal spectral variance and a scarcity of training samples. First, we use a recurrent neural network to minimize seasonal urban spectral variance. Second, we introduce an automated transfer strategy to maximize information gain from limited training samples when applied to new target cities in similar climate zones. Compared with other state-of-the-art methods, our method achieved comparable or even better accuracy: the average change detection accuracy during 1984–2016 is 89% for Beijing, 94% for New York, 93% for Melbourne, and 89% for Munich, and the overall accuracy of single-year urban maps is approximately 96 ± 3% among the four target cities. The results demonstrate the practical potential and suitability of the proposed framework. The method is a promising tool for detecting urban change in massive remote sensing data sets with limited training data.

6. Interannual Variation and Regime Shift of the Evaporative Moisture Sources for Wintertime Precipitation Over Southern China

   Zifan Yang, Wenyu Huang, Tianpei Qiu, and 3 more authors

   Journal of Geophysical Research: Atmospheres, 2018

   Abs DOI

   The evaporative moisture source for wintertime precipitation over southern China during 1979–2017 is determined using a Lagrangian methodology. The five most important moisture source regions, in descending order, are the South China Sea, southern China (local moisture recycling), southeastern Asia, the East China Sea, and the western North Pacific. The dominant mode of variability of moisture contribution appears at the interannual time scale, which is closely related to the El Niño Southern Oscillation variability. The conventional El Niño signal favors moisture transport from the surrounding moisture source regions through an enhancement of an anticyclonic anomaly over the western North Pacific, while El Niño Modoki drives moisture recycling over southern China by cutting off access to moisture sources from surrounding regions. A regime shift of the evaporative moisture source is identified to occur around 1991/1992, after which there was a significant eastward migration of the moisture source. This migration is closely related to the presence of a cyclonic anomaly over the South China Sea. Both the observational and experimental results reveal that the appearance of this cyclonic anomaly is due to the warming over the Maritime Continent during the last two decades. The moisture contribution from the South China Sea is modulated by the circulation pattern rather than evaporation from ocean surface The two types of El Nino have distinct impacts on the moisture contribution An eastward migration of the moisture source occurred during 1991/1992, owing to the Maritime Continent warming

7. The Evaluation of SMAP Enhanced Soil Moisture Products Using High-Resolution Model Simulations and In-Situ Observations on the Tibetan Plateau

   Chengwei Li, Hui Lu, Kun Yang, and 7 more authors

   Remote Sensing, 2018

   Abs DOI

   The Soil Moisture Active Passive (SMAP) mission was designed to provide a global mapping of soil moisture (SM) measured by L-band passive and active microwave sensors. In this study, we evaluate the newly released SMAP enhanced SM products over the Tibetan Plateau by performing comparisons among SMAP standard products, in-situ observations and Community Land Model (CLM) simulations driven by high-resolution meteorological forcing. At local scales, the enhanced SMAP products, the standard products and CLM simulations all generally compare well with the in-situ observations. The SMAP products show stronger correlations (0.64–0.88) but slightly larger unbiased root mean square errors (ubRMSE, \textbackslashtextasciitilde0.06) relative to the CLM simulations (0.58–0.79 and 0.037–0.047, for correlation and ubRMSE, respectively). At the regional scale, both SMAP products show similar spatial distributions of SM on the TP (Tibetan Plateau), although, as expected, the enhanced product provides more fine details. The SMAP enhanced product is in good agreement with model simulations with respect to temporal and spatial variations in SM over most of the TP. Regions with low correlation between SMAP enhanced products and model simulations are mainly located in the northwestern TP and regions of complex topography, where meteorological stations are sparse and non-existent or elevation is highly variable. In such remote regions, CLM simulations may be problematic due to inaccurate land cover maps and/or uncertainties in meteorological forcing. The independent, high-resolution observations provided by SMAP could help to constrain the model simulation and, ultimately, improve the skill of models in these problematic regions.

8. Possible mechanisms for four regimes associated with cold events over East Asia

   Zifan Yang, Wenyu Huang, Bin Wang, and 3 more authors

   Climate Dynamics, 2018

   Abs DOI

   Circulation patterns associated with cold events over East Asia during the winter months of 1948–2014 are classified into four regimes by applying a k-means clustering method based on the area-weighted pattern correlation. The earliest precursor signals for two regimes are anticyclonic anomalies, which evolve into Ural and central Siberian blocking-like circulation patterns. The earliest precursor signals for the other two regimes are cyclonic anomalies, both of which evolve to amplify the East Asian trough (EAT). Both the blocking-like circulation patterns and amplified EAT favor the initialization of cold events. On average, the blocking-related regimes tend to last longer. The lead time of the earliest precursor signal for the central Siberian blocking-related regime is only 4 days, while those for the other regimes range from 16 to 18 days. The North Atlantic Oscillation plays essential roles both in triggering the precursor for the Ural blocking-related regime and in amplifying the precursors for all regimes. All regimes preferentially occur during the positive phase of the Eurasian teleconnection pattern and the negative phase of the El Niño–Southern Oscillation. For three regimes, surface cooling is primarily due to reduced downward infrared radiation and enhanced cold advection. For the remaining regime, which is associated with the southernmost cooling center, sensible and latent heat release and horizontal cold advection dominate the East Asian cooling.

9. A Large Eddy Model Study on the Effect of Overshooting Convection on Lower Stratospheric Water Vapor

   Wenjun Sang, Qian Huang, Wenshou Tian, and 6 more authors

   Journal of Geophysical Research: Atmospheres, 2018

   Abs DOI

   Using a cloud‐resolving large eddy model (LEM), we investigate how overshooting convection affects the water vapor content in the lower stratosphere. We design and conduct a series of sensitivity experiments to diagnose the effects of dynamical and thermodynamical background conditions on the transport of water vapor into the lower stratosphere associated with overshooting convection. The three‐dimensional LEM simulations capture the bulk properties of the target case and track microphysical processes using a three‐phase microphysical parameterization. The model results indicate that the net effect of overshooting convection on lower stratospheric water content is moistening, primarily due to gravity wave breaking and ice sublimation. The contributions of small‐scale turbulent mixing to water vapor transport from the overshooting turret into the stratosphere are relatively weak. Sensitivity experiments show that convective intensity (as measured by updraft velocity) is directly related to the effect of overshooting convection on lower stratospheric humidity. This impact is quantified for the idealized target case. Changes in vertical wind shear near the tropopause have no significant impact on the extent of overshooting but have important impacts on cross‐tropopause water vapor exchange via their modulation of gravity wave breaking. Larger vertical wind shear in the tropopause layer inhibits the transport of water vapor and ice into the lower stratosphere by overshooting convection. Idealized simulations of overshooting tropical convection indicate net moistening of the lower stratosphere Stratospheric moistening results from ice sublimation and gravity wave breaking near the tropopause Effects of near‐tropopause vertical wind shear are mainly through modulation of gravity wave breaking rather than turbulent mixing

10. Leaf anatomical traits determine the 18O enrichment of leaf water in coastal halophytes

    Jie Liang, Jonathan S. Wright, Xiaowei Cui, and 3 more authors

    Plant, Cell & Environment, 2018

    Abs DOI

    Anatomical adaptations to high‐salinity environments in mangrove leaves may be recorded in leaf water isotopes. Recent studies observed lower 18O enrichment (ΔL) of leaf water with respect to source water in three mangrove species relative to adjacent freshwater trees, but the factors that govern this phenomenon remain unclear. To resolve this issue, we investigated leaf traits and ΔL in 15 species of true mangrove plants, 14 species of adjacent freshwater trees, and 4 species of semi‐mangrove plants at five study sites along south‐eastern coast of China. Our results confirm that ΔL was generally 3–4‰ lower for mangrove species than for adjacent freshwater or semi‐mangrove species. We hypothesized that higher leaf water content (LWC) and lower leaf stomatal density (LS) both played important roles in reducing ΔL in mangroves relative to nearby freshwater plants. Both differences acted to elongate effective leaf mixing length (L) in mangroves by about 200% and lower stomatal conductance by about 30%. Péclet models based on both LWC and LS could accurately predict ΔL. Our findings highlight the potential species‐specific anatomical determinants of ΔL (or L), which has important implications for the interpretation of environmental information from metabolites produced by leaf water isotopes in palaeoclimate research. Mangrove forests evolve typical leaf structures to adapt to high‐salinity environments, and many of these adaptations may leave imprints in leaf water isotopes. Our comprehensive field study confirmed that the isotope enrichment of leaf water (ΔL) in mangroves were generally lower than those of the adjacent nonmangrove at multiple‐site and multiple‐species scales and that leaf anatomies play key roles in determining these differences in ΔL. The potential species‐specific anatomical determinants of ΔL based on a large pool of species can be incorporated into the leaf water model, which will have considerable potential applications not only for palaeoclimate reconstruction but also in distinguishing what kinds of plants with strong Péclet effect.

11. Contributions of Atmospheric Transport and Rain–Vapor Exchange to Near-Surface Water Vapor in the Zhanjiang Mangrove Reserve, Southern China: An Isotopic Perspective

    Xiang Lai, Jonathon S. Wright, Wenyu Huang, and 3 more authors

    Atmosphere, 2018

    Abs DOI

    Coastal mangroves are increasingly recognized as valuable natural resources and important sites of water and carbon exchange. In this study, we examine atmospheric water cycling in the boundary layer above a coastal mangrove forest in southern China. We collected site observations of isotopic ratios in water vapor and precipitation along with core meteorological variables during July 2017. Our evaluation of these data highlights the influences of large-scale atmospheric transport and rain–vapor exchange in the boundary layer water budget. Rain–vapor exchange takes different forms for different types of rainfall events. The evolution of isotopic ratios in water vapor suggests that substantial rain recycling occurs during the passage of large-scale organized convective systems, but that this process is much weaker during rainfall associated with less organized events of local origin. We further examine the influences of large-scale transport during the observation period using a Lagrangian trajectory-based moisture source analysis. More than half (63%) of the boundary layer moisture during the study period traced back to the South China Sea, consistent with prevailing southerly to southwesterly flow. Other important moisture sources included mainland Southeast Asia and the Indian Ocean, local land areas (e.g., Hainan Island and the Leizhou Peninsula), and the Pacific Ocean. Together, these five regions contributed more than 90% of the water vapor. The most pronounced changes in isotopic content due to large-scale transport during the study period were related to the passage of Tropical Storm Talas. The outer rain bands of this tropical cyclone passed over the measurement site on 15–17 July, causing a sharp reduction in the heavy isotopic content of boundary layer water vapor and a substantial increase in deuterium excess. These changes are consistent with extensive isotopic distillation and rain–vapor exchange in downdrafts associated with the intense convective systems produced by this storm.

12. On the Formation Mechanism for Wintertime Extreme Precipitation Events Over the Southeastern Tibetan Plateau

    Wenyu Huang, Tianpei Qiu, Zifan Yang, and 4 more authors

    Journal of Geophysical Research: Atmospheres, 2018

    Abs DOI

    The formation mechanism for wintertime extreme precipitation events over the southeastern Tibetan Plateau (SETP) is explored. The crucial step in the development of these events was the emergence of a cyclonic anomaly above the Tibetan Plateau. Wave activity fluxes along a Rossby wave train embedded in the subtropical jet stream (i.e., the circumglobal teleconnection) played the dominant role in producing this cyclonic anomaly, supported by weaker wave activity fluxes along a second Rossby wave train originating over Scandinavia. The cyclonic anomaly then moved over the SETP, favored strong updrafts, large‐scale moisture convergence, and intense precipitation. Extreme precipitation was more likely to ensue when the arrival of the cyclonic anomaly was preceded by persistent warm anomalies over the SETP, which favored moisture accumulation there. Temperatures above the SETP dropped sharply around the onset of the extreme precipitation. A heat budget analysis indicates that adiabatic cooling associated with convective ascent along the downstream edge of the cyclone played a leading role in this temperature drop, while a cold air intrusion associated with an anticyclonic anomaly over western Siberia (one center of action along the second wave train) played a complementary role. An Eulerian moisture budget analysis shows that variations in precipitable water delayed the onset and enhanced the intensity of these events, with moisture for precipitation delivered to the SETP mainly through the western and southern boundaries. A companion Lagrangian moisture source analysis reveals that the land areas south of the Tibetan Plateau typically contributed 78.7% of the moisture supply for these events. The circumglobal teleconnection pattern played a key role in the development of extreme precipitation over the southeastern Tibetan Plateau The evolution of precipitable water delayed the timing and increased the intensity of extreme precipitation events Land areas to the south of the Tibetan Plateau contributed 78.7% of the moisture for the extreme precipitation

13. Moisture Sources for Wintertime Extreme Precipitation Events Over South China During 1979–2013

    Wenyu Huang, Xinsheng He, Zifan Yang, and 4 more authors

    Journal of Geophysical Research: Atmospheres, 2018

    Abs DOI

    A Lagrangian approach is applied to explore the evaporative moisture sources within the boundary layer for 103 wintertime extreme precipitation events over South China during 1979–2013. Oceanic sources provided about 67.7% of the moisture for these extreme precipitation events, with terrestrial sources providing the remaining parts. The five most important moisture source regions were the South China Sea (30.9% of the total moisture source within boundary layer), the western North Pacific (20.2%), the East China Sea (14.9%), South China (i.e., moisture recycling, in which local evapotranspiration supplies moisture for precipitation; 14.6%), and southeastern Asia (11.5%). Characteristic trajectories linking moisture from the key source regions to South China are identified. All of these trajectories entered China south of the Yangtze River, with characteristic time scales ranging from 2.8 to 5.7 days. The critical circulation patterns for moisture transport from different source regions are also determined. Cyclonic anomalies over South China and its surrounding continental areas favored moisture transport from the South China Sea and southeastern Asia. To leading order, changes in the relative contributions of these and other key moisture source regions were associated with changes in the location or development pattern of these cyclonic anomalies. Beyond these relationships, the presence of anticyclonic anomalies over the western North Pacific favored moisture transport from the western North Pacific, while the absence of these anomalies favored moisture recycling within South China and moisture transport from the East China Sea. Moisture supply from the South China Sea played a leading role in wintertime extreme precipitation events over South China Moisture delivered from the South China Sea reflected a mixture of moisture originating over the South China Sea and western North Pacific Characteristic transport time scales for moisture from the key source regions ranged from 2.8 to 5.7 days

14. On the Utility (or Futility) of Using Stable Water Isotopes to Constrain the Bulk Properties of Tropical Convection

    Suqin Q. Duan, Jonathon S. Wright, and David M. Romps

    Journal of Advances in Modeling Earth Systems, 2018

    Abs DOI

    Atmospheric water‐vapor isotopes have been proposed as a potentially powerful constraint on convection, which plays a critical role in Earth’s present and future climate. It is shown here, however, that the mean tropical profile of HDO in the free troposphere does not usefully constrain the mean convective entrainment rate or precipitation efficiency. This is demonstrated using a single‐column analytical model of atmospheric water isotopes. The model has three parameters: the entrainment rate, the precipitation efficiency, and the distance that evaporating condensates fall. At a given relative humidity, the possible range of HDO is small: its range is comparable to both the measurement uncertainty in the mean tropical profile and the structural uncertainty of a single‐column model. Therefore, the mean tropical HDO profile is unlikely to add information about convective processes in a bulk‐plume framework that cannot already be learned from relative humidity alone. Several of the the physical processes related to rain clouds are still quite uncertain, and that uncertainty is the largest impediment to developing more accurate forecasts of future climate. It has been proposed that measurements of the heavy water isotope HDO could be a useful means of probing those cloud processes. Here, a simple model is developed for testing the sensitivity of HDO to different cloud processes. The results from this model reveal that HDO is largely insensitive to changes in these parameters at a fixed relative humidity. Unfortunately, then, the average amount of HDO in the atmosphere does not provide any significant constraint on cloud processes that is not already provided by the relative humidity alone. An analytical model is derived for profiles of H2O and HDO in radiative‐convective equilibrium Mean HDO is insensitive to entrainment and precipitation efficiency at fixed relative humidity Mean HDO is not a useful source of information on bulk convective processes below the TTL

15. Connections Between a Late Summer Snowstorm Over the Southwestern Tibetan Plateau and a Concurrent Indian Monsoon Low‐Pressure System

    Wenhao Dong, Yanluan Lin, Jonathon S. Wright, and 7 more authors

    Journal of Geophysical Research: Atmospheres, 2018

    Abs DOI

    Snowstorms cause more damage to the livelihoods and livestock of more than 2 million herders (\textbackslashtextasciitilde80% of the total population) on the Tibetan Plateau than any other natural disaster. In this study, we investigate an extreme snowstorm over the southwestern Tibetan Plateau (SWTP) that occurred on 18–21 September 2008. We explore sources and transport pathways of moisture to this storm using multiple data sets and a series of regional model simulations. The results show that this snowstorm results from dynamical coupling between an upper‐level trough and a concurrent low‐pressure system over northern India. This tropical‐extratropical interaction provides the dynamical mechanism for the snowstorm to occur: the upper level trough favors southward cold advection upstream of the SWTP while the low‐pressure system over northern India provides an abundant supply of moisture. Model‐based sensitivity tests indicate that large amounts of moisture are transported over the SWTP via both the up‐and‐over and upslope transport pathways. Our findings corroborate that low‐pressure systems over northern India can deliver abundant moisture into the SWTP. The coupling of this abundant supply of moisture with an upper‐level trough further leads to the occurrence of an early snow disaster in this case, aggravating already harsh conditions and causing severe damage. Improved understanding of the interactions between these two types of synoptic systems and the climatic conditions that influence their occurrence would aid the development of effective strategies for climate change adaptation and sustainable husbandry, as well as the mitigation and relief of future snow disasters in this region. A snowstorm over the southwestern Tibetan Plateau arises from the interaction of an upper‐level trough and a monsoonal low‐pressure system Moisture for the snowstorm is supplied by the low‐pressure system via both up‐and‐over and upslope transport Daily snowfall and snowfall frequency increase over the southwestern Tibetan Plateau in late summer when LPSs approach northern India

16. Regional disparities in warm season rainfall changes over arid eastern–central Asia

    Wenhao Dong, Yanluan Lin, Jonathon S. Wright, and 15 more authors

    Scientific Reports, 2018

    Abs DOI

    Multiple studies have reported a shift in the trend of warm season rainfall over arid eastern–central Asia (AECA) around the turn of the new century, from increasing over the second half of the twentieth century to decreasing during the early years of the twenty-first. Here, a closer look based on multiple precipitation datasets reveals important regional disparities in these changes. Warm-season rainfall increased over both basin areas and mountain ranges during 1961–1998 due to enhanced moisture flux convergence associated with changes in the large-scale circulation and increases in atmospheric moisture content. Despite a significant decrease in warm-season precipitation over the high mountain ranges after the year 1998, warm season rainfall has remained large over low-lying basin areas. This discrepancy, which is also reflected in changes in river flow, soil moisture, and vegetation, primarily results from disparate responses to enhanced warming in the mountain and basin areas of AECA. In addition to changes in the prevailing circulation and moisture transport patterns, the decrease in precipitation over the mountains has occurred mainly because increases in local water vapor saturation capacity (which scales with temperature) have outpaced the available moisture supply, reducing relative humidity and suppressing precipitation. By contrast, rainfall over basin areas has been maintained by accelerated moisture recycling driven by rapid glacier retreat, snow melt, and irrigation expansion. This trend is unsustainable and is likely to reverse as these cryospheric buffers disappear, with potentially catastrophic implications for local agriculture and ecology.

2017

1. Assessment of upper tropospheric and stratospheric water vapor and ozone in reanalyses as part of S-RIP

   Sean M. Davis, Michaela I. Hegglin, Masatomo Fujiwara, and 11 more authors

   Atmospheric Chemistry and Physics, 2017

   Abs DOI

   Reanalysis data sets are widely used to understand atmospheric processes and past variability, and are often used to stand in as “observations” for comparisons with climate model output. Because of the central role of water vapor (WV) and ozone (O3) in climate change, it is important to understand how accurately and consistently these species are represented in existing global reanalyses. In this paper, we present the results of WV and O3 intercomparisons that have been performed as part of the SPARC (Stratosphere–troposphere Processes and their Role in Climate) Reanalysis Intercomparison Project (S-RIP). The comparisons cover a range of timescales and evaluate both inter-reanalysis and observation-reanalysis differences. We also provide a systematic documentation of the treatment of WV and O3 in current reanalyses to aid future research and guide the interpretation of differences amongst reanalysis fields. The assimilation of total column ozone (TCO) observations in newer reanalyses results in realistic representations of TCO in reanalyses except when data coverage is lacking, such as during polar night. The vertical distribution of ozone is also relatively well represented in the stratosphere in reanalyses, particularly given the relatively weak constraints on ozone vertical structure provided by most assimilated observations and the simplistic representations of ozone photochemical processes in most of the reanalysis forecast models. However, significant biases in the vertical distribution of ozone are found in the upper troposphere and lower stratosphere in all reanalyses. In contrast to O3, reanalysis estimates of stratospheric WV are not directly constrained by assimilated data. Observations of atmospheric humidity are typically used only in the troposphere, below a specified vertical level at or near the tropopause. The fidelity of reanalysis stratospheric WV products is therefore mainly dependent on the reanalyses’ representation of the physical drivers that influence stratospheric WV, such as temperatures in the tropical tropopause layer, methane oxidation, and the stratospheric overturning circulation. The lack of assimilated observations and known deficiencies in the representation of stratospheric transport in reanalyses result in much poorer agreement amongst observational and reanalysis estimates of stratospheric WV. Hence, stratospheric WV products from the current generation of reanalyses should generally not be used in scientific studies.

2. Evaluation of multiple forcing data sets for precipitation and shortwave radiation over major land areas of China

   Fan Yang, Hui Lu, Kun Yang, and 6 more authors

   Hydrology and Earth System Sciences, 2017

   Abs DOI

   Precipitation and shortwave radiation play important roles in climatic, hydrological and biogeochemical cycles. Several global and regional forcing data sets currently provide historical estimates of these two variables over China, including the Global Land Data Assimilation System (GLDAS), the China Meteorological Administration (CMA) Land Data Assimilation System (CLDAS) and the China Meteorological Forcing Dataset (CMFD). The CN05.1 precipitation data set, a gridded analysis based on CMA gauge observations, also provides high-resolution historical precipitation data for China. In this study, we present an intercomparison of precipitation and shortwave radiation data from CN05.1, CMFD, CLDAS and GLDAS during 2008–2014. We also validate all four data sets against independent ground station observations. All four forcing data sets capture the spatial distribution of precipitation over major land areas of China, although CLDAS indicates smaller annual-mean precipitation amounts than CN05.1, CMFD or GLDAS. Time series of precipitation anomalies are largely consistent among the data sets, except for a sudden decrease in CMFD after August 2014. All forcing data indicate greater temporal variations relative to the mean in dry regions than in wet regions. Validation against independent precipitation observations provided by the Ministry of Water Resources (MWR) in the middle and lower reaches of the Yangtze River indicates that CLDAS provides the most realistic estimates of spatiotemporal variability in precipitation in this region. CMFD also performs well with respect to annual mean precipitation, while GLDAS fails to accurately capture much of the spatiotemporal variability and CN05.1 contains significant high biases relative to the MWR observations. Estimates of shortwave radiation from CMFD are largely consistent with station observations, while CLDAS and GLDAS greatly overestimate shortwave radiation. All three forcing data sets capture the key features of the spatial distribution, but estimates from CLDAS and GLDAS are systematically higher than those from CMFD over most of mainland China. Based on our evaluation metrics, CLDAS slightly outperforms GLDAS. CLDAS is also closer than GLDAS to CMFD with respect to temporal variations in shortwave radiation anomalies, with substantial differences among the time series. Differences in temporal variations are especially pronounced south of 34∘ N. Our findings provide valuable guidance for a variety of stakeholders, including land-surface modelers and data providers.

3. Development of a global gridded Argo data set with Barnes successive corrections

   Hong Li, Fanghua Xu, Wei Zhou, and 4 more authors

   Journal of Geophysical Research: Oceans, 2017

   Abs DOI

   A new 11 year (2004–2014) monthly 1° gridded Argo temperature and salinity data set with 49 vertical levels from the surface to 1950 m depth (named BOA-Argo) is generated for use in ocean research and modeling studies. The data set is produced based on refined Barnes successive corrections by adopting flexible response functions based on a series of error analyses to minimize errors induced by nonuniform spatial distribution of Argo observations. These response functions allow BOA-Argo to capture a greater portion of mesoscale and large-scale signals while compressing small-sale and high-frequency noise relative to the most recent version of the World Ocean Atlas (WOA). BOA-Argo data set is evaluated against other gridded data sets, such as WOA13, Roemmich-Argo, Jamestec-Argo, EN4-Argo, and IPRC-Argo in terms of climatology, independent observations, mixed-layer depth, and so on. Generally, BOA-Argo compares well with other Argo gridded data sets. The RMSEs and correlation coefficients of compared variables from BOA-Argo agree most with those from the Roemmich-Argo. In particular, more mesoscale features are retained in BOA-Argo than others as compared to satellite sea surface heights. These results indicate that the BOA-Argo data set is a useful and promising adding to the current Argo data sets. The proposed refined Barnes method is computationally simple and efficient, so that the BOA-Argo data set can be easily updated to keep pace with tremendous daily increases in the volume of Argo temperature and salinity data.

4. Evaluation of the Common Land Model (CoLM) from the Perspective of Water and Energy Budget Simulation: Towards Inclusion in CMIP6

   Chengwei Li, Hui Lu, Kun Yang, and 5 more authors

   Atmosphere, 2017

   Abs DOI

   Land surface models (LSMs) are important tools for simulating energy, water and momentum transfer across the land–atmosphere interface. Many LSMs have been developed over the past 50 years, including the Common Land Model (CoLM), a LSM that has primarily been developed and maintained by Chinese researchers. CoLM has been adopted by several Chinese Earth System Models (GCMs) that will participate in the Coupled Model Intercomparison Project Phase 6 (CMIP6). In this study, we evaluate the performance of CoLM with respect to simulating the water and energy budgets. We compare simulations using the latest version of CoLM (CoLM2014), the previous version of CoLM (CoLM2005) that was used in the Beijing Normal University Earth System Model (BNU-GCM) for CMIP5, and the Community Land Model version 4.5 (CLM4.5) against global diagnostic data and observations. Our results demonstrate that CLM4.5 outperforms CoLM2005 and CoLM2014 in simulating runoff (R), although all three models overestimate runoff in northern Europe and underestimate runoff in North America and East Asia. Simulations of runoff and snow depth (SNDP) are substantially improved in CoLM2014 relative to CoLM2005, particularly in the Northern Hemisphere. The simulated global energy budget is also substantially improved in CoLM2014 relative to CoLM2005. Simulations of sensible heat (SH) based on CoLM2014 compare favorably to those based on CLM4.5, while root-mean-square errors (RMSEs) in net surface radiation indicate that CoLM2014 (RMSE = 16.02 W m−2) outperforms both CoLM2005 (17.41 W m−2) and CLM4.5 (23.73 W m−2). Comparisons at regional scales show that all three models perform poorly in the Amazon region but perform relatively well over the central United States, Siberia and the Tibetan Plateau. Overall, CoLM2014 is improved relative to CoLM2005, and is comparable to CLM4.5 with respect to many aspects of the energy and water budgets. Our evaluation confirms CoLM2014 is suitable for inclusion in Chinese GCMs, which will increase the diversity of LSMs considered during CMIP6.

5. Rainforest-initiated wet season onset over the southern Amazon

   Jonathon S. Wright, Rong Fu, John R. Worden, and 5 more authors

   Proceedings of the National Academy of Sciences, 2017

   Abs DOI

   Although it is well established that transpiration contributes much of the water for rainfall over Amazonia, it remains unclear whether transpiration helps to drive or merely responds to the seasonal cycle of rainfall. Here, we use multiple independent satellite datasets to show that rainforest transpiration enables an increase of shallow convection that moistens and destabilizes the atmosphere during the initial stages of the dry-to-wet season transition. This shallow convection moisture pump (SCMP) preconditions the atmosphere at the regional scale for a rapid increase in rain-bearing deep convection, which in turn drives moisture convergence and wet season onset 2–3 mo before the arrival of the Intertropical Convergence Zone (ITCZ). Aerosols produced by late dry season biomass burning may alter the efficiency of the SCMP. Our results highlight the mechanisms by which interactions among land surface processes, atmospheric convection, and biomass burning may alter the timing of wet season onset and provide a mechanistic framework for understanding how deforestation extends the dry season and enhances regional vulnerability to drought.

6. Indian Monsoon Low‐Pressure Systems Feed Up‐and‐Over Moisture Transport to the Southwestern Tibetan Plateau

   Wenhao Dong, Yanluan Lin, Jonathon S. Wright, and 4 more authors

   Journal of Geophysical Research: Atmospheres, 2017

   Abs DOI

   As an integral part of the South Asian summer monsoon system, monsoon low-pressure systems (LPSs) bring large amounts of precipitation to agrarian north and central India during their passage across the subcontinent. In this study, we investigate the role of LPSs in supplying moisture from north and central India to the southwestern Tibetan Plateau (SWTP) and quantify the contribution of these systems to summer rainfall over the SWTP. The results show that more than 60% of total summer rainfall over the SWTP is related to LPS occurrence. LPSs are associated with a 15% rise in average daily rainfall and a 10% rise in rainy days over the SWTP. This relationship is maintained primarily through up-and-over transport, in which convectively lifted moisture over the Indian subcontinent is swept over the SWTP by southwesterly winds in the middle troposphere. LPSs play two roles in supplying up-and-over moisture transport. First, these systems elevate large amounts of water vapor and condensed water to the midtroposphere. Second, the circulations associated with LPSs interact with the background westerlies to induce southwesterly flow in the midtroposphere, transporting elevated moisture and condensate over the Himalayan Mountains. Our findings indicate that LPSs are influential in extending the northern boundary of the South Asian monsoon system across the Himalayas into the interior of the SWTP. The strength of this connection depends on both LPS characteristics and the configuration of the midtropospheric circulation, particularly the prevailing westerlies upstream of the SWTP.

7. On the cooccurrence of wintertime temperature anomalies over eastern Asia and eastern North America

   Ruyan Chen, Wenyu Huang, Bin Wang, and 3 more authors

   Journal of Geophysical Research: Atmospheres, 2017

   Abs DOI

   The cooccurrence of wintertime temperature anomalies over eastern Asia and eastern North America is examined. The winter days during 1948–2014 are assigned to nine regimes by applying the self-organizing map clustering method to the area-averaged land surface temperature anomalies over these two regions. About half of the winter days are associated with concurrent temperature anomalies. The occurrence of the concurrent/nonconcurrent regimes is closely related to the large-scale circulation conditions. The Eurasian teleconnection pattern and the Pacific-North American teleconnection pattern are two dominant large-scale circulation modes associated with the cooccurrence of the temperature anomalies, through their impacts on the intensities of the corresponding troughs. The precursor analysis reveals that the lead time of the early signals for the concurrent cold anomalies is about 4 days longer than that for the concurrent warm anomalies. In addition, the average lead time of the precursor signals for the wintertime temperature anomalies over eastern Asia is longer than that over eastern North America.

8. Potential vorticity regimes over East Asia during winter

   Wenyu Huang, Ruyan Chen, Bin Wang, and 3 more authors

   Journal of Geophysical Research: Atmospheres, 2017

   Abs DOI

   Nine potential vorticity (PV) regimes over East Asia are identified by applying a Self‐Organizing Map and Hierarchical Ascendant Classification regime analysis to the daily PV reanalysis fields on the 300 K isentropic surface for December–March 1948–2014. According to the surface temperature anomalies over East Asia, these nine regimes are further classified into three classes, i.e., cold class (three regimes), warm class (four regimes), and neutral class (two regimes). The PV‐based East Asian winter monsoon index (EAWMI) is used to study the relationship between PV distributions and the temperature anomalies. The magnitude of cold (warm) anomalies over the land areas of East Asia increases (decreases) quasi‐linearly with the EAWMI. Regression analysis reveals that cold temperature anomalies preferentially occur when the EAWMI exceeds a threshold at ∼0.2 PVU (where 1 PVU ≡ 10−6 m2 K kg−1 s−1). PV inversion uncovers the mechanisms behind the relationships between the PV regimes and surface temperature anomalies and reveals that cold (warm) PV regimes are associated with significant warming (cooling) in the upper troposphere and lower stratosphere. On average, cold regimes have longer durations than warm regimes. Interclass transition probabilities are much higher for paths from warm/neutral regimes to cold regimes than for paths from cold regimes to warm/neutral regimes. Besides, intraclass transitions are rare within the warm or neutral regimes. The PV regime analysis provides insight into the causes of severe cold spells over East Asia, with blocking circulation patterns identified as the primary factor in initiating and maintaining these cold spells. Wintertime circulation regimes over East Asia are analyzed based on isen‐ tropic potential vorticity fields The response in temperature to variations in potential vorticity distributions is characteristic of a first baroclinic mode While warm and neutral regimes often transit into cold regimes, cold regimes rarely transit into warm or neutral regimes

2016

1. Validation of Aura MLS retrievals of temperature, water vapour and ozone in the upper troposphere and lower–middle stratosphere over the Tibetan Plateau during boreal summer

   Xiaolu Yan, Jonathon S. Wright, Xiangdong Zheng, and 3 more authors

   Atmospheric Measurement Techniques, 2016

   Abs DOI

   We validate Aura Microwave Limb Sounder (MLS) version 3 (v3) and version 4 (v4) retrievals of summertime temperature, water vapour and ozone in the upper troposphere and lower–middle stratosphere (UTLS; 10–316 hPa) against balloon soundings collected during the Study of Ozone, Aerosols and Radiation over the Tibetan Plateau (SOAR-TP). Mean v3 and v4 profiles of temperature, water vapour and ozone in this region during the measurement campaigns are almost identical through most of the stratosphere (10–68 hPa), but differ in several respects in the upper troposphere and tropopause layer. Differences in v4 relative to v3 include slightly colder mean temperatures from 100 to 316 hPa, smaller mean water vapour mixing ratios in the upper troposphere (215–316 hPa) and a more vertically homogeneous profile of mean ozone mixing ratios below the climatological tropopause (100–316 hPa). These changes substantially improve agreement between ozonesondes and MLS ozone retrievals in the upper troposphere, but slightly worsen existing cold and dry biases at these levels. Aura MLS temperature profiles contain significant cold biases relative to collocated temperature measurements in several layers of the lower–middle stratosphere and in the upper troposphere. MLS retrievals of water vapour volume mixing ratio generally compare well with collocated measurements, excepting a substantial dry bias (-32±11 % in v4) that extends through most of the upper troposphere (121–261 hPa). MLS retrievals of ozone volume mixing ratio are biased high relative to collocated ozonesondes in the stratosphere (18–83 hPa), but are biased low at 100 hPa. The largest relative biases in ozone retrievals (approximately +70 %) are located at 83 hPa. MLS v4 offers substantial benefits relative to v3, particularly with respect to water vapour and ozone. Key improvements include larger data yields, reduced noise in the upper troposphere and smaller fluctuations in the bias profile at pressures larger than 100 hPa. The situation for temperature is less clear, with cold biases and noise levels in the upper troposphere, both larger in v4 than in v3. Several aspects of our results differ from those of validations conducted in other locations. These differences are often amplified by monsoon onset, indicating that the Asian monsoon anticyclone poses unique challenges for remote sensing that impact the quality of MLS retrievals in this region.

2. Introduction to the SPARC Reanalysis Intercomparison Project (S-RIP) and overview of the reanalysis systems

   Masatomo Fujiwara, Jonathon S. Wright, Gloria L. Manney, and 33 more authors

   Atmospheric Chemistry and Physics, 2016

   Abs DOI

   The climate research community uses atmospheric reanalysis data sets to understand a wide range of processes and variability in the atmosphere, yet different reanalyses may give very different results for the same diagnostics. The Stratosphere–troposphere Processes And their Role in Climate (SPARC) Reanalysis Intercomparison Project (S-RIP) is a coordinated activity to compare reanalysis data sets using a variety of key diagnostics. The objectives of this project are to identify differences among reanalyses and understand their underlying causes, to provide guidance on appropriate usage of various reanalysis products in scientific studies, particularly those of relevance to SPARC, and to contribute to future improvements in the reanalysis products by establishing collaborative links between reanalysis centres and data users. The project focuses predominantly on differences among reanalyses, although studies that include operational analyses and studies comparing reanalyses with observations are also included when appropriate. The emphasis is on diagnostics of the upper troposphere, stratosphere, and lower mesosphere. This paper summarizes the motivation and goals of the S-RIP activity and extensively reviews key technical aspects of the reanalysis data sets that are the focus of this activity. The special issue “The SPARC Reanalysis Intercomparison Project (S-RIP)” in this journal serves to collect research with relevance to the S-RIP in preparation for the publication of the planned two (interim and full) S-RIP reports.

3. A potential vorticity‐based index for the East Asian winter monsoon

   Wenyu Huang, Bin Wang, and Jonathon S. Wright

   Journal of Geophysical Research: Atmospheres, 2016

   Abs DOI

   A novel dynamically based index that reflects the strength of the regional potential vorticity (PV) intrusion on the 300 K isentropic surface is proposed as a reliable measure of East Asian winter monsoon (EAWM) intensity. The index captures essential aspects of the EAWM, including its climatic influences on East Asia, its continuous weakening trend since the 1980s, and its close relationships with the Siberian high, Arctic Oscillation, and El Niño. The use of a potential vorticity framework enables the definition of a new metric called continuous PV intrusion duration (CPVID), which can be used to monitor and explain wintertime weather extremes like the extreme snowfall event that occurred in south China during January 2008. The CPVID of March is comparable to that of December, indicating that data from this month should be included in estimates of the strength of the EAWM.

4. Summer rainfall over the southwestern Tibetan Plateau controlled by deep convection over the Indian subcontinent

   Wenhao Dong, Yanluan Lin, Jonathon S. Wright, and 10 more authors

   Nature Communications, 2016

   Abs DOI

   Despite the importance of precipitation and moisture transport over the Tibetan Plateau for glacier mass balance, river runoff and local ecology, changes in these quantities remain highly uncertain and poorly understood. Here we use observational data and model simulations to explore the close relationship between summer rainfall variability over the southwestern Tibetan Plateau (SWTP) and that over central-eastern India (CEI), which exists despite the separation of these two regions by the Himalayas. We show that this relationship is maintained primarily by ‘up-and-over’ moisture transport, in which hydrometeors and moisture are lifted by convective storms over CEI and the Himalayan foothills and then swept over the SWTP by the mid-tropospheric circulation, rather than by upslope flow over the Himalayas. Sensitivity simulations confirm the importance of up-and-over transport at event scales, and an objective storm classification indicates that this pathway accounts for approximately half of total summer rainfall over the SWTP. While precipitation over the Tibetan Plateau is a vital resource for glacier mass balance, river runoff and local ecology, the controlling mechanisms are poorly understood. Here, the authors combine observations and simulations and show that convective storms over India sweep moisture up and over the plateau.

5. On the Non-Stationary Relationship between the Siberian High and Arctic Oscillation

   Wenyu Huang, Bin Wang, Jonathon S. Wright, and 1 more author

   PLOS ONE, 2016

   Abs DOI

   An area-weighted k-means clustering method based on pattern correlations is proposed and used to explore the relationship between the Siberian High (SH) and Arctic Oscillation (AO) during the winter months (December-January-February) of 1948–2014. Five regimes are identified. Four of these five regimes (comprising 171 of 201 months) show a negative correlation between the SH and AO indices, while the last regime (30 months) shows a positive correlation. The location of the SH shifts southward into China under two of the four negative-correlation regimes (117 months), with pressure variations over the center of activity for the SH opposite to pressure variations over the climatological center of the SH (which is used to define the SH index). Adjusting the SH index to account for these spatial shifts suggests positive rather than negative correlations between major variations in the SH and AO under these regimes. Under one of the two remaining negative-correlation regimes, pressure anomalies are weak over the Arctic Ocean. In total, only one regime comprising 21 of 201 months strictly obeys the negative correlation between the SH and AO reported by previous studies. The climate regime characterized by an intensified SH is associated with a greater frequency of cold surges over northern and southeastern China, and the weakening of the East Asian winter monsoon during the 1980s was accompanied by a sharp reduction in the occurrence of this regime.

2015

1. Relationships between convective structure and transport of aerosols to the upper troposphere deduced from satellite observations

   Sudip Chakraborty, Rong Fu, Jonathon S. Wright, and 1 more author

   Journal of Geophysical Research: Atmospheres, 2015

   Abs DOI

   We estimate the extent of upper tropospheric aerosol layers (UT ALs) surrounding mesoscale convective systems (MCSs) and explore the relationships between UT AL extent and the morphology, location, and developmental stage of collocated MCSs in the tropics. Our analysis is based on satellite data collected over equatorial Africa, South Asia, and the Amazon basin between June 2006 and June 2008. We identify substantial variations in the relationships between convective properties and aerosol transport by region and stage of convective development. The most extensive UT ALs over equatorial Africa are associated with mature MCSs, while the most extensive UT ALs over South Asia and the Amazon are associated with growing MCSs. Convective aerosol transport over the Amazon is weaker than that observed over the other two regions despite similar transport frequencies, likely due to the smaller sizes and shorter mean lifetimes of MCSs over the Amazon. Variations in UT ALs in the vicinity of tropical MCSs are primarily explained by variations in the horizontal sizes of the associated MCSs and are largely unrelated to aerosol loading in the lower troposphere. We also identify potentially important relationships with the number of convective cores, vertical wind shear, and convective fraction during the growing and mature stages of MCS development. Relationships between convective properties and aerosol transport are relatively weak during the decaying stage of convective development. Our results provide an interpretive framework for devising and evaluating numerical model experiments that examine relationships between convective properties and ALs in the upper troposphere.

2013

1. Large differences in reanalyses of diabatic heating in the tropical upper troposphere and lower stratosphere

   J. S. Wright, and S. Fueglistaler

   Atmospheric Chemistry and Physics, 2013

   Abs DOI

   We present the time mean heat budgets of the tropical upper troposphere (UT) and lower stratosphere (LS) as simulated by five reanalysis models: the Modern-Era Retrospective Analysis for Research and Applications (MERRA), European Reanalysis (ERA-Interim), Climate Forecast System Reanalysis (CFSR), Japanese 25-yr Reanalysis and Japan Meteorological Agency Climate Data Assimilation System (JRA-25/JCDAS), and National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis 1. The simulated diabatic heat budget in the tropical UTLS differs significantly from model to model, with substantial implications for representations of transport and mixing. Large differences are apparent both in the net heat budget and in all comparable individual components, including latent heating, heating due to radiative transfer, and heating due to parameterised vertical mixing. We describe and discuss the most pronounced differences. Discrepancies in latent heating reflect continuing difficulties in representing moist convection in models. Although these discrepancies may be expected, their magnitude is still disturbing. We pay particular attention to discrepancies in radiative heating (which may be surprising given the strength of observational constraints on temperature and tropospheric water vapour) and discrepancies in heating due to turbulent mixing (which have received comparatively little attention). The largest differences in radiative heating in the tropical UTLS are attributable to differences in cloud radiative heating, but important systematic differences are present even in the absence of clouds. Local maxima in heating and cooling due to parameterised turbulent mixing occur in the vicinity of the tropical tropopause.

2012

1. Geographic and seasonal distributions of CO transport pathways and their roles in determining CO centers in the upper troposphere

   L. Huang, R. Fu, J. H. Jiang, and 2 more authors

   Atmospheric Chemistry and Physics, 2012

   Abs DOI

   Past studies have identified a variety of pathways by which carbon monoxide (CO) may be transported from the surface to the tropical upper troposphere (UT); however, the relative roles that these transport pathways play in determining the distribution and seasonality of CO in the tropical UT remain unclear. We have developed a method to automate the identification of two pathways ("local convection" and "advection within the lower troposphere (LT) followed by convective vertical transport") involved in CO transport from the surface to the UT. This method is based on the joint application of instantaneous along-track, co-located, A-Train satellite measurements. Using this method, we find that the locations and seasonality of the UT CO maxima in the tropics were strongly correlated with the frequency of local convective transport during 2007. We also find that the "local convection" pathway (convective transport that occurred within a fire region) typically transported significantly more CO to the UT than the "LT advection → convection" pathway (advection of CO within the LT from a fire region to a convective region prior to convective transport). To leading order, the seasonality of CO concentrations in the tropical UT reflected the seasonality of the "local convection" transport pathway during 2007. The UT CO maxima occurred over Central Africa during boreal spring and over South America during austral spring. Occurrence of the "local convection" transport pathway in these two regions also peaked during these seasons. During boreal winter and summer, surface CO emission and convection were located in opposite hemispheres, which limited the effectiveness of transport to the UT. During these seasons, CO transport from the surface to the UT typically occurred via the "LT advection → convection" pathway.

2011

1. Properties of air mass mixing and humidity in the subtropics from measurements of the D/H isotope ratio of water vapor at the Mauna Loa Observatory

   David Noone, Joseph Galewsky, Zachary D. Sharp, and 14 more authors

   Journal of Geophysical Research: Atmospheres (1984–2012), 2011

   Abs DOI

   Water vapor in the subtropical troposphere plays an important role in the radiative balance, the distribution of precipitation, and the chemistry of the Earth’s atmosphere. Measurements of the water vapor mixing ratio paired with stable isotope ratios provide unique information on transport processes and moisture sources that is not available with mixing ratio data alone. Measurements of the D/H isotope ratio of water vapor from Mauna Loa Observatory over 4 weeks in October–November 2008 were used to identify components of the regional hydrological cycle. A mixing model exploits the isotope information to identify water fluxes from time series data. Mixing is associated with exchange between marine boundary layer air and tropospheric air on diurnal time scales and between different tropospheric air masses with characteristics that evolve on the synoptic time scale. Diurnal variations are associated with upslope flow and the transition from nighttime air above the marine trade inversion to marine boundary layer air during daytime. During easterly trade wind conditions, growth and decay of the boundary layer are largely conservative in a regional context but contribute ∼12% of the nighttime water vapor at Mauna Loa. Tropospheric moisture is associated with convective outflow and exchange with drier air originating from higher latitude or higher altitude. During the passage of a moist filament, boundary layer exchange is enhanced. Isotopic data reflect the combination of processes that control the water balance, which highlights the utility for baseline measurements of water vapor isotopologues in monitoring the response of the hydrological cycle to climate change.

2. The influence of summertime convection over Southeast Asia on water vapor in the tropical stratosphere

   J. S. Wright, R. Fu, S. Fueglistaler, and 2 more authors

   Journal of Geophysical Research: Atmospheres (1984–2012), 2011

   Abs DOI

   The relative contributions of Southeast Asian convective source regions during boreal summer to water vapor in the tropical stratosphere are examined using Lagrangian trajectories. Convective sources are identified using global observations of infrared brightness temperature at high space and time resolution, and water vapor transport is simulated using advection-condensation. Trajectory simulations are driven by three different reanalysis data sets, GMAO MERRA, ERA-Interim, and NCEP/NCAR, to establish points of consistency and evaluate the sensitivity of the results to differences in the underlying meteorological fields. All ensembles indicate that Southeast Asia is a prominent boreal summer source of tropospheric air to the tropical stratosphere. Three convective source domains are identified within Southeast Asia: the Bay of Bengal and South Asian subcontinent (MON), the South China and Philippine Seas (SCS), and the Tibetan Plateau and South Slope of the Himalayas (TIB). Water vapor transport into the stratosphere from these three domains exhibits systematic differences that are related to differences in the bulk characteristics of transport. We find air emanating from SCS to be driest, from MON slightly moister, and from TIB moistest. Analysis of pathways shows that air detrained from convection over TIB is most likely to bypass the region of minimum absolute saturation mixing ratio over the equatorial western Pacific; however, the impact of this bypass mechanism on mean water vapor in the tropical stratosphere at 68 hPa is small (<0.1 ppmv). This result contrasts with previously published hypotheses, and it highlights the challenge of properly quantifying fluxes of atmospheric humidity.

2010

1. Diagnosis of Zonal Mean Relative Humidity Changes in a Warmer Climate

   Jonathon S. Wright, Adam Sobel, and Joseph Galewsky

   Journal of Climate, 2010

   Abs DOI

   The zonal mean relative humidity response to a doubling of CO2 in a climate model is examined using a global climate model and an offline tracer transport model. Offline tracer transport model simulations are driven by the output from two configurations of the climate model, one with 1979 concentrations of atmospheric greenhouse gases and one with doubled CO2. A set of last saturation tracers is applied within the tracer transport model to diagnose the dynamics responsible for features in the water vapor field. Two different methods are used to differentiate the effects of circulation and transport shifts from spatially inhomogeneous temperature changes. The first of these uses the tracer transport model and is achieved by decoupling the input temperature and circulation fields; the second uses the reconstruction of humidity from the last saturation tracers and is achieved by decoupling the tracer concentrations from their saturation specific humidities. The responses of the tropical and subtropical relative humidities are found to be largely dependent on circulation and transport changes, particularly a poleward expansion of the Hadley cell, a deepening of the height of convective detrainment, a poleward shift of the extratropical jets, and an increase in the height of the tropopause. The last saturation tracers are used to illustrate the influence of changes in transport pathways within the GCM on the zonal mean relative humidity, particularly in the tropical upper troposphere and subtropical dry zones. Relative humidity changes near the extratropical tropopause and in the lower troposphere are largely dependent on changes in the distribution and gradients of temperature. Increases in relative humidity near the extratropical tropopause in both hemispheres are coincident with increases in the occurrence of local saturation and high cloud cover.

2009

1. A statistical analysis of the influence of deep convection on water vapor variability in the tropical upper troposphere

   J. S. Wright, R. Fu, and A. J. Heymsfield

   Atmospheric Chemistry and Physics, 2009

   Abs DOI

   The factors that control the influence of deep convective detrainment on water vapor in the tropical upper troposphere are examined using observations from multiple satellites in conjunction with a trajectory model. Deep convection is confirmed to act primarily as a moisture source to the upper troposphere, modulated by the ambient relative humidity (RH). Convective detrainment provides strong moistening at low RH and offsets drying due to subsidence across a wide range of RH. Strong day-to-day moistening and drying takes place most frequently in relatively dry transition zones, where between 0.01% and 0.1% of Tropical Rainfall Measuring Mission Precipitation Radar observations indicate active convection. Many of these strong moistening events in the tropics can be directly attributed to detrainment from recent tropical convection, while others in the subtropics appear to be related to stratosphere-troposphere exchange. The temporal and spatial limits of the convective source are estimated to be about 36–48 h and 600–1500 km, respectively, consistent with the lifetimes of detrainment cirrus clouds. Larger amounts of detrained ice are associated with enhanced upper tropospheric moistening in both absolute and relative terms. In particular, an increase in ice water content of approximately 400% corresponds to a 10–90% increase in the likelihood of moistening and a 30–50% increase in the magnitude of moistening.

2. Influence of condensate evaporation on water vapor and its stable isotopes in a GCM

   Jonathon S. Wright, Adam H. Sobel, and Gavin A. Schmidt

   Geophysical Research Letters, 2009

   Abs DOI

   The direct effect of condensate evaporation on atmospheric water vapor and its isotopic composition is assessed in a climate model. The model contains two parallel hydrologic cycles, an active one which influences the model physics and dynamics and a passive one which does not. Two model simulations are performed, one in which passive cloud and precipitation can evaporate and one in which they cannot. The active hydrologic cycles, and thus the simulated circulations and temperatures, are identical in both simulations. Eliminating passive condensate evaporation reduces the specific humidity in the passive cycle by around 5%; this reduction varies from a few percent to 25% of the control value, depending on location. Zonal mean water vapor in the lower and middle troposphere is enriched in HDO relative to the control case, and is depleted in the upper troposphere.

2006

1. Short circuit of water vapor and polluted air to the global stratosphere by convective transport over the Tibetan Plateau

   Rong Fu, Yuanlong Hu, Jonathon S. Wright, and 7 more authors

   Proceedings of the National Academy of Sciences, 2006

   Abs DOI

   During boreal summer, much of the water vapor and CO entering the global tropical stratosphere is transported over the Asian monsoon/Tibetan Plateau (TP) region. Studies have suggested that most of this transport is carried out either by tropical convection over the South Asian monsoon region or by extratropical convection over southern China. By using measurements from the newly available National Aeronautics and Space Administration Aura Microwave Limb Sounder, along with observations from the Aqua and Tropical Rainfall-Measuring Mission satellites, we establish that the TP provides the main pathway for cross-tropopause transport in this region. Tropospheric moist convection driven by elevated surface heating over the TP is deeper and detrains more water vapor, CO, and ice at the tropopause than over the monsoon area. Warmer tropopause temperatures and slower-falling, smaller cirrus cloud particles in less saturated ambient air at the tropopause also allow more water vapor to travel into the lower stratosphere over the TP, effectively short-circuiting the slower ascent of water vapor across the cold tropical tropopause over the monsoon area. Air that is high in water vapor and CO over the Asian monsoon/TP region enters the lower stratosphere primarily over the TP, and it is then transported toward the Asian monsoon area and disperses into the large-scale upward motion of the global stratospheric circulation. Thus, hydration of the global stratosphere could be especially sensitive to changes of convection over the TP.