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This study investigated the relationship between the pre-summer soil moisture (SM) over the Indo-China Peninsula (ICP) and the summer precipitation in the middle and lower reaches of the Yangtze River basin (MLR-YRB) under different SST backgrounds. When spring soil over the ICP was dry (wet), summer precipitation in most parts of the MLR-YRB was significantly higher (lower) under normal SST backgrounds (in the inner-quartile years). However, the SM anomalies produced a limited influence on summer precipitation under strong SST conditions (in the outer-quartile years). Further analysis indicates that in the inner-quartile years, the thermal heating anomalies induced by abnormal SM evidently increased (decreased) the geopotential heights above the western tropical Pacific, and enhanced (weakened) the low-level anticyclonic circulation, thereby increasing (decreasing) the warm and moist southerly flow from the ICP and western Pacific regions to the MLR-YRB, which was conducive to more (less) precipitation in the MLR-YRB. In the outer-quartile years, thermal anomalies of the ICP had very weak effects on both the wind fields above the western Pacific and the water vapor flux to the MLR-YRB, thereby leading to a limited increase (decrease) in summer precipitation. The different responses of the local and surrounding atmospheric circulations to pre-summer SM anomalies over the ICP against different SST backgrounds produced distinct impacts on the distribution of summer precipitation in the MLR-YRB. The dry (wet) anomalies of spring SM in the ICP could also increase (decrease) the zonal thermal contrast between the ICP and the South China Sea (SCS) and impact the SCS summer monsoon differently in the inner-quartile and outer-quartile years. Based on numerical experiments with the Community Earth System Model (CESM), this work further confirmed the observational results and explored the physical mechanism by which the ICP spring soil moisture affects the MLR-YRB summer precipitation under various SST backgrounds. The simulation results show that a dry (wet) ICP spring soil significantly increased (decreased) the MLR-YRB rainfall in the following summer. This negative correlation was stronger in the inner-quartile years. The atmospheric response to changes in the surface thermal conditions was more significant in the inner-quartile years. This surface heating anomaly over the ICP led to anomalous southwest wind over the SCS, which could enhance the Asian summer monsoon and increase summer precipitation in the MLR-YRB.

Previous studies detected significant negative correlations between the nonuniform land surface warming and the decadal weakened activities of the summer extratropical cyclones (ECs) over East Asia and the East Asian summer monsoon (EASM) after the early 1990s. Here such relationships are further examined and the possible mechanisms are explored via numerical sensitivity experiments with a regional climate model (RegCM4.5). The positive/negative sensible heat flux (SH) anomalies were added as a forcing to a key region near 50°N of East Asia in RegCM4.5 to simulate the observed ground surface temperature (GST) anomalies. The model results suggest that the nonuniform land surface warming over the Lake Baikal area (50°–60°N, 90°–120°E) can indeed cause the weakening of the extratropical cyclogenesis and affect the decadal weakening of the EASM. Warm (cold) GST forcing over the key GST region can lead to decreasing (increasing) atmospheric baroclinicity and related energy conversion of the EC activity over the key EC region (40°–50°N, 90°–120°E), resulting in an evidently weakening (enhancing) of the ECs over East Asia. Meanwhile, precipitation shows a dipole pattern with significantly suppressed (enhanced) precipitation in northern and northeastern China, and slightly enhanced (suppressed) rainfall south of 40°N of East Asia, mainly over the East China Sea. Lake Baikal and its adjacent areas are occupied by a strong anticyclonic (cyclonic) circulation while the southeast coastal areas of China have a relatively weak cyclonic (anticyclonic) circulation accompanied with an anomalous northeasterly (southwesterly) wind to the southeast of the anticyclonic circulation, which is opposite to (coincident with) the atmospheric circulation anomalies that are associated with the second mode of the EASM.

To evaluate and clarify the daily precipitation characteristics (i.e. amount, frequency and intensity) of the regional climate models (RCMs) in China, long-term simulations were carried out using RegCM4.5 and Weather Research and Forecasting model (WRF), which were nested within the European Centre for Medium-Range Weather Forecasts (ECMWF)’s 20th century reanalysis (ERA-20C) between 1901 and 2010. The 2 RCMs were initially run at a resolution of 50 km. Analyses mainly compared the model-simulated climatic means and interannual variations of precipitation characteristics with those of dense and high-quality station observations (STN) from 1961-2010. Both models satisfactorily reproduced the seasonal mean precipitation amount, but they overestimated its frequency and underestimated its intensity. Extreme rainfall frequency was also underestimated by both RCMs. In winter (DJF), the interannual variabilities in dry days, light precipitation and moderate precipitation were well represented by both models. However, they poorly reproduced the counterparts of extreme precipitation in winter. In summer (JJA), the 2 RCMs performed well in simulating the interannual variability of extreme precipitation. Comparably, RegCM outperformed WRF in reproducing the spatial patterns of precipitation amount, interannual variations in extreme precipitation and rain events. By contrast, WRF better represented precipitation frequency in different sub-regions overall. Moreover, when the horizontal resolution of RegCM was increased from 50 to 25 km, there was a slight improvement in the representation of precipitation amount and intensity. Our results show that RCMs perform well in reproducing actual climatic means and interannual variations of daily precipitation characteristics in China, and that high-resolution RCM simulations can produce improved results for precipitation amount and intensity.

Soil moisture is a key variable of the land surface and its variations are an important issue in climate studies. In this study, we employed the Community Earth System Model (CESM) for 20 ensemble member simulations of the 50-year period from 1965 to 2014 and used canonical correlation analysis (CCA) and multiple regression to analyze the spatiotemporal characteristics of soil moisture predictability. The effects of soil moisture persistence and sea surface temperature (SST) as an external forcing on its predictability were analyzed. Results show that the soil moisture predictability due to its persistence is 1–2 months and considering SST as an external forcing can significantly increase its predictability. Regions that exhibit a significant increase in predictability are mainly tropical regions, North America and Western Asia during winter and spring. In tropical regions, SST increases the predictability of soil moisture by influencing the local surface temperature and precipitation. In other regions, the effects of SST on wind speed, cloud cover, and surface evaporation also contribute significantly to the increase in soil moisture predictability. The results were validated through the analysis based on soil moisture data from land data assimilation system and observed precipitation.

International audience; The Indian summer monsoon (ISM) is one of the world's strongest monsoon systems that brings about eighty percent of the annual rainfall to the Indian subcontinent and impacts the livelihood of more than a quarter of the world's population. Meanwhile, Asia is the world's second largest dust source-with major deserts in the Middle East, Central and East Asia. The interactions between the Asian dust and the ISM have received increasing attention in recent decades. Dust particles can modulate the circulation and precipitation of the ISM through absorption of solar and terrestrial radiation when suspending in the atmosphere and when deposited in snow and ice at surface and by acting as nuclei of liquid and ice clouds. In turn, the ISM can affect dust emissions, transport, and deposition through atmospheric circulation and wet scavenging. This review provides a) an overview of several physical mechanisms behind the interactions between the ISM rainfall and the Asian dust particularly the Middle East dust, b) a new hypothesis to explain the observed positive correlation between the Middle East dust and the ISM rainfall, and c) a summary of current progress and challenges in dust simulation in climate models. Finally, we propose future research directions aimed at improving dust-monsoon simulations in terms of dust long-term variability, absorbing property, and anthropogenic contributions.

Through a series of model simulations with an atmospheric general circulation model coupled to three different land surface models, this study investigates the impacts of land model ensembles and coupled model ensemble on precipitation simulation. It is found that coupling an ensemble of land models to an atmospheric model has a very minor impact on the improvement of precipitation climatology and variability, but a simple ensemble average of the precipitation from three individually coupled land-atmosphere models produces better results, especially for precipitation variability. The generally weak impact of land processes on precipitation should be the main reason that the land model ensembles do not improve precipitation simulation. However, if there are big biases in the land surface model or land surface data set, correcting them could improve the simulated climate, especially for well-constrained regional climate simulations.

North China is one of the most densely populated regions in the world. To its west, north, and northwest, the world's largest afforestation project has been going on for decades. At the same time, North China has been suffering from air pollution because of its large fossil fuel consumption. Here we show that the changes in land cover and aerosol concentration are coupled with the variations of land surface temperature, cloud cover, and surface solar radiation during the summer 2000–2013. Model experiments show that the interannual variation of aerosol concentration in North China is mainly a result of the varying atmospheric circulation. The increasing vegetation cover due to afforestation has enhanced surface evapotranspiration (ET) and cooled the local surface, and precipitation is observed to be increasing with ET. The model with prescribed increasing vegetation cover can simulate the increasing ET but cannot reproduce the increasing precipitation. Although this may be caused by model biases, the lack of aerosol processes in the model could also be a potential cause.

The seasonal responses of the Indian summer monsoon (ISM) to dust aerosols in local (the Thar Desert) and remote (the Middle East and western China) regions are studied using the WRF Model coupled with online chemistry (WRF-Chem). Ensemble experiments are designed by perturbing model physical and chemical schemes to examine the uncertainties of model parameterizations. Model results show that the dust-induced increase in ISM total rainfall can be attributed to the remote dust in the Middle East, while the contributions from local and remote dust are very limited. Convective rainfall shows a spatially more homogeneous increase than stratiform rainfall, whose responses follow the topography. The magnitude of dust-induced increase in rainfall is comparable to that caused by anthropogenic aerosols. The Middle East dust aerosols tend to enhance the southwesterly monsoon flow, which can transport more water vapor to southern and northern India, while the anthropogenic aerosols tend to enhance the southeasterly monsoon flow, resulting in more water vapor and rainfall over northern India. Both dust and anthropogenic aerosol-induced rainfall responses can be attributed to their heating effect in the mid-to-upper troposphere, which enhances monsoon circulations. The heating effect of dust over the Iranian Plateau seems to play a bigger role than that over the Tibetan Plateau, while the heating of anthropogenic aerosols over the Tibetan Plateau is more important. Moreover, dust aerosols can decrease rainfall over the Arabian Sea through their indirect effect. This study addresses the relative roles of dust and anthropogenic aerosols in altering the ISM rainfall and provides insights into aerosol–ISM interactions.

Since winter 2011, a record-breaking drought has occurred in California. Studies found that the drought is mainly caused by a persistent high-pressure system off the U.S. West Coast, which is linked to Pacific sea surface temperature anomalies. The water cycles associated with the droughts and floods are still not clearly understood. Here we show that the atmospheric circulation off the West Coast not only controls the atmospheric convergence and formation of precipitation but also largely determines surface wind speed, which further affects the evaporation over the eastern North Pacific, the major evaporative moisture source for California precipitation. Because of this mechanism, the ocean evaporation over the eastern North Pacific has been reduced during the recent drought. However, the ocean evaporation anomalies have little direct influence on California precipitation, especially during dry years, mainly because of their weak amplitudes. The California droughts cannot be readily attributed to the reduced ocean evaporation. The association between increased Pacific evaporation and floods over California is somewhat stronger.

The responses of atmospheric variability to Tibetan Plateau (TP) snow cover (TPSC) at seasonal, interannual and decadal time scales have been extensively investigated. However, the atmospheric response to faster subseasonal variability of TPSC has been largely ignored. Here, we show that the subseasonal variability of TPSC, as revealed by daily data, is closely related to the subsequent East Asian atmospheric circulation at medium-range time scales (approximately 3–8 days later) during wintertime. TPSC acts as an elevated cooling source in the middle troposphere during wintertime and rapidly modulates the land surface thermal conditions over the TP. When TPSC is high, the upper-level geopotential height is lower, and the East Asia upper-level westerly jet stream is stronger. This finding improves our understanding of the influence of TPSC at multiple time scales. Furthermore, our work highlights the need to understand how atmospheric variability is rapidly modulated by fast snow cover changes., The atmospheric response to subseasonal variability of Tibetan Plateau snow cover has been largely ignored. Here the authors show that the fast subseasonal variability of Tibetan Plateau snow cover is closely related to the subsequent East Asian atmospheric circulation at medium-range time scales.

Interactions between soil moisture, evapotranspiration (ET), atmospheric moisture fluxes and precipitation are complex. It is difficult to attribute the variations of one variable to another. In this study, we investigate the influence of atmospheric moisture fluxes and land surface soil moisture on local precipitation, with a focus on the southern United States (U.S.), a region with a strong humidity gradient and intense moisture fluxes. Experiments with the Weather Research and Forecasting model show that the variation of moisture flux convergence (MFC) is more important than that of soil moisture for precipitation variation over the southern U.S. Further analyses decompose the precipitation change into several contributing factors and show that MFC affects precipitation both directly through changing moisture inflow (wet areas) and indirectly by changing the precipitation efficiency (transitional zones). Soil moisture affects precipitation mainly by changing the precipitation efficiency, and secondly through direct surface ET contribution. The greatest soil moisture effects are over transitional zones. MFC is more important for the probability of heavier rainfall; soil moisture has much weaker impact on rainfall probability and its roles are similar for the probability of intermediate-to-heavy rainfall (>10 mm day−1). Although MFC is more important than soil moisture for precipitation over most regions, the impact of soil moisture could be large over certain transitional regions. At the submonthly time scale, the African Sahel appears to be the only major region where soil moisture has a greater impact than MFC on precipitation. This study provides guidance to understanding and further investigation of the roles of local land surface processes and large-scale circulations on precipitation.

We present the first systematic study to quantify the impact of land initialization on seasonal temperature prediction in the Northern Hemisphere, emphasizing the role of land snow data assimilation (DA). Three suites of ensemble seasonal integrations are conducted for coupled land–atmosphere runs. The land component is initialized using datasets from (1) no DA, (2) assimilating MODIS snow cover fraction (SCF), and (3) assimilating both MODIS SCF and GRACE terrestrial water storage. Results show that snow DA improves temperature predictions especially in the Tibetan Plateau (by 5–20%) and high latitudes. Improvements at low latitudes are seen immediately and last up to 60 days, whereas improvements at high latitudes only appear later in transitional seasons. At high latitudes, assimilating GRACE data results in marked and prolonged improvements (by ~25%) due to large initial snow mass changes. This study has great implications for future land DA and seasonal climate prediction studies.

The absorptive properties of dust aerosols largely determine the magnitude of their radiative impacts on the climate system. Currently, climate models use globally constant values of dust imaginary refractive index (IRI), a parameter describing the dust absorption efficiency of solar radiation, although it is highly variable. Here we show with model experiments that the dust-induced Indian summer monsoon (ISM) rainfall differences (with dust minus without dust) change from −9% to 23% of long-term climatology as the dust IRI is changed from zero to the highest values used in the current literature. A comparison of the model results with surface observations, satellite retrievals, and reanalysis data sets indicates that the dust IRI values used in most current climate models are too low, tending to significantly underestimate dust radiative impacts on the ISM system. This study highlights the necessity for developing a parameterization of dust IRI for climate studies., King Abdullah University of Science and Technology, University of Texas at Austin. Jackson School of Geosciences