Soil moisture influence on summertime surface air temperature over East Asia

Soil moisture influence on surface air temperature in summer is statistically quantified across East Asia using the Global Land Data Assimilation System soil moisture and observational temperature. The analysis uses a soil moisture feedback parameter computed based on lagged covariance ratios. It is found that significant negative soil moisture feedbacks on temperature mainly appear over the transition zones between dry and wet climates of northern China and Mongolia. Over these areas, the feedbacks account for typically 5–20% of the total temperature variance, with the feedback parameter of ?0.2°C to ?0.5°C (standardized soil moisture)^?1. Meanwhile, positive feedbacks may exist over some areas of Northeast Asia but are much less significant. These findings emphasize the importance of soil moisture-temperature feedbacks in influencing summer climate variability and have implications for seasonal temperature forecasting.     

Global changes in extreme events: regional and seasonal dimension

This study systematically analyzes the complete IPCC AR4 (CMIP3) ensemble of GCM simulations with respect to changes in extreme event characteristics at the end of the 21st century compared to present-day conditions. It complements previous studies by investigating a more comprehensive database and considering seasonal changes beside the annual time scale. Confirming previous studies, the agreement between the GCMs is generally high for temperature-related extremes, indicating increases of warm day occurrences and heatwave lengths, and decreases of cold extremes. However, we identify issues with the choice of indices used to quantify heatwave lengths, which do overall not affect the sign of the changes, but strongly impact the magnitude and patterns of projected changes in heatwave characteristics. Projected changes in precipitation and dryness extremes are more ambiguous than those in temperature extremes, despite some robust features, such as increasing dryness over the Mediterranean and increasing heavy precipitation over the Northern high latitudes. We also find that the assessment of projected changes in dryness depends on the index choice, and that models show less agreement regarding changes in soil moisture than in the commonly used ‘consecutive dry days’ index, which is based on precipitation data only. Finally an analysis of the scaling of changes of extreme temperature quantiles with global, regional and seasonal warming shows that much of the extreme quantile changes are due to a seasonal scaling of the regional annual-mean warming. This emphasizes the importance of the seasonal time scale also for extremes. Changes in extreme quantiles of temperature on land scale with changes in global annual mean temperature by a factor of more than 2 in some regions and seasons, implying large changes in extremes in several countries, even for the commonly discussed global 2°C-warming target.     

Observational evidence for the relationship between spring soil moisture and June rainfall over the Indian region

Understanding the relationship between gradually varying soil moisture (SM) conditions and monsoon rainfall anomalies is crucial for seasonal prediction. Though it is an important issue, very few studies in the past attempted to diagnose the linkages between the antecedent SM and Indian summer monsoon rainfall. This study examined the relationship between spring (April–May) SM and June rainfall using observed data during the period 1979–2010. The Empirical Orthogonal Function (EOF) analyses showed that the spring SM plays a significant role in June rainfall over the Central India (CI), South India (SI), and North East India (NEI) regions. The composite anomaly of the spring SM and June rainfall showed that excess (deficit) June rainfall over the CI was preceded by wet (dry) spring SM. The anomalies in surface-specific humidity, air temperature, and surface radiation fluxes also supported the existence of a positive SM-precipitation feedback over the CI. On the contrary, excess (deficit) June rainfall over the SI and NEI region were preceded by dry (wet) spring SM. The abnormal wet (dry) SM over the SI and NEI decreased (increased) the 2-m air temperature and increased (decreased) the surface pressure compared to the surrounding oceans which resulted in less (more) moisture transport from oceans to land (negative SM-precipitation feedback over the Indian monsoon region).     

Possible association of the western Tibetan Plateau snow cover with the decadal to interdecadal variations of northern China heatwave frequency

Northern China has been subject to increased heatwave frequency (HWF) in recent decades, which deteriorates the local droughts and desertification. More than half a billion people face drinking water shortages and worsening ecological environment. In this study, the variability in the western Tibetan Plateau snow cover (TPSC) is observed to have an intimate linkage with the first empirical orthogonal function mode of the summer HWF across China. This distinct leading mode is dominated by the decadal to inter-decadal variability and features a mono-sign pattern with the extreme value center prevailing over northern China and high pressure anomalies at mid- and upper troposphere over Mongolia and the adjacent regions. A simplified general circulation model is utilized to examine the possible physical mechanism. A reduced TPSC anomaly can induce a positive geopotential height anomaly at the mid- and upper troposphere and subsequently enhance the climatological high pressure ridge over Mongolia and the adjacent regions. The subsidence associated with the high pressure anomalies tends to suppress the local cloud formation, which increases the net radiation budget, heats the surface, and favors more heatwaves. On the other hand, the surface heating can excite high pressure anomalies at mid- and upper troposphere. The latter further strengthens the upper troposphere high pressure anomalies over Mongolia and the adjacent regions. Through such positive feedback effect, the TPSC is tied to the interdecadal variations of the northern China HWF.     

中国夏季气温对东亚土壤湿度异常响应的统计评估

基于欧亚夏季土壤湿度变化特征及其与中国夏季气温的相关分析,选取东亚地区作为土壤湿度异常影响中国夏季气温的陆面关键区,采用广义平衡反馈分析方法(GEFA)探讨了我国夏季气温对东亚地区土壤湿度异常的可能响应,并初步讨论了相关的物理过程。结果表明:中国夏季气温与东亚地区初夏和同期的土壤湿度异常具有密切的联系;进一步分析表明,夏季气温距平场对土壤湿度第一模态的响应最显著:当东亚中纬度及我国东部地区土壤湿度异常偏干时,夏季气温表现为一致增暖;而土壤湿度第二模态对长江流域至我国西部地区的气温有较弱的强迫作用;气温对第三模态的响应主要表现为华南地区的显著降温。并以对气温影响最为显著的土壤湿度异常第一模态为例,初步探讨了气温对土壤湿度异常响应的可能物理过程。当贝加尔湖以南以及我国东部的土壤偏干时,地表异常加热容易引起我国北方高层大气出现明显正异常和低层的反气旋性异常环流,上述环流异常容易导致温度偏高,同时不利于该区域降水的发生,进而导致土壤湿度偏低,上述正反馈机制可能是该区域土壤湿度与大气之间联系的一种可能途径。     

基于多模式集合的中国未来热浪趋势研究北大核心CSCD

全球变化导致极端天气事件频发,尤其是高温热浪严重影响我国农业生态系统及人类健康。关于热浪事件的定义一直存在着许多争议,对热浪变化趋势空间分布特征的认识有待进一步提高。本文使用气温日较差、绝对温度与相对温度相结合的热浪指标,基于9个CMIP6气候模式的多模式集合结果,评估了可持续发展情景(SSP1-2.6)、中度发展情景(SSP2-4.5)及常规排放情境(SSP5-8.5)下未来中国高温热浪事件的时空分布及变化特征。结果表明:(1)SSP1-2.6情景下未来热浪事件在2050年前后达到顶峰,之后趋于稳定,而在SSP2-4.5情景下,热浪频次、日数及最长持续时间均呈现上升态势,SSP5-8.5情景下热浪的增长趋势及严重程度均为最高;(2)华南、华中地区未来面临更大的热浪风险,SSP5-8.5情景下的热浪频次及强度约是SSP1-2.6的2倍及以上,SSP2-4.5约是SSP1-2.6的1.5倍;(3)西部干旱/半干旱地区、内蒙古东部干旱地区出现较大范围的热浪,结合本文中热浪定义,预示着夜间变暖是全球变暖的一个重要特征。研究结果有助于理解可持续发展、中等强迫情景下我国未来的热浪频次和强度的变化特征,为区域发展节能减排方案的制定提供有效参考。     

Sources of Subseasonal Prediction Skill for Heatwaves over the Yangtze River Basin Revealed from Three S2S Models

Based on the reforecast data (1999–2010) of three operational models [the China Meteorological Administration (CMA), the National Centers for Environmental Prediction of the U.S. (NCEP) and the European Centre for Medium-Range Weather Forecasts (ECMWF)] that participated in the Subseasonal to Seasonal Prediction (S2S) project, we identified the major sources of subseasonal prediction skill for heatwaves over the Yangtze River basin (YRB). The three models show limited prediction skills in terms of the fraction of correct predictions for heatwave days in summer; the Heidke Skill Score drops quickly after a 5-day forecast lead and falls down close to zero beyond the lead time of 15 days. The superior skill of the ECMWF model in predicting the intensity and duration of the YRB heatwave is attributable to its fidelity in capturing the phase evolution and amplitude of high-pressure anomalies associated with the intraseasonal oscillation and the dryness of soil moisture induced by less precipitation via the land–atmosphere coupling. The effects of 10–30-day and 30–90-day circulation prediction skills on heatwave predictions are comparable at shorter forecast leads (10 days), while the biases in 30–90-day circulation amplitude prediction show close connection with the degradation of heatwave prediction skill at longer forecast leads (> 15–20 days). The biases of intraseasonal circulation anomalies further affect precipitation anomalies and thus land conditions, causing difficulty in capturing extremely hot days and their persistence in the S2S models.     

Regional variability of convection over northern India during the pre-monsoon season

In general, the overall differences in activity and timing of convection are a result of the influence of large-scale regional and synoptic flow patterns on the local mesoscale environment. The linkage between the space?Ctime variability of observed clouds and rainfall, with large-scale circulation patterns and mesoscale variables over north India during the pre-monsoon season (March to May) is the focus of this paper. We use harmonic analysis to identify the first hour of rainfall for 42 stations spread over the north Indian region during the pre-monsoon summer season (March to May), from 1980 to 2000. The variability is observed to be systematic, with large regions having similar timing for occurrence of rainfall. The stations located in the foothills of the Himalayas have a late night to early morning maximum of first hour rainfall. In the northwestern plains, the first hour of rainfall mostly starts in the early afternoon to evening hours. Further eastward, the rainfall occurs in the late evening hours. Overall, there is a gradient in the occurrence of first rainfall events from late afternoon hours in the southern sections of the north Indian region to nocturnal maxima in the higher altitude regions. Five of these stations, located in different regions of homogenous timing of rainfall occurrence, were selected to analyze in detail the variable trigger for convection. Our results indicate that convective episodes occur mostly in association with the passage of westerly troughs over this region. These upper atmosphere troughs enable moisture to flow from the surrounding oceanic regions to the dry inland regions and also provide some dynamic support to the episodes of convection. However, the actual occurrence of convection is triggered by local factors, giving rise to the mesoscale structure of the weather systems during this season. Specifically, over the plains of northwest India, convection is triggered in a moistened environment by diurnal solar heating. The late night to early morning convection over the foothills is triggered by the orography, when the moistened airflow is normally incident on the mountain slopes. Further eastward, the primary trigger for localized moist convection is downdrafts from south-eastward propagating convective systems that originate at a north?Csouth dry line over north India. These systems propagate with a speed of about 15?m?s^?1. The above results are supported by geostationary satellite brightness temperature data for March to May 2008.     

青藏高原土壤湿度对中国夏季降水与气温影响的敏感试验

青藏高原为全球气候变化中的敏感区域。利用WRF3.5.1中尺度模式,选取青藏高原为关键区域,设计干、湿土壤湿度两组敏感试验,以探讨青藏高原土壤湿度异常对中国夏季短期区域气候产生的可能影响。结果表明：模式模拟的短期区域气候对土壤湿度十分敏感,湿土壤导致东北、内蒙古东北部以及华东地区降水增多,与此同时,全国大部分地区气温下降,且各地区表现较一致;干土壤导致西北、华北、华中以及西南除四川西部以外的地区降水减小,全国气温除华中地区以外,普遍升高。湿土壤对降水主要表现为正反馈作用,对气温表现为负反馈作用。干土壤则反之。     

淮河流域陆面水文过程的数值模拟

采用生物-大气传输模式（BATS模型）模拟了淮河流域山丘区和平原区在1998年汛期的暴雨洪水过程,从陆地-大气间水量交换的角度揭示了径流量、土壤含水量、土壤质地、植被分布的内在联系。结果表明：对于山丘区和平原区而言,根系层土壤含水量、土壤质地以及土壤颜色的变化对径流量的影响具有相似性,但是敏感性不同;而山丘区和平原区深层土壤含水量和植被覆盖率变化对径流量影响的作用正好相反。这些结果显示,由于山丘区与平原区的不同气候和下垫面条件,而造成两者水文性质的差异性,反映了大气-水文之间关系与作用的不同特征。     

A CLIMATOLOGY OF DEEP CONVECTION OVER SOUTH CHINA AND THE ADJACENT WATERS DURING SUMMER

Due to the higher temporal and spatial resolution and the better integrality of long-term satellite infrared(IR) Brightness Temperature(TBB) data,a climatology of deep convection during summer over South China and the adjacent waters is presented in this paper based on the 1-hourly infrared IR TBB data during June-August of 1996-2007(except 2004).The results show that the geographic distribution of deep convection denoted by TBB ≤-52℃ over South China and the adjacent waters are basically consistent with previous statistical results based on surface thunderstorm observations and low-orbit satellite lightning observations.The monthly,ten-day,five-day and diurnal variations of deep convection in this region are focused on in this paper.There are 5 active deep-convection areas in June-August.The monthly variations of the deep convection are closely associated with the large-scale atmospheric circulations.The deep convection over the land areas of South China is more active in June while that over the South China Sea is more active in July and August.The development of deep convection is prominently intermittent and its period is about 3 to 5 five-day periods.However,the deep convection over the coastal areas in South China remains more active during summer and has no apparent intermittence.The ten-day and five-day variations of deep convection show that there are different variations of deep convection over different areas in South China and the adjacent waters.The tendency of deep convection over the land areas of South China is negatively correlated with that over the South China Sea.The diurnal variations of deep convection show that the sea-land breeze,caused by the thermal differences between land and sea,and the mountain-valley breeze,caused by the thermal differences between mountains and plains or basins,cause deep convection to propagate from sea to land in the afternoon and from land to sea after midnight,and the convection over mountains propagates from mountains to plains after midnight.The different diurnal variations of deep convection over different underlying surfaces show that not only there are general mountainous,marine and multi-peak deep convection,but also there is longer-duration deep convection over coastal areas and other deep convection triggered and maintained by larger-scale weather systems in South China during summer.     

高分辨率WRF模式中土壤湿度扰动对短期高温天气模拟影响的个例研究

本文利用WRFV3.6中尺度预报模式就土壤湿度扰动对2003年7月22~23日和29~30日短期高温天气过程的影响进行了高分辨率模拟研究。结果表明:(1)WRF模式地表气温对土壤湿度扰动有较强的敏感性,且随着土壤湿度的增加(减小)而降低(升高)。同时,模式中土壤湿度对地面气温影响的强度对模式分辨率具有较高的依赖性。(2)不同模式分辨率下气温随土壤湿度变化的规律一致;由于更高分辨率的地形资料的应用,提高分辨率可在较大程度上改善模拟效果。(3)不同土壤湿度试验模拟的地表感热、潜热通量可直接影响气温变化;(4)土壤湿度扰动通过间接影响高温发展的近地层各物理过程使得地表气温发生变化。这些过程中,对流(平流)过程在全天表现为增温(冷却)的作用,强度在白天均随土壤湿度的减小而增加。在较干的土壤条件下,非绝热增温在白天的主导地位加强;在夜间,非绝热冷却的强度减弱,且小于占据主导的对流绝热增温的强度。以上结果表明,在模拟和预报高温天气时土壤湿度非常重要,也意味着通过土壤湿度扰动的集合预报方法来改进模式高温模拟预报具有较大的潜力。     

热带大西洋海温异常季节内演变对中国江南地区夏季持续性高温事件影响的初步研究

中国江南地区是高温热浪灾害的高影响区.以往的一些研究发现了不同海域海温异常在年际或年代际尺度上的变化对中国南方夏季平均温度异常的影响效应.但是,关于这些关键海域海温季节内尺度变化对江南地区高温事件发生和维持影响的研究尚不多见.为此,本文利用中国站点观测、美国气象环境预报中心和美国国家大气研究中心(NCEP/NCAR)再...     

山东半岛夏季降水异常的环流型及影响因子分析

利用1961—2016年华东地区106个气象观测站的日降水数据和再分析资料,分析引起山东半岛夏季降水异常的大气环流型及其与前期下垫面因子（海温和土壤湿度）的关系,结果发现：1）当孟加拉湾出现西南风异常,日本列岛以南和贝加尔湖西南侧地区分别呈反气旋和气旋式环流异常时,加强了向山东半岛的水汽输送,配合区域大气上升运动异常最终导致山东半岛夏季降水偏多;反之,当孟加拉湾出现西北风异常,日本列岛以南和贝加尔湖西南地区分别呈气旋和反气旋式环流异常时山东半岛降水偏少。2）孟加拉湾和北太平洋中部关键区的对流层整层位势高度与下垫面海温自春季持续至夏季存在显著正相关,当两个地区的整层位势高度均呈正异常时,分别对应夏季孟加拉湾的强西风气流和日本列岛以南的反气旋环流异常。3）区域土壤湿度异常引起的感热和潜热通量异常,可能是引起贝加尔湖关键区位势高度和山东半岛局地对流异常的原因：贝加尔湖西南地区土壤湿度偏大时,其上空对流层位势高度为负异常;山东半岛地区土壤湿度偏大时,其上空对流层大气出现异常上升运动。4）利用关键区春季下垫面因子（海温和土壤湿度）建立山东半岛夏季降水的统计预测模型,留一交叉检验的距平同号率达到75%。这些结果可为山东半岛夏季降水预测提供重要参考。     

Diurnal Variations of Precipitation over North China Regulated by the Mountain-plains Solenoid and Boundary-layer Inertial Oscillation

The mountain-plains solenoid(MPS) and boundary-layer inertial oscillation(BLO) are two typical regional forcings at the diurnal time scale. Their relative role in regulating the diurnal variations of summer rainfall over North China and their change under different monsoon conditions are studied using a 19-yr archive of satellite rainfall and reanalysis data. It is shown that both a strong MPS and BLO can increase nocturnal rainfall in the North China plains but exhibit evident regional differences. The MPS-induced nocturnal rainfall is relatively confined to the plains adjacent to mountains from late night to morning, due to the upward branch of the nighttime MPS. In contrast, the BLO-induced nocturnal rainfall strengthens from early evening and is more extensive in early morning over the open plains further east. The contrasting effect in the evening is related to the convergent(divergent) easterly anomaly in the plains under the BLO(MPS). The BLO also induces the relatively strong enhancement of moisture convergence and high humidity by the southerly anomaly at late night. On strong monsoon days, the nocturnal rainfall amount associated with the MPS and BLO increases considerably in the plains.Both regional forcings become effective in regulating the rainfall diurnal cycle with enhanced moisture convergence under monsoon conditions. Their induced diurnal amplitudes of moisture convergence can be comparable to the daily mean by monsoon flow. The regional forcings thus couple with monsoon flow to strengthen rainfall in the plains, particularly from late night to morning. The results highlight that a combination of regional and large-scale forcings can strongly regulate the warm-season climate.     

Equilibrium mechanism analysis on the physical processes of IAP3.0

Based on the detection of the dynamic and thermodynamic functions of the physical processes in IAP3.0, the equilibrium mechanisms of the temperature, moisture and wind are analyzed. The negative feedback between the longwave radiation and temperature is verified. The cooling regulation of net radiation on temperature is mostly balanced by the heating of precipitation, the leading actions on temperature of other processes such as vertical diffusion, shallow cumulus convection and friction are merely available for lower air. The moisture consumption of precipitation is compensated on the whole by the provision of shallow cumulus convection, which sustains the moisture conservation to a high degree. The wind field is directly regulated by the momentum redistribution of cumulus, the dry adiabatic convection and vertical diffusion. Yet, the prominent influences of these processes are generally confined to the lower level. The east wind at low latitudes and the west wind at high latitudes are both weakened by the regulations and furthermore, by virtue of the transportation of mean meridional circulation, such a variation exactly maintains the angular momentum conservation.     

Characteristics of large-scale atmospheric circulation related to extreme monthly rainfall anomalies in the Pampa Region, Argentina, under non-ENSO conditions

Summary There is a widely held view that the Pampa region (PR) dry and wet periods are predominantly a consecuence of the El Ni?o-Southern oscillation (ENSO) phenomenom. The current paper focuses on non-ENSO rainfall anomalies for the period 1948–2000, the more recent of which have had catastrophic consequences throughout the region. We analyze horizontal water vapor transport, pressure and circulation anomalies occurring in Southern South America (SSA) during this type of event. Positive and negative (wet and dry) extreme events during the rainy and dry seasons in the region were registered. Based on NCEP reanalysis data it was established that under rainfall deficit, anomalies of similar intensity occurred simultaneously in the PR and in central Chile, whereas under excess rainfall the anomalies were mostly confined to the PR. The existence of a cyclone-anticyclone pair in the anomalous circulation pattern over mid latitudes of the Atlantic and Pacific oceans and straddling the southern portion of the continent maintains an intense and extense meridional circulation over the continental plains, which leads to the abnormal values in moisture transport and rainfall rate. The atmospheric water balance equation calculated for the PR indicates that anomalous water vapor is carried in from the continental equatorial region and from the subtropical Atlantic, its magnitude varying in accordance with the season and the sign of the anomaly. Furthermore, evidence of the important role of transient terms corroborates their contribution to the anomalous total moisture flux divergence under rainfall deficit during the dry season. The mean sea-level pressure anomaly fields of the extreme cases were further examined by principal component analysis to discern those circulation features directly linked to rainfall deviations.     

湖南省夏季高温热浪时空分布特征及其成因

利用1960~2014年夏季(6~8月)湖南省77个气象站逐日最高气温资料,对该地区高温热浪的时空分布特征进行分析研究。研究结果表明:湖南省大部分地区常年遭受高温热浪袭击,年均发生1.53次,部分地区年均出现2次以上。高频次、高强度的高温热浪主要集中于湘中偏东(衡阳盆地一带)地区,株洲、长沙、益阳等地为次大值区,湘西地区高温热浪的发生频次及强度则低于全省平均值。近年来,湖南高温热浪的影响范围不断扩大,炎热程度不断增强。近55年高温热浪频次及强度具有显著的阶段性特征,20世纪80年代以前呈减少(减弱)趋势,此后呈增多(增强)趋势,21世纪以来高温热浪增多增强尤其明显。对一次持续时间长、影响范围广的极端高温热浪过程研究发现:南亚高压东伸扩展,西太副高异常偏西、偏北且稳定少动,下沉运动强盛等条件的配合有利于湖南省持续性高温的发生。     

Contrasts of Atmospheric Circulation and Associated Tropical Convection between Huaihe River Valley and Yangtze River Valley Mei-yu Flooding

The significant differences of atmospheric circulation between flooding in the Huaihe and Yangtze River valleys during early mei-yu(i.e.,the East Asian rainy season in June) and the related tropical convection were investigated.During the both flooding cases,although the geopotential height anomalies always exhibit equivalent barotropic structures in middle to high latitudes at middle and upper troposphere,the phase of the Rossby wave train is different over Eurasian continent.During flooding in the Huaihe River valley,only one single blocking anticyclone is located over Baikal Lake.In contrast,during flooding in the Yangtze River valley,there are two blocking anticyclones.One is over the Ural Mountains and the other is over Northeast Asia.In the lower troposphere a positive geopotential height anomaly is located at the western ridge of subtropical anticyclone over Western Pacific(SAWP) in both flooding cases,but the location of the height anomaly is much farther north and west during the Huaihe River mei-yu flooding.Furthermore,abnormal rainfall in the Huaihe River valley and the regions north of it in China is closely linked with the latent heating anomaly over the Arabian Sea and Indian peninsula.However,the rainfall in the Yangtze River valley and the regions to its south in China is strongly related to the convection over the western tropical Pacific.Numerical experiments demonstrated that the enhanced latent heating over the Arabian Sea and Indian peninsula causes water vapor convergence in the region south of Tibetan Plateau and in the Huaihe River valley extending to Japan Sea with enhanced precipitation;and vapor divergence over the Yangtze River valley and the regions to its south with deficient precipitation.While the weakened convection in the tropical West Pacific results in moisture converging over the Yangtze River and the region to its south,along with abundant rainfall.     

RCP4.5情景下中国未来干湿变化预估

本文采用国际耦合模式比较计划第五阶段(CMIP5)中21个气候模式的试验数据, 利用土壤湿度以及由其他8个地表气象要素计算所得的干旱指数, 预估了RCP4.5(Representative Concentration Pathway 4.5)情景下21世纪中国干湿变化。结果表明:全球气候模式对1986～2005年中国现代干湿分布具备模拟能力, 尽管在西部地区模式与观测间存在一定的差异。在RCP4.5情景下, 21世纪中国区域平均的标准化降水蒸散发指数和土壤湿度均有减小趋势, 与之对应的是短期和长期干旱发生次数增加以及湿润区面积减小。从2016到2100年, 约1.5%～3.5%的陆地面积将从湿润区变成半干旱或半湿润区。空间分布上, 干旱化趋势明显的区域主要位于西北和东南地区, 同时短期和长期干旱发生次数在这两个地区的增加幅度也最大, 未来干旱化的发生时间也较其他地区要早;只在东北和西南地区未来或有变湿倾向, 但幅度较小。在季节尺度上, 北方地区变干主要发生在暖季, 南方则主要以冷季变干为主。造成中国干旱化的原因主要是由降水与蒸散发所表征的地表可用水量减少。