季节尺度区域气候模拟适应调整时间选取问题的数值试验

文中利用区域气候模式RegCM3,以1998年夏季中国异常气候事件为例,对季节尺度区域气候模拟适应调整时间的选取问题进行了数值研究。共做了11个试验,每个试验的适应调整时间最短为10 d,最长为6个月,以检验适应调整时间长短对夏季中国异常气候事件模拟结果的影响。结果表明:对于大气变量而言,模式通常在经过4—8 d的适应调整时间后,就进入"气候模态"运行,此后模拟误差主要是由于模式对区域内大气过程描述能力不足造成的,对适应调整时间选取不再敏感,这进一步证实了区域气候模拟是一个边值问题的观点。各气候区平均降水量模拟结果受适应调整时间影响也不大,但不同的适应调整时间对降水分布格局模拟将产生一定影响,降水分布模拟结果随适应调整时间的不同存在一定的不确定性,这种不确定性通常出现在强降水发生区域。总之,对于季节尺度降水模拟,适应调整时间大于2个月效果更好。对降水分布格局模拟误差和东亚夏季风系统进退过程之间关系的进一步分析发现,模式对受夏季风系统影响比较大的区域模拟的降水相关系数变化性也比较大,因此,发展合适的积云对流参数化方案以提高受夏季风系统直接影响区域强降水过程的描述能力是改进区域气候模式对中国区域夏季气候模拟效果的有效途径。     

Regional Features and Seasonality of Land–Atmosphere Coupling over Eastern China

Land-atmosphere coupling is a key process of the climate system, and various coupling mechanisms have been proposed before based on observational and numerical analyses. The impact of soil moisture(SM) on evapotranspiration(ET) and further surface temperature(ST) is an important aspect of such coupling. Using ERA-Interim data and CLM4.0 offline simulation results, this study further explores the relationships between SM/ST and ET to better understand the complex nature of the land-atmosphere coupling(i.e., spatial and seasonal variations) in eastern China, a typical monsoon area. It is found that two diagnostics of land-atmosphere coupling(i.e., SM-ET correlation and ST-ET correlation) are highly dependent on the climatology of SM and ST. By combining the SM-ET and ST-ET relationships, two "hot spots" of land-atmosphere coupling over eastern China are identified: Southwest China and North China. In Southwest China, ST is relatively high throughout the year, but SM is lowest in spring, resulting in a strong coupling in spring. However, in North China, SM is relatively low throughout the year, but ST is highest in summer, which leads to the strongest coupling in summer. Our results emphasize the dependence of land-atmosphere coupling on the seasonal evolution of climatic conditions and have implications for future studies related to land surface feedbacks.     

Numerical Experiments on the Spin-up Time for Seasonal-Scale Regional Climate Modeling

In this paper,the numerical experiments on the issue of spin-up time for seasonal-scale regional climate modeling were conducted with the newly Regional Climate Model (RegCM3),in the case of the abnormal climate event during the summer of 1998 in China.To test the effect of spin-up time on the regional climate simulation results for such abnormal climate event,a total of 11 experiments were performed with different spin-up time from 10 days to 6 months,respectively.The simulation results show that,for the meteorological variables in the atmosphere,the model would be running in"climate mode"after 4-8-day spin-up time,then, it is independent of the spin-up time basically,and the simulation errors are mainly caused by the model's failure in describing the atmospheric processes over the model domain.This verifies again that the regional climate modeling is indeed a lateral boundary condition problem as demonstrated by earlier research work. The simulated mean precipitation rate over each subregion is not sensitive to the spin-up time,but the precipitation scenario is somewhat different for the experiment with different spin-up time,which shows that there exists the uncertainty in the simulation to precipitation scenario,and such a uncertainty exhibits more over the areas where heavy rainfall happened.Generally,for monthly-scale precipitation simulation,a spin-up time of 1 month is enough,whereas a spin-up time of 2 months is better for seasonal-scale one. Furthermore,the relationship between the precipitation simulation error and the advancement/withdrawal of East Asian summer monsoon was analyzed.It is found that the variability of correlation coefficient for precipitation is more significant over the areas where the summer monsoon is predominant.Therefore,the model's capability in reproducing precipitation features is related to the heavy rainfall processes associated with the advancement/withdrawal of East Asian summer monsoon,which suggests that it is necessary to develop a more reliable parameterization scheme to capture the convective precipitation of heavy rainfall pro- cesses associated with the activities of East Asian summer monsoon,so as to improve the climate modeling over China.     

区域大气模式中陆面子模式起转过程的研究

模式的起转过程(spin-up)是指在非平衡初值或扰动的条件下,模式进行调整而达到平衡态的过程.以黑河实验(HEIFE) 1991年6月20日到7月20日张掖站观测的地表能量通量和土壤温度资料为基础,用大量数值实验研究了RAMS (Regional Atmospheric Modeling System)陆面子模式起转过程所需的时间范围.数值实验中,为细致考虑土壤初始参数对此过程的影响,共设计了40余组不同的初始土壤水、热参数.通过模拟结果分析,把RAMS陆面模式起转过程的表现归纳为4大类,并说明了这个过程的持续时间可能因为土壤初始参数的不同而产生较大的差异,其范围可能为数小时至一周以上.在判断RAMS模拟地表、低空物理量时,土壤含水量是判断模式是否达到平衡态的重要因子.     

2004年南海夏季风的爆发及中南半岛陆面-过程的可能影响Ⅱ： 数值试验

在对南海夏季风的爆发及中南半岛陆面过程的可能影响进行了诊断分析的基础上,应用MM5/NOAHLSM模式,研究了中南半岛陆气相互作用对2004年南海夏季风爆发过程的可能影响。结果发现:在南海夏季风爆发前,中南半岛南海地区低层气温差确实出现低值,甚至负值;尽管短期内中南半岛土壤湿度和降水的变化没有引起季风爆发日期的改变,但对季风爆发的强度有影响。土壤湿度和降水变化引起的干异常可导致地表感热通量的增大和地表温度的升高,致使中南半岛与南海之间低层的温差异常(负温差)减小,季风爆发强度减弱;不同的是,湿异常可引起季风爆发强度增强。这一结果说明,在南海夏季风爆发前期,中南半岛上空对流活动和降水异常及其引起的土壤湿度的异常变化在一定程度上会影响到季风爆发的过程。文章还比较了不同温湿地表条件下低层大气状态的差异和地表能量、水分平衡过程的不同,分析了陆气相互作用对季风活动产生影响的物理机制。     

地表反照率与高原夏季风爆发关系分析

利用2000～2016年MODIS地表反照率和ECMWF/ERA-Interim再分析资料,选取有代表性的高原季风指数DPMI,统计分析了青藏高原地表反照率与高原季风之间的联系,结果表明:1)11月高原地表反照率大小与次年高原夏季风爆发存在密切关系:11月高原地表反照率偏低(高),次年4月高原夏季风爆发偏早(晚),强度偏强(弱)。2)可能的影响机制为:当前期11月高原地表反照率偏低时,后期高原主体对大气的感热加热信号更强,从而引起4月高原上空近地面层上升运动明显加强,这有利于热量向高空传输,导致对流层加热作用加强,高原上空对流层温度偏高,使得高原季风环流系统加强,最终导致高原季风季节变化相应提前;反之亦然。     

Influence of soil moisture in eastern China on the East Asian summer monsoon

The sensitivity of the East Asian summer monsoon to soil moisture anomalies over China was investigated based on ensembles of seasonal simulations(March–September) using the NCEP GCM coupled with the Simplified Simple Biosphere Model(NCEP GCM/SSi B). After a control experiment with free-running soil moisture, two ensembles were performed in which the soil moisture over the vast region from the lower and middle reaches of the Yangtze River valley to North China(YRNC) was double and half that in the control, with the maximum less than the field capacity. The simulation results showed significant sensitivity of the East Asian summer monsoon to wet soil in YRNC. The wetter soil was associated with increased surface latent heat flux and reduced surface sensible heat flux. In turn, these changes resulted in a wetter and colder local land surface and reduced land–sea temperature gradients, corresponding to a weakened East Asian monsoon circulation in an anomalous anticyclone over southeastern China, and a strengthened East Asian trough southward over Northeast China. Consequently, less precipitation appeared over southeastern China and North China and more rainfall over Northeast China. The weakened monsoon circulation and strengthened East Asian trough was accompanied by the convergence of abnormal northerly and southerly flow over the Yangtze River valley, resulting in more rainfall in this region.In the drier soil experiments, less precipitation appeared over YRNC. The East Asian monsoon circulation seems to show little sensitivity to dry soil anomalies in NCEP GCM/SSi B.     

THE CLIMATIC CHARACTERISTICS OF SUMMER MONSOON ONSET OVER THE SOUTH CHINA SEA I.40-YEAR AVERAGE

By using 40-year NCEP reanalysis daily data (1958-1997), we have analyzed the climatic characteristics of summer monsoon onset in the South China Sea (105°E ~ 120°E, 5°N ~ 20°N, to be simplified as SCS in the text followed) pentad by pentad (5 days). According to our new definition, in the monsoon area of the SCS two of the following conditions should be satisfied: 1) At 850hPa, the southwest winds should be greater than 2m/s. 2) At 850 hPa, θ_se should be greater than 335°K. The new definition means that the summer monsoon is the southwest winds with high temperature and high moisture. The onset of the SCS summer monsoon is defined to start when one half of the SCS area (105°E ~ 120°E,5°N ~ 20°N) is controlled by the summer monsoon. The analyzed results revealed the following: 1) The summer monsoon in the SCS starts to build up abruptly in the 4th pentad in May. 2) The summer monsoon onset in the SCS is resulted from the development and intensification of southwesterly monsoon in the Bay of Bengal. 3) The onset of the summer monsoon and establishment of the summer monsoon rainfall season in the SCS occur simultaneously. 4) During the summer monsoon onset in the SCS, troughs deepen and widen quickly in the lower troposphere of the India; the subtropical high in the Western Pacific moves eastward off the SCS in the middle troposphere; the easterly advances northward over the SCS in the upper troposphere.     

东亚中纬度夏季陆面增暖与土壤湿度异常的联系

利用1979—2015年ERA-Interim再分析土壤湿度、云量资料,NCEP/NCAR再分析环流资料和CPC土壤湿度资料,分析了东亚中纬度夏季陆面热力异常的时空分布特征及其与前期春季土壤湿度异常的联系,探讨了前期春季土壤湿度影响东亚中纬度夏季陆面增暖的可能途径。结果表明,东亚中纬度夏季土壤表层温度呈全区一致增暖趋势,其中贝加尔湖及以南地区温度变化最剧烈、增暖最迅速,且1990年代中期前后存在一个明显由冷向暖的年代际转变。进一步分析发现,春、夏季西西伯利亚到贝加尔湖北部地区的土壤湿度与夏季贝加尔湖及以南地区的土壤表层温度在年代际和年际尺度上均存在紧密联系:西西伯利亚到贝加尔湖北部地区土壤湿度异常偏高,通常对应贝加尔湖及南部地区夏季土壤表层温度偏高。西西伯利亚到贝加尔湖北部地区春、夏土壤湿度异常可以引起夏季大气环流异常,从而对东亚夏季中纬度陆面热力异常产生影响:春、夏土壤湿度偏高时,贝加尔湖及其南部地区上空位势高度为正异常,对应为反气旋性异常环流,云量减少,有利于东亚中纬度陆面增暖;反之,则对应为气旋性异常环流,不利于陆面增暖。     

Possible Impacts of the Arctic Oscillation on the Interdecadal Variation of Summer Monsoon Rainfall in East Asia

The influences of the wintertime AO (Arctic Oscillation) on the interdecadal variation of summer monsoon rainfall in East Asia were examined. An interdecadal abrupt change was found by the end of the 1970s in the variation of the AO index and the leading principal component time series of the summer rainfall in East Asia, The rainfall anomaly changed from below normal to above normal in central China, the southern part of northeastern China and the Korean peninsula around 1978. However,the opposite interdecadal variation was found in the rainfall anomaly in North China and South China.The interdecadal variation of summer rainfall is associated with the weakening of the East Asia summer monsoon circulation. It is indicated that the interdecadal variation of the AO exerts an influence on the weakening of the monsoon circulation. The recent trend in the AO toward its high-index polarity during the past two decades plays important roles in the land-sea contrast anomalies and wintertime precipitation anomaly. The mid- and high-latitude regions of the Asian continent are warming, while the low-latitude regions are cooling in winter and spring along with the AO entering its high-index polarity after the late 1970s. In the meantime, the precipitation over the Tibetan Plateau and South China is excessive, implying an increase of soil moisture. The cooling tendency of the land in the southern part of Asia will persist until summer because of the memory of soil moisture. So the warming of the Asian continent is relatively slow in summer. Moreover, the Indian Ocean and Pacific Ocean which are located southward and eastward of the Asian land, are warming from winter to summer. This suggests that the contrast between the land and sea is decreased in summer. The interdecadal decrease of the land-sea heat contrast finally leads to the weakening of the East Asia summer monsoon circulation.     

Land surface sensitivity of monsoon depressions formed over Bay of Bengal using improved high-resolution land state

Monsoon depressions (MDs) constitute a large fraction of the total rainfall during the Indian summer monsoon season. In this study, the impact of high-resolution land state is addressed by assessing the evolution of inland moving depressions formed over the Bay of Bengal using a mesoscale modeling system. Improved land state is generated using High Resolution Land Data Assimilation System employing Noah-MP land-surface model. Verification of soil moisture using Soil Moisture and Ocean Salinity (SMOS) and soil temperature using tower observations demonstrate promising results. Incorporating high-resolution land state yielded least root mean squared errors with higher correlation coefficient in the surface and mid tropospheric parameters. Rainfall forecasts reveal that simulations are spatially and quantitatively in accordance with observations and provide better skill scores. The improved land surface characteristics have brought about the realistic evolution of surface, mid-tropospheric parameters, vorticity and moist static energy that facilitates the accurate MDs dynamics in the model. Composite moisture budget analysis reveals that the surface evaporation is negligible compared to moisture flux convergence of water vapor, which supplies moisture into the MDs over land. The temporal relationship between rainfall and moisture convergence show high correlation, suggesting a realistic representation of land state help restructure the moisture inflow into the system through rainfall-moisture convergence feedback.     

An Overview of Research and Forecasting on Rainfall Associated with Landfalling Tropical Cyclones

The ability to forecast heavy rainfall associated with landfalling tropical cyclones (LTCs) can be improved with a better understanding of the mechanism of rainfall rates and distributions of LTCs. Research in the area of LTCs has shown that associated heavy rainfall is related closely to mechanisms such as moisture transport, extratropical transition (ET), interaction with monsoon surge, land surface processes or topographic effects, mesoscale convective system activities within the LTC, and boundary layer energy transfer etc.. LTCs interacting with environmental weather systems, especially the westerly trough and mei-yu front, could change the rainfall rate and distribution associated with these mid-latitude weather systems. Recently improved technologies have contributed to advancements within the areas of quantitative precipitation estimation (QPE) and quantitative precipitation forecasting (QPF). More specifically, progress has been due primarily to remote sensing observations and mesoscale numerical models which incorporate advanced assimilation techniques. Such progress may provide the tools necessary to improve rainfall forecasting techniques associated with LTCs in the future.     

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).     

中南半岛春季土壤湿度异常对亚洲热带夏季风建立和发展的影响

利用WRF区域模式模拟分析了中南半岛地区春季土壤湿度异常对亚洲热带夏季风建立和发展的影响,结果表明:亚洲热带夏季风对中南半岛春季土壤湿度的响应是不对称的,当中南半岛春季土壤湿度偏高时,中南半岛及孟加拉湾周边地区呈现异常东风,伴随降水减少,季风减弱;而中南半岛春季土壤湿度偏低时,孟加拉湾及周边地区西风减弱,降水减少,季风也对应减弱。通过进一步分析物理机制得到,中南半岛春季土壤湿度异常偏高使季风建立初期感热减小,陆表温度明显降低,从而导致海陆温差逐渐降低,使季风减弱;而中南半岛春季土壤湿度异常偏低使整个中南半岛区域蒸发减少,导致地表向上输送的水汽减少,减弱季风环流和降水。此外,通过分析850 h Pa纬向风及对流层中上层经向温度梯度两项季风暴发指数,探讨了中南半岛春季土壤湿度异常对孟加拉湾东部季风暴发时间的影响,结果表明:中南半岛春季土壤湿度偏高时,孟加拉湾东部季风暴发时间大约推迟10天左右,而土壤湿度较低对亚洲热带夏季风暴发时间影响甚微。     

东亚海陆热力差指数及其与环流和降水的年际变化关系

利用 196 1～ 1999年海温和地温月平均资料 ,定义了一个海陆热力差指数 ,来表示东亚季风环流的纬向和经向海陆热力差异的变化强度 ,研究了夏季指数与东亚夏季风环流场和中国东部夏季降水的年际变化关系。结果表明 :(1)海陆热力差指数可用来表示东亚夏季风的强弱变化。强指数年东亚季风区低空西南夏季风气流和高层的东风气流明显偏强 ,表明这一年夏季风偏强 ,弱指数年反之。 (2 )海陆热力差指数能较好地反映东部季风区夏季降水的异常状况。强指数年 ,雨带偏北 ,江淮流域和长江中下游明显干旱 ,华南、华北降水偏多 ,弱指数年反之。这一降水异常特征可以从强弱海陆热力差指数年的环流场得到解释。 (3)海陆热力差指数所反映的东亚夏季风具有明显的准 2a和 3～ 6a周期的年际振荡 ,但其振幅和周期具有显著的年代际异常     

南海夏季风撤退期的气候特征I——40年平均

近年来广州地区的气溶胶污染日趋严重,灰霾天气主要出现在10月～次年4月。大气灰霾导致能见度恶化。自1980年代初开始,该地区的能见度急剧恶化,灰霾天气显著增加,其中有3次大的波动,分别代表广州地区经济发展相伴随的粉尘污染、硫酸盐加粉尘污染、光化学过程的细粒子加硫酸盐加粉尘的复合污染时期。雾和轻雾造成的低能见度的长期变化趋势,没有由于人类活动影响或经济发展影响带来的趋势性变化,其波动主要反映了气候波动固有的年际和年代际变化。广州地区能见度的恶化主要与细粒子有关,PM10有一半季均值超过国家二级标准的日均值浓度限值,而PM2.5季均值全部超过美国国家标准的日均值浓度限值,细粒子浓度甚高。另外PM2.5占PM10的比重非常高,可达62%～69%,尤其是旱季比雨季更高。     

Water vapor transport around the Tibetan Plateau and its effect on summer rainfall over the Yangtze River valley

The water vapor transport around the Tibetan Plateau(TP) and its effect on the rainfall in the Yangtze River valley(YRV) in summer are investigated by decomposing the moisture transport into rotational and divergent components.Based on the ERA-Interim and PREC/L(Precipitation Reconstruction over Land) data from 1985 to 2014,the vertically integrated features of the two components are examined.The results show that the divergent part dominates the western TP while the rotational part dominates the rest of the TP,implying that moisture may be mostly locally gathered in the western TP but could be advected to/from elsewhere over the rest of the TP.The divergent and rotational moisture fluxes exhibit great temporal variability along the southern periphery of the TP,showing sensitivity of water vapor to the steep topography there.Correlation analysis reveals that it is over the southeastern corner of the TP and to its south that a significant correlation between rotational zonal moisture transport and summer rainfall in the YRV appears,suggesting that the southeastern corner of the TP may serve as a moisture transport bridge between the South Asian(Indian) monsoon and the East Asian monsoon.Further composite analysis indicates that anomalous eastward(westward) zonal water vapor transport from the South Asian monsoon via the southeastern corner of the TP favors more(less) precipitation in the YRV in summer.     

1981～2020年青藏高原春季土壤湿度时空变化特征及其与高原季风的关系

本研究利用欧洲中心ERA5再分析资料的逐日土壤湿度（土壤体积含水量）、降水量、位势高度场以及风场数据,重点分析了1981～2020年高原春季浅层（0～7 cm）土壤湿度的时空变化特征,并探讨了青藏高原土壤湿度与高原季风的关系。青藏高原春季土壤湿度西北偏干,东南部相对偏湿的分布特征。对高原春季土壤湿度进行经验正交函数（EOF）分析后发现,其第一模态呈中部与东、西部反向变化特征,该模态存在准3年（2～4年）的振荡周期,这一周期特征在2000～2010年表现的更为显著;第二模态呈南北反向分布,较好地表征高原地区气候带与下垫面覆盖状况。研究发现,高原夏季风与高原春季土壤湿度变化之间存在密切的隔季相关,高原夏季风异常变化是翌年春季土壤湿度变化的主要原因。     

Impact of moisture flux convergence and soil moisture on precipitation: a case study for the southern United States with implications for the globe

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.     

A STUDY ON THE RELATIONSHIP BETWEEN SPRING SOIL MOISTURE OVER CHINA AND EAST ASIA SUMMER MONSOON

The correlation analysis has been used to study the relationship between spring soil moisture over China and East Asian summer monsoon (EASM). It is shown that EASM has a strong positive correlation with spring soil moisture over southwest China and the Great Bend region of the Yellow River. A standard soil moisture index (SMI) has been defined using the observed soil moisture of the two regions. The results show that SMI has a strong correlation with EASM. The years of strong (weak) SMI are associated with stronger (weaker) summer monsoon circulation. In the years of strong SMI, the west Pacific subtropical high is much northward in position and weaker in intensity; the westerlies zone is also more to the north. All of these make EASM circulation move northward and cause the rainfall belt to relocate to North China and Northeast China. SMI can reflect the variation of the summer rainfall anomaly over eastern China. In the years of strong SMI, the rainfall belt is mainly located over the northem part of China.However, during the weak years, the summer rainfall belt is largely located over the mid- and lower- reaches of the Yangtze River. Additionally, the SMI has obvious oscillations of quasi 4-6 years and quasi 2 years. Moreover, negative SMI predicts EASM better than positive SMI.