欧亚大陆积雪与2010年中国春末夏初降水的关系

利用欧亚大陆地区的积雪覆盖日数资料、中国160个台站逐月降水资料以及美国国家环境预报中心和国家大气研究中心(NCEP/NCAR)再分析资料,采用奇异值分解(SVD)方法和数值试验研究了欧亚大陆2-4月积雪与后期5-6月中国降水的关系。结合2010年积雪和降水的分布特征可以发现,SVD第二模态基本反映了2010年积雪异常与降水异常的时滞耦合关系。欧亚大陆积雪在欧洲、青藏高原东部和东亚地区异常偏多时,我国华南降水增多,长江中下游降水减少,欧亚大陆东、西部积雪异常偏多与后期中国华南降水存在正相关关系。根据统计分析得到的空间分布特征和积雪参数的观测数据,选用大气环流模式CAM3.1进行数值试验,模拟结果与统计分析结果比较一致。积雪异常通过反照率效应和水文效应引起地表及其上的大气热力状况异常,进而引起位势高度场、风场等各个大气环流要素场的调整,导致后期华南降水异常偏多。欧亚大陆积雪异常很可能是2010年春末夏初华南降水异常的一个重要诱发因子。     

积雪分布及其对中国气候影响的研究进展

对北半球不同地区的积雪分布状况、积雪异常影响中国气候的事实以及影响机理等问题的研究成果进行了较系统的回顾与总结。青藏高原、蒙古高原、欧洲阿尔卑斯山脉及北美中西部是北半球积雪分布的关键区,其中青藏高原是北半球积雪异常变化最强烈的区域。中国积雪分布范围广泛,其中新疆、东北和青藏高原是3个大值区。总体来看,北半球积雪有减少的趋势,而中国积雪却有弱的增加趋势。冬、春季高原积雪与欧亚积雪对中国夏季降水的影响是相反的。积雪影响中国气候的机理解释为：冬季积雪反照率效应起主要作用,春夏季积雪水文效应起主要作用。积雪被视为中国短期气候预测的一个重要物理因子,继续加强该领域的研究对于提高中国短期气候预测的准确率将有重要意义。     

中国冬季积雪特征及欧亚大陆积雪对中国气候影响

该文首先回顾了有关中国冬季积雪的研究进展,包括中国冬季积雪的空间分布气候特征以及季节、年际和年代际变化,中国冬季降雪特征,气象因子对中国冬季积雪水量平衡的影响,外强迫和大气环流系统在积雪形成中的作用等。冬春季欧亚大陆积雪对同期和后期中国气候影响的相关研究说明与欧亚大陆积雪异常相关联的中国气候异常以及积雪通过改变土壤湿度、表面温度和辐射分布,引起大气环流异常,进而对中国气候产生影响的物理过程。应用美国环境预测中心 (NCEP) 第2版气候预测系统 (CFSv2) 的回报试验结果,对CFSv2在欧亚大陆积雪变化及其与中国气候关系的可预报性方面的分析表明,CFSv2能够较好地回报出春季欧亚积雪的年际和年代际变异及其与中国夏季降水之间的联系。文章最后提出了在积雪及其气候效应研究方面一些有待解决的问题。     

欧亚大陆冷季积雪与亚洲夏季风的关系:区域特征与季节性

主要回顾了欧亚大陆冷季积雪与亚洲夏季风的关系,特别是积雪对季风演变和强度的影响。从Blanford(1884)着眼喜马拉雅山测站积雪开始,到20世纪70年代卫星测量的大范围雪盖资料的问世,直至近几十年来全球气候模式模拟与资料分析的结合,回顾了人们在不同时期对积雪-季风关系的不同认识以及对积雪影响季风的物理过程的不同理解。一方面,积雪通过反照率效应影响温度、温度梯度和大气环流包括季风环流;另一方面,积雪通过融化效应影响大气,特别是增强大气异常信号的持续性。在这些过程中,陆面积雪与大气运动的相互作用是一个正反馈的过程。另外,特别关注不同地区和季节的欧亚大陆积雪对不同亚洲区域季风的不同影响,以及积雪在季风演变过程中对不同阶段季风特征的作用。虽然积雪与季风的关系非常复杂,加深对这些复杂关系的认识,对理解季风系统的整体变化以及改进季风预报都尤为重要。     

欧亚大陆积雪对我国春季气候可预报性的影响

利用大气环流模式IAP9L_CoLM,通过两组集合后报试验,考察了欧亚大陆积雪对我国春季气候可预报性的影响。一组试验为常规后报试验,积雪是由模式陆面过程预报得到的,另一组试验为积雪试验,模式积分过程中欧亚大陆雪水当量由微波遥感积雪资料替代,一天替换一次。通过分析两组试验后报结果的差异,来考察欧亚大陆积雪对我国春季（3~5月）气候可预报性的影响。分析表明:欧亚大陆积雪模拟水平的改善能提高春季欧亚大陆中高纬环流场（海平面气压场和中、高层位势高度场）的可预报性,模式对我国春季气温异常的年际变化和空间分布的可预报能力也有显著增强。对我国春季降水,虽然预报技巧较低,但引入较真实的欧亚积雪作用后,由于中高纬环流场预报技巧的改进导致降水的预测能力也有所改进。个例分析也表明,欧亚中高纬春季积雪异常模拟水平的改善导致了欧亚中高纬贝加尔湖及以南区域环流场可预报性的提高,最终使中国东部区域春季气候异常模拟技巧得以改善。以上结果也证实,欧亚大陆积雪是影响东亚区域春季气候的一个重要因子,要提高模式对中国春季气候的预报技巧,积雪模拟水平的改进是非常必要的。     

青藏高原冬春积雪影响亚洲夏季风的研究进展

青藏高原积雪作为亚洲夏季降水异常的一个预测因子已有很长历史。统计分析表明,青藏高原冬春积雪与印度夏季降水反相关,与我国长江中下游夏季降水多为正相关,不同作者对此现象给出了一些可能的解释,如地表反照率效应和积雪一水文效应。但积雪的这种跨季节滞后影响的具体物理过程仍有争议。另一方面,青藏高原冬春积雪资料存在不确定性,这对相关物理机制的理解造成了很大困难。通过回顾和梳理前人在该领域的研究成果,提出了高原积雪影响亚洲季风的未来研究方向。     

中国春季降水异常及其与热带太平洋海面温度和欧亚大陆积雪的联系

利用降水观测资料, 研究了1979～2004年中国春季 (3～5月) 标准化累积降水异常的时空特征及其与前冬、 春热带太平洋海面温度和春季欧亚大陆积雪的关系。中国春季标准化累积降水量EOF第一模态最大变率位于中国东部中纬度地区, 主要反映了中国东部中纬度地区春季降水的变化特征。同时, 中国东部春季降水异常具有南、 北反相变化的特征。当长江以南大部分地区的降水偏少时, 长江以北地区的降水偏多。春季降水异常具有显著的年际变化, 但在1980年代末出现年代际转型, 即年际变化的振幅明显增大变强、 周期变长。从华北到长江流域中纬度地区的春季降水异常特征与前冬热带太平洋海面温度有密切的关系。当前冬、 春热带东太平洋海温偏暖, 西太平洋海温偏冷时, 中国东部从华北到长江流域中纬度地区的春季降水偏多, 反之亦然。虽然当春季欧亚大陆楚科奇半岛和青藏高原积雪偏多, 贝加尔湖到中国东北地区的积雪偏少时, 对应着中国东部从华北到长江流域中纬度地区的降水偏多, 但当去掉ENSO信号后, 这种关系并不显著。说明EOF第一模态所反映的中国东部从华北到长江流域中纬度地区春季降水与欧亚大陆积雪的相关关系可能是前冬热带太平洋海面温度异常的一个体现。     

青藏高原南、北积雪异常与中国东部夏季降水关系的数值试验研究

青藏高原冬、春积雪有着显著的南、北空间差异,本文利用通用地球系统模式（CESM）设计了增加高原南、北冬、春积雪的敏感性试验,结果表明:当高原南部冬、春积雪异常偏多,长江及其以北地区夏季降水偏多,华南大部分地区夏季降水偏少;而当高原北部冬、春积雪异常偏多,华北及东北地区夏季降水偏多,长江下游南部地区夏季降水偏少,雨带更偏北。青藏高原南、北部冬、春积雪异常影响中国东部夏季降水的物理机制的分析结果表明,高原不同区域（南部和北部）冬、春积雪异常引起的非绝热加热异常效应都可持续到夏季,且北部积雪异常持续时间更长。高原南部和北部积雪异常偏多均会减弱高原北侧上空大气的水平温度梯度,进而减弱高原北侧西风急流的位置及强度,进而影响下游出口区处急流的强度和位置,且高原北部积雪异常偏多的影响更大。当高原南部积雪异常偏多,急流出口区的西风急流加强且偏南;而高原北部积雪异常偏多,出口区的西风急流减弱且偏北。相应地,对流层中层500 hPa西太平洋副热带高压减弱,低层850 hPa异常反气旋环流,影响中国东部地区水汽输送,从而影响了中国东部地区夏季雨带的变化。当高原南部积雪异常偏多,异常反气旋性环流位于东海附近,有利于更多水汽输送至长江流域,华南水汽输送减少;当高原北部积雪异常偏多,异常反气旋性环流相对偏北,更有利于华北及东北水汽输送,雨带偏北。     

陆面过程模式中积雪过程的参数化及初步试验

为了用气候模式研究冰雪圈变化的气候效应，我们在已经发展的土壤－植被－大气模式基础上，考虑积雪改变表水文和反照率的参数化，建立了包含雪盖问题的陆面过程模式。利用中国西北黑河地区的ＨＥＩＦＥ实测气象和辐射资料，检验了模式对大气降雨和降雪的响应，结果表明，该模式描写的各种物理过程合理，一些可观测要素（如地面温度和地表净辐射通量）的演变特征与实况相当一致。     

冬季高原积雪和欧亚积雪对我国夏季旱涝不同影响关系的环流特征分析

冬季高原积雪和欧亚积雪异常对我国夏季旱涝有一定的影响作用,但是它们与我国夏季降水的相关分布基本上是相反的.通过冬季积雪与北半球500hPa高度场的相关分析,从春季和夏季平均环流场对前期冬季高原积雪和欧亚积雪异常的不同响应,来探讨冬季高原积雪和欧亚积雪与我国夏季降水不同相关关系的原因,也为积雪因子在我国汛期旱涝预测中的应用提供一定的物理基础.     

A Modeling Study of the Effects of Anomalous Snow Cover over the Tibetan Plateau upon the South Asian Summer Monsoon

The effect of anomalous snow cover over the Tibetan Plateau upon the South Asian summer monsoon is investigated by numerical simulations using the NCAR regional climate model (RegCM2) into which gravity wave drag has been introduced. The simulations adopt relatively realistic snow mass forcings based on Scanning Multi-channel Microwave Radiometer (SNINIR) pentad snow depth data. The physical mechanism and spatial structure of the sensitivity of the South Asian early summer monsoon to snow cover anomaly over the Tibetan Plateau are revealed. The main results are summarized as follows. The heavier than normal snow cover over the Plateau can obviously reduce the shortwave radiation absorbed by surface through the albedo effect, which is compensated by weaker upward sensible heat flux associated with colder surface temperature, whereas the effects of snow melting and evaporation are relatively smaller.The anomalies of surface heat fluxes can last until June and become unobvious in July. The decrease of the Plateau surface temperature caused by heavier snow cover reaches its maximum value from late April to early May. The atmospheric cooling in the mid-upper troposphere over the Plateau and its surrounding areas is most obvious in May and can keep a fairly strong intensity in June. In contrast, there is warming to the south of the Plateau in the mid-lower troposphere from April to June with a maximum value in May.The heavier snow cover over the Plateau can reduce the intensity of the South Asian summer monsoon and rainfall to some extent, but this influence is only obvious in early summer and almost disappears in later stages.     

A Modeling Study of the Effects of Anomalous Snow Cover over the Tibetan Plateau upon the South Asian Summer Monsoon

The effect of anomalous snow cover over the Tibetan Plateau upon the South Asian summer monsoon is investigated by numerical simulations using the NCAR regional climate model (RegCM2) into which gravity wave drag has been introduced. The simulations adopt relatively realistic snow mass forcings based on Scanning Multi-channel Microwave Radiometer (SMMR) pentad snow depth data. The physical mechanism and spatial structure of the sensitivity of the South Asian early summer monsoon to snow cover anomaly over the Tibetan Plateau are revealed. The main results are summarized as follows. The heavier than normal snow cover over the Plateau can obviously reduce the shortwave radiation absorbed by surface through the albedo effect, which is compensated by weaker upward sensible heat flux associated with colder surface temperature, whereas the effects of snow melting and evaporation are relatively smaller.The anomalies of surface heat fluxes can last until June and become unobvions in July. The decrease of the Plateau surface temperature caused by heavier snow cover reaches its maximum value from late April to early May. The atmospheric cooling in the mid-upper troposphere over the Plateau and its surrounding areas is most obvious in May and can keep a fairly strong intensity in June. In contrast, there is warming to the south of the Plateau in the mid-lower troposphere from April to June with a maximum value in May.The heavier snow cover over the Plateau can reduce the intensity of the South Asian summer monsoon and rainfall to some extent, but this influence is only obvious in early summer and almost disappears in later stages.     

青藏高原冬季积雪影响我国夏季降水的模拟研究

利用区域气候模式 (NCC_RegCM1.0) 对青藏高原前冬积雪对次年夏季中国降水的影响进行了数值模拟研究, 所得结果与实际观测的积雪和降水的关系较为吻合, 即长江流域、 新疆地区夏季多雨, 华北和华南少雨, 这与我国最近二十年来维持的 “南涝北旱” 雨型较为一致。因此, 可以认为青藏高原冬季多雪, 是引起中国东部夏季降水出现 “南涝北旱” 的一个重要原因。本文揭示了青藏高原冬季积雪影响我国夏季降水的可能物理机制。青藏高原冬季多雪, 会导致青藏高原地面感热热源减弱, 这种热源的减弱在冬季导致冬季风偏强, 可以影响到我国华南、 西南及孟加拉湾地区。同时, 由于高原热源的减弱可持续到夏季, 成为东亚夏季风和南亚夏季风减弱的一个原因。在积雪初期, 地面反射通量的增加起了主要作用; 在积雪融化后, “湿土壤” 在延长高原积雪对天气气候的影响过程中起了重要作用。初期的反射通量增加减少了太阳辐射的吸收、 融雪时的融化吸热, 以及后期的湿土壤与大气的长期相互作用, 作为异常冷源, 减弱了春夏季高原热源, 是高原冬季积雪影响夏季风并进而影响我国夏季降水的主要机理。本文的模拟结果表明, 青藏高原冬季积雪的显著影响时效可以一直持续到6月份。     

2010年6月中国南方持续性强降水过程：天气系统演变和青藏高原热力作用的影响

2010年6月中国南方发生持续性强降水,其强度与2008年6月相当,超过近年来其他年份。但是,与2008年6月相比,2010年6月对流层中低层低值系统活动在青藏高原至长江中下游地区异常频繁,副热带高压(副高)位置异常偏西、强度偏强,导致低层异常风场辐合区及强降水区域相对偏北。分析2010年6月14—24日中国南方连续出现的4次持续性强降水过程,发现南亚高压、对流层中层的中纬度槽脊和西太平洋副高以及低层切变线和东移低涡是造成持续性强降水的主要天气系统。利用WRF模式对2010年6月强降水过程实施显式对流集合模拟试验,在控制试验重现观测到的地面降水和天气系统特征的基础上,在敏感性试验中将青藏高原的地表短波反照率修改为1.0,对比两组模拟试验的结果表明:控制试验中青藏高原的地表感热加热作用使得高原及其周边地区的大气温度发生变化,相应的热成风平衡调整使得对流层低层至高层大气环流和天气系统特征发生显著变化,增强了中国南方的持续性降水。200 hPa青藏高原西部形成反气旋性环流异常,东部形成气旋性环流异常,青藏高原东部南下的冷空气加强,中国南方辐散增强;500 hPa青藏高原北部的脊加强,中国东部的槽加深,副高西北侧的西南风明显增强,从青藏高原向下游传播的正涡度也显著加强;850 hPa的低涡强烈发展并逐步东移,华南沿海的西南低空急流更为强盛,导致降水区的水汽辐合、上升运动及降水强度都增强。     

Light-absorbing particles in snow and ice: Measurement and modeling of climatic and hydrological impact

Light absorbing particles(LAP, e.g., black carbon, brown carbon, and dust) influence water and energy budgets of the atmosphere and snowpack in multiple ways. In addition to their effects associated with atmospheric heating by absorption of solar radiation and interactions with clouds, LAP in snow on land and ice can reduce the surface reflectance(a.k.a., surface darkening), which is likely to accelerate the snow aging process and further reduces snow albedo and increases the speed of snowpack melt. LAP in snow and ice(LAPSI) has been identified as one of major forcings affecting climate change, e.g.in the fourth and fifth assessment reports of IPCC. However, the uncertainty level in quantifying this effect remains very high. In this review paper, we document various technical methods of measuring LAPSI and review the progress made in measuring the LAPSI in Arctic, Tibetan Plateau and other mid-latitude regions. We also report the progress in modeling the mass concentrations, albedo reduction, radiative forcing, and climatic and hydrological impact of LAPSI at global and regional scales. Finally we identify some research needs for reducing the uncertainties in the impact of LAPSI on global and regional climate and the hydrological cycle.     

春季欧亚积雪异常影响中国夏季降水的数值试验

 利用NCAR的新一代GCM CAM3.1版本模式,研究了欧亚大陆春季积雪异常对北半球大气环流和中国夏季降水的影响。结果表明,春季积雪异常通过改变其后夏季的土壤湿度和温度分布,造成对流层厚度场的异常,激发一个从欧洲西部到东亚的500 hPa高度场异常波列。我国南、北方处于符号相反的高度场异常区,同时降水也呈现南北相异的态势,这表明春季欧亚积雪异常是影响我国夏季降水分布的一个重要因子。     

NUMERICAL EXPERIMENTS ON THE EFFECT OF QINGHAIXIZANG PLATEAU SNOW COVER ON SUMMER MONSOON FORMATION

The calculating schemes of underlying surface processes in the model described by Li et al.(1989)are modified with inclusion of simple land surface processes and oceanic mixed layer processes,then a simulation on the zonal wind along 90°E from the Northern to the Southern Hemisphere with moun-tains is performed.Comparisons of the results and the observations show that the modified model not onlyhas an excellent stability in calculation but also can better display the seasonal change of the wind field,theability of the present model is improved as compared with that of the previous one.Based on the simulations,the authors investigate the effects of Qinghai-Xizang Plateau snow cover on theformation of South Asian monsoon by thickcning the snow depth and by increasing the snow albedo.Themain results arc as follows:The summer meridional circulation over the south of the Plateau and its vicinityis weakeued,and the precipitation reduced.However,over the northern tropics,the circulation is enhanced,and the ecipitation is increased,and the land and the air above it become warmer,the tropical easterly jetis weakened.     

The Persistent Heavy Rainfall over Southern China in June 2010：Evolution of Synoptic Systems and the Effects of the Tibetan Plateau Heating

This study investigates influencing weather systems for and the effect of Tibetan Plateau （TP）’s surface heating on the heavy rainfall over southern China in June 2010, focusing on the four persistent heavy rainfall events during 14-24 June 2010. The ma jor weather systems include the South Asian high, midlatitude trough and ridge, western Pacific subtropical high in the middle troposphere, and shear lines and eastward-moving vortices in the lower troposphere. An ensemble of convection-permitting simulations （CTL） is carried out with the WRF model for these rainfall events, which successfully reproduce the observed evolution of precipitation and weather systems. Another ensemble of simulations （SEN） with the surface albedo over the TP and its southern slope changed artificially to one, i.e., the surface does not absorb any solar heating, otherwise it is identical to CTL, is also performed. Comparison between CTL and SEN suggests that the surface sensible heating of TP in CTL significantly affects the temperature distributions over the plateau and its surroundings, and the thermal wind adjustment consequently changes atmospheric circulations and properties of the synoptic systems, leading to intensified precipitation over southern China. Specifically, at 200 hPa, anticyclonic and cyclonic anomalies form over the western and eastern plateau, respectively, which enhances the southward cold air intrusion along the eastern TP and the divergence over southern China;at 500 hPa, the ridge over the northern plateau and the trough over eastern China are strengthened, the southwesterly flows along the northwestern side of the subtropical high are intensified, and the positive vorticity propagation from the plateau to its downstream is also enhanced significantly;at 850 hPa, the low-pressure vortices strongly develop and move eastward while the southwesterly low-level jet over southern China strengthens in CTL, leading to increased water vapor convergence and upward motion over the precipitation region.     

Assessing the Impacts of Initial Snow Conditions over the Tibetan Plateau on China Precipitation Prediction Using a Global Climate Model

Two ensemble experiments were conducted using a general atmospheric circulation model. These experiments were used to investigate the impacts of initial snow anomalies over the Tibetan Plateau（TP） on China precipitation prediction. In one of the experiments, the initial snow conditions over the TP were climatological values; while in the other experiment, the initial snow anomalies were snow depth estimates derived from the passive microwave remote-sensing data. In the current study, the difference between these two experiments was assessed to evaluate the impact of initial snow anomalies over the TP on simulated precipitation. The results indicated that the model simulation for precipitation over eastern China had certain improvements while applying a more realistic initial snow anomaly, especially for spring precipitation over Northeast China and North China and for summer precipitation over North China and Southeast China. The results suggest that seasonal prediction could be enhanced by using more realistic initial snow conditions over TP, and microwave remote-sensing snow data could be used to initialize climate models and improve the simulation of eastern China precipitation during spring and summer. Further analyses showed that higher snow anomalies over TP cooled the surface, resulting in lower near- surface air temperature over the TP in spring and summer. The surface cooling over TP weakened the Asian summer monsoon and brought more precipitation in South China in spring and more precipitation to Southeast China during summer.