CHARACTERISTICS OF VARIATION IN SURFACE ALBEDO AND SNOW FORCING OVER THE TIBETAN PLATEAU

The combination of field experiments and satellite observations is the fundamental way tounderstand the characteristics of spatial-temporal variation in surface albedo over the Tibetan(Qinghai-Xizang) Plateau. Under the condition without snow cover, the relatively regular annualvariation cycle of the surface albedo can be expressed by an empirical formula. The effect of snowcover on the surface albedo in winter can be expressed by introducing two variables of snow forcingand sensitivity parameter. The existing satellite retrieved results of surface albedo may provide thedigital grid data for describing the geographical distribution. However, some satellite retrievedsurface albedos available over the Tibetan Plateau are obviously too low in winter. Taking thesatellite derived results in summer as the background field representative of geographicaldistribution and combining the empirical formula of annual cycle based on the surface observations,a dynamic model of surface albedo is developed for the need of modeling the climatic influence ofthe underlying surface forcing of the Tibetan Plateau.     

东亚地区云和地表反照率对硫酸盐直接辐射强迫的影响

文中利用区域气候模式深入探讨了东亚地区云及地表反照率对硫酸盐直接辐射强迫的影响 ,同时定量估算了东亚地区云区气溶胶的直接辐射强迫 ,讨论了硫酸盐对地表和大气短波辐射平衡产生的不同影响。研究表明 :云对气溶胶的直接辐射强迫具有很强的减弱作用 ,这种减弱作用不仅取决于云覆盖份数 ,而且取决于云的光学厚度。就区域平均而言 ,文中模拟的东亚地区气溶胶直接辐射强迫为 - 0 .0 97W /m2 ,占总直接辐射强迫的 10 .4%左右。表明云对硫酸盐直接辐射具有很强的减弱作用 ,在估算其总的直接辐射强迫时 ,云区的贡献不可忽视。较高的地表反照率会减弱硫酸盐的直接辐射强迫 ,而较低地表反照率则会增加硫酸盐的辐射强迫。硫酸盐气溶胶对大气辐射平衡影响非常小 ,但对地表辐射平衡产生重要影响 ,影响程度与大气几乎一致。     

中国短期气候变化的一个重要原因

通过数值试验对高原地表反射率变化的气候效应进行了敏感性研究，同时与观测的近４０年中国区域气候变化趋势作了对比分析。结果表明，高原主体地表反射率增加是我国短期气候变化的重要控制因子之一，它能造成东亚夏季风和高原夏季风的显著减弱，使夏季我国东部季风区北方变暖，南方变冷，季风降水普遍减少     

NUMERICAL EXPERIMENTS ON THE INFLUENCES OF GENERAL CIRCULATION ANOMALY OVER THE TIBETAN PLATEAU AND SURFACE ALBEDO CHANGE IN NORTHWEST CHINA ON SUMMER PRECIPITATION

An advanced three-level global atmospheric general circulation model has been used to studythe summer precipitation anomaly in Northwest China.based on the synoptic fact and thestatistical analysis of the precipitation,the surface albedo in Northwest China,and the synopticsystems over the Tibetan(Qinghai-Xizang)Plateau.The results show that either the anticycloneintensified over the plateau or the surface alhedo enhanced in Northwest China results in summerprecipitation reduction east of Northwest China.Especially.when both of them appearsimultaneously,summer precipitation was obviously reduced and severe drought occurred in mostareas of Northwest China.Moreover.the simulated difference of precipitation rate of NorthwestChina is similar to the actural precipitation distribution in Northwest China in 1995,which is themost severe drought year in Northwest China in the past fifty years.So the tendency in droughtseverity intensified,drought frequency accelerated,drought persistence period extended,anddrought areas expanded in Northwest China in recent years is maybe a result of the influences ofhuman activities(e.g.vegetation was reduced,and desertification worsened)on droughtcirculation pattens over the Tibetan Plateau.     

青藏高原地表反照率计算研究

根据改进的甚高分辨率扫描辐射仪（ＡＶＨＲＲ）５个观测波段的光谱特征，经多次试验，设计了一组从卫星观测的地－气系统的辐射测值中提取晴空资料的多通道门槛值判识法和提取月平均反照率的合成法；并对１９９２年ＮＯＡＡ－１２卫星获取的ＡＶＨＲＲ资料进行计算处理，分析提取晴空数据，在此基础上按卫星轨道覆盖周期合成计算得到的晴空行星反照率和地表反照率，并且计算了逐月的地表反照率。对计算结果做了初步分析和认真比较。     

青藏高原东部一次大雪过程的Q矢量分析

应用Q矢量理论对1996年1月16－18日青藏高原东部大雪过程进行了诊断分析，结果表明：在降雪前14h，Q矢量散度出现了整层辐合，锋生函数正值区南部等值线密集处与大雪区有较好的对应关系。假相当位温（θse)的24h变量正值中心与强降雪中心相接近。     

青藏高原大气热源和冬春积雪与中国东部降水的年代际变化关系

利用NCEP 1950—2004年逐日再分析资料,采用倒算法,对青藏高原大气热源的长期变化进行了计算,结果发现,青藏高原及附近地区上空大气春夏季热源在过去50年里,尤其是最近20年,表现为持续减弱的趋势。而1960—2004年青藏高原50站的冬春雪深却出现了增加,尤其是春季雪深在1977年出现了由少到多的突变。用SVD方法对高原积雪和高原大气热源关系的分析表明,二者存在非常显著的反相关关系,即高原冬春积雪偏多,高原大气春夏季热源偏弱。高原大气春夏季热源和中国160站降水的SVD分析表明,高原大气春夏季热源和夏季长江中下游降水呈反相关,与华南和华北降水呈正相关;而高原冬春积雪和中国160站降水的SVD分析显示,高原冬春积雪和夏季长江流域降水呈显著正相关,与华南和华北降水呈反相关。在年代际尺度上,青藏高原大气热源和冬春积雪与中国东部降水型的年代际变化(南涝北旱)有很好的相关。最后讨论了青藏高原大气热源影响中国东部降水的机制。青藏高原春夏季热源减弱,使得海陆热力差异减小,致使东亚夏季风强度减弱,输送到华北的水汽减少,而到达长江流域的水汽却增加;同时,高原热源减弱,使得副热带高压偏西,夏季雨带在长江流域维持更长时间。导致近20年来长江流域降水偏多,华北偏少,形成"南涝北旱"雨型。高原冬春积雪的增加,降低了地表温度,减弱了地面热源,并进而使得青藏高原及附近地区大气热源减弱。     

青藏高原东部积雪日数特征的分析

本文利用青藏高原曲麻来、托托河和玉树三站的32年逐日积雪实测资料,进行了统计分析,得出了它们的时间变化特征,并对Elnino现象的发生作了简单的联系。     

IMPACT OF UPLIFT OF TIBETAN PLATEAU AND CHANGE OF LAND-OCEAN DISTRIBUTION ON CLIMATE OVER ASIA*

Using an improved CCM1/NCAR climate dynamic model and a combination distribution of land-ocean-vegetation during 40-50 MaBP,a series of numerical experiments representing different stages of the Tibetan Plateau uplifting and different land-ocean distributions are designed to discuss the influence of the Plateau uplifting and land-ocean distribution variation on Asian climate change.It is shown that Tibetan Plateau uplifting can firstly increase the precipitation in China during the period from initial uplift to half height of modern Tibetan Plateau and then decrease the rainfall during the time from the half height to the present plateau. At the same time.the uplifting can reduce surface air temperature over China.Besides.the effects of the uplift and land-ocean distribution change on the variation of winter and summer Asian monsoon circulation are also discussed.     

青藏高原积雪的分布特征及其对地面反照率的影响

通过对１９８３年７月至１９９０年６月青藏高原主体５８个格点积雪资料进行ＥＯＦ分析发现，青藏高原主体积雪分布以西部兴都库什山脉。天山山脉以及南部喜马拉雅山脉为主；高原中部唐古拉山脉、北部昆仑山脉和东部巴颜喀拉山脉的积雪相对较少，青藏高原西部、南部的积雪变化与中部、北部和东部的积雪变化趋势存在反位相关系。另外，本文还对积雪对高原地面反照率的影响作了简单分析。     

HEIFE绿洲区太阳总辐射和地表反射率的分光谱特征

太阳辐射的分光测量在许多研究和应用领域中都具有十分重要的意义．本文利用HEIFE临泽子站进行的太阳总辐射和反射辐射的分光观测资料，研究了黑河绿洲区太阳总辐射和地表反射率的光谱特征．研究结果表明：在太阳总辐射的光谱能量分布中，紫外和可见光部分的比倒偏小，近红外部分则偏太．在其从冬到夏的季节变化中紫外和可见光部分是由小到大，近红外部分剐是由大到小。在其日变化中0．5μm以下波段是中午大早晚小；0．7μm以上波段是中午小，早晚大；0．5-0．7μm波段的日变化不明显。紫外波段的相对日变化幅度最大．地表反射率的光谱特性有明显的季节变化，变化特征与地表植被状况有关．各波段地表反射率的日变化型有一定的季节差异，其中波长小于0．7μm部分较为明显，并与地表植被状况有关．在裸露地表各波段反射率与太阳高度角的关系中，近红外波段随太阳高度角增加而增大，紫外和可见光波段则随太阳高度角增大而减小．全波段反射率随太阳高度角增大而增大主要是近红外波段的贡献．这是裸露地表光谱反射率的重要特征之一。     

CHARACTERISTICS OF VEGETATION COVER, ROUGHNESS AND ALBEDO DISTRIBUTION OVER CHINA*

To build land surface dataset for climate model,with application of remote sensing technique as well as the Geographic Information System(GIS),the data of surface type,roughness and albedo over China in 1997 were retrieved,resolutions being 10 km×10 km.Based on these data,an analysis is conducted on the geographic distributions and seasonal variations of surface vegetation cover and roughness as well as albedo over China.Results show that surface vegetation cover is mainly located to the south of Yangtze River,in Southwest and Northeast China andsparse vegetation cover is in the Northwest.The variation of land surface cover affects the variations of land surface roughness and albedo.High albedo occurred in the north of Xinjiang Autonomous Region,the north of Northeast China and the Qinghai-Xizang Plateau in winter,in correspondence with the location of snow cover.For most part of China,surface roughness decreases and albedo increases in winter,while the roughness increases and the albedo decreases in summer,which could mainly result from land surface cover(snow cover and vegetation cover)and soil moisture changes.This shows that the geographic distribution and seasonal variation of the albedo are almost opposite to those of the roughness,in agreement with theoretical results.Temporally,the amplitude of surface roughness change is quite small in comparison with the roughness itself.     

A PRELIMINARY STUDY OF SNOW MASS VARIATIONS IN CHINA OVER THE PAST 30 YEARS*

Daily snow data for 2300 climate stations covering the period from 1951 through 1980 have been used to monitor and diagnose secular variations,year-to-year fluctuations,and the spatial characteristics of snow variation trends in China.An examination of time series reveals that there is a strong teleconnction to ENSO,to major volcanic eruptions,as well as to the CO₂-induced warming.The country-wide snow mass variations are positively correlated with global mean temperature,increasing during the current warming period and decreasing during the recent cooling period prior to the mid 1960s.A synchronous relationship exists between El Nino/Southern Oscillation and snowy winter in China.The year-to-year snow fluctuations seem to be generally out of phase with volcanic activity.The anomaly map shows that snow mass increased in high altitudes and moist regions,while it decreased in arid lowland and the southern boundary zone during the warming period.The potential CO₂-induced changes in snow mass will further aggravate the regional differentiation between high mountains and lowlands,between moist and arid regions.The number of snow cover days will decrease in the northern lowlands,and snowfall will increase in the Qinghai-Xizang Plateau,high mountains,and the lower reaches of the Changjiang(Yangtze) River.     

青藏高原地面加热场日变化对亚洲季风区大气环流的影响

利用1982-1996年每天两次的NCEP丙分析资料，研究青藏高原地面加热场的日变化对亚洲季风区环流的影响。结果表明；青藏高原地面加热场的日变化是引起亚洲季风区大气环流日变化的主要因子，青藏高原地区，阿拉伯海，盂加拉湾和菲律宾附近地区是四个主要的日变化显著区，青藏高原地区是垂直运动的负值日变化中心，其它三个区域的日变化与青藏高原地区的日变化有反相关系，这种特征一年四季都存在，但各显著区域范围的大小，中心位置及环流日变化的强度随季节有不同程度的变化，青藏高原加热场日变化对我国东部地区环流的影响主要发生在夏季。     

MAIN HEATING MODES OVER THE TIBETAN PLATEAU IN JULY AND THE CORRELATION PATTERNS OF CIRCULATION AND PRECIPITATION OVER EAST ASIA*

Based on the 1958-1999 monthly averaged NCEP/NCAR reanalysis data,the REOF analysis is applied to obtain the main spatial modes of normalized atmospheric heating source over the Tibetan Plateau(TP) in July.Results show that the four leading modes are located over the northeast TP,southwest TP.Kashmir and southeast TP respectively,and the cumulative variances are no more than one third of the total.It indicates that the heating source distribution is very complicated over the TP in July.In other words.it is difficult to depict the heating spatial distribution with a few modes.By using wavelet analysis,a 2-4-year variation period is identified in these modes.Moreover,correlation coefficients between each RPC and zonal wind U, meridional wind V.zonal moisture flux Q_v,meridional moisture flux Q_v,and precipitation rate over East Asia are calculated to construct correlation fields,Results show that different heating modes over the TP correspond to different circulation,moisture flux as well as precipitation patterns,Precipitation over North China(or Kashmir) is negatively(or positively) correlated with REOF1.Similarly.notable negative(or positive) correlation can be found between the rainfall over south part of Southwest China.South China,and the Philippines(or Japan) and the REOF3. Due to high localization of diabatic heating over the TP.it is not enough to study the influence of TP thermal forcing on the climate with an area averaged heating index.     

CHARACTERISTICS OF SEASONAL VARIATION OF RAINFALL OVER THE TIBETAN PLATEAU DURING SUMMER 1998 AND ITS IMPACT ON EAST ASIAN WEATHER*

The seasonal variation of rainy season over the Tibetan Plateau in summer 1998 is analyzed by using daily observational rainfall data for Lhasa from 1955 to 1996,and rainfall data at 70 stations from January to August of 1998 over the Tibetan Plateau (TP) and adjacent regions,as well as TBB data from May to August of 1998.The onset date of rainy season for Lhasa is climatologically 6 June.Among the analyzed years,the earliest onset date is 6 May,while the latest may delay to 2 July.The obvious inter-decadal variation can be found in the series of onset date.The onset date of summer 1998 over middle TP (onset date of Lhasa) is 24 June,which is relatively later than the normal case.The onset for rainy season of 1998 started over southeast and northeast parts of TP and then propagated westward and northward.The convection over east and west parts of TP shows that there is a quasi 12-15 day oscillation.In June,the convection over middle and lower reaches of Yangtze River is formed by the westward propagation of convection over subtropical western Pacific.while in July.it is formed by the eastward propagation of convection over TP.Besides,it is also found that there exists good negative and obvious advance and lag correlation between the convection over the middle and western TP and that over the subtropical western Pacific and southern China.Therefore it can be inferred that a feedback zonal circulation with a quasi two-three week oscillation exists between the ascending region of TP and descending region of subtropical western Pacific,i.e.the convection over TP may affect the subtropical high over western Pacific and vice versa.     

青藏高原云对地气系统长波射出辐射(OLR)强迫的气候研究

利用地球辐射平衡试验（ＥＲＢＥ）和国际卫星云气修计划（ＩＳＣＣＰ）提供的地气系统长波射出辐射（ＯＬＲ）和云量资料,计算并讨论了青藏高原地气系统各季和年平均总云量对ＯＬＲ的强迫及其所产生的温室效应,揭示了高、低示了高、低云对ＯＬＲ强迫的特点。结果表明：高原的ＯＬＲ云强迫与总云量、高云量都有较好的相关关系,且季节变化明显;ＯＬＲ云强迫和云温室效应的地理分布与高原总云量的分布较为一致;云强迫的年变化一同     

青藏高原地气系统云辐射强迫的气候学特征

利用ＥＲＢＥ－Ｓ４和ＩＳＣＣＰ－Ｃ２月平均资料着重分析了青藏高原这一特殊气候区域地气系统云辐射强拓的气候学特征，分析结果表明，冬，夏季云对气系统辐射强迫的场分布形势有明显的差异，对于地气系统长波辐射，冬季高原主体云强迫高值区，夏季云强迫空间变化平缓，高原主体平均云的温室效应春季最大，秋季最小，云使地气系统射出长波辐射年平均减少４５．６Ｗ／ｍ＾２对于地气系统短波辐射，冬季高原地区云强迫相对高值区，夏