CHARACTERISTICS OF ATMOSPHERIC HEATING AND ATMOSPHERIC CIRCULATION DURING ACTIVE PERIOD OF 500 hPa HIGH OVER THE TIBETAN PLATEAU IN SUMMER

In correspondence with the establishment of the“upper high and lower high”pressure pattern due to the activitiesof 500 hPa high over the Tibetan Plateau in summer,a series of changes of the East Asia atmospheric circulation willtake place.In this paper,the distributions of divergence and vertical velocity of 500 hPa high,the evolutions of atmos-pheric heat source,the variations of vorticity and zonal wind at 100 hPa level and vertical meridional cell over theTibetan Plateau etc.are statistically analyzed.Thus,we can see that the ascending motion and the convective heatingover the Tibetan Plateau,the South Asia high and the westerly jet on the north of the Plateau at 100 hPa level are weak-ened.The northern branch and the southern branch of the easterly jet on the south of the Plateau merge into a singlewhole and situate on the south of the former northern branch.In the meantime,thermodynamic land-sea discrepancy inSouth Asia and the convective heating over the Bay of Bengal is enhanced.It will play an important role in the mainte-nance of the easterly jet and the South Asia monsoon.     

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.     

夏季青藏高原500hPa高压活动期间大气加热状况与大气环流特征

由于青藏高原500hPa层出现高压系统的活动,使高原大气产生“上高下高”的气压场结构,从而东亚大气环流也发生某些相应的变化。本文统计分析了高原500hPa高压的散度与垂直速度分布、高原大气热源的演变和100hPa层涡度、纬向风以及经圈环流的变化等。结果认为,由于夏季高原500hPa高压的活动,使高原上空垂直上升运动和对流加热受到抑制,100hPa南亚高压强度减弱、位置北抬、有向西部型过渡的特征。高原北侧西风急流减弱,东风急流南支与北支合并后位于原北文东风急流位置以南,侵入高原南麓的西南季风减弱。与此同时,孟加拉湾上空上升运动有所增强,其对流加热对维持东风急流乃至南亚季风将起重要作用。     

青藏高原500hPa高压的统计特征

本文普查1978－1987年共10年的天气图，找出了青藏高原500hPa高压活动的个例336次。从而分析了它们的季节分布、维持时间、高原上高压活动的高频区以及高压移动路径等。同时选出高压个例18个，对其结构和天气影响也作了进一步的研究。     

华东高温期的大气环流特征分析

利用NCEP/NCAR再分析资料和地面气象站点资料,对1951—2005年华东高温日数较多（少）月和高温过程各阶段的500 hPa位势高度、850 hPa温度和相对湿度及700 hPa垂直速度的环流特征进行对比分析。结果表明：在高温日数较多月,副高西伸,850 hPa温度较高而相对湿度较低,华东中南部下沉气流明显,华东多为位势高度、温度和垂直速度正距平及相对湿度负距平;而在高温日数较少月,副高东退,850 hPa温度较低而相对湿度较高,整个华东为上升气流,并为位势高度、温度和垂直速度负距平及相对湿度正距平。与高温前期和衰退期相比,高温盛期副高北抬西进,华东850 hPa温度较高而相对湿度较低,华东南部下沉气流强盛,各要素距平值增加,多数要素距平中心移向华东或其周边。高温间断期则比盛期各要素距平减弱。大气环流要素场及其距平场的变动可以作为预报华东7—8月高温日数多寡和高温进程的参考依据。     

THE IMPACT OF MID-LATITUDE CIRCULATION OVER THE TIBETAN PLATEAU ON THE TRACK OF TYPHOON OVER THE NORTHWEST PACIFIC*

There is a close relationship between variation of middle latitude synoptic system over the Tibetan Plateau and movement of typhoon over the Northwest Pacific.This paper tries to discuss the mechanism of it through the numerical simulation and dynamic diagnosis.25-year statistical results during the period from 1970 to 1995 indicate that the relationship between middle latitude circulation over the Plateau and the track of typhoon over the Northwest Pacific are as follows.When longwave trough in 500 hPa isobaric surface over the Plateau.it is favourable to typhoon's westward movement:on the contrary,large scale anticyclone over the Plateau is favourable to typhoon's recurvature.We simulated the typhoon under these two flow patterns with MM5 model.Numerical simulation results showed that:(a)Longwave trough on 500 hPa surface over the Plateau makes subtropical high extended westward so as to typhoon south to high moves westward in easterly steering flow.(b)Anticyclone over the Plateau will bring about the longwave trough developing along the east coast of Asia,it'will usually cause typhoon recurring in front of the developed trough.In addition,the results of dynamic diagnosis show the physical mechanism on impact of synoptic system over the Plateau on the downstream synoptic system,that is,the transport of disturbance kinetic energy over the upstream Plateau makes the downstream trough developed.And then it affects the steering flow of typhoon.Because of existence of the high ridge over the Plateau.the energy transport channel moves to north.On the contrary,the trough exists over the Plateau,the energy transport channel moves to south.The southerly in front of trough over the Plateau and the southerly in the east part of typhoon transport low potential vorticity of low latitude into subtropical high.That is beneficial to development of subtropical high and affects typhoon's movement.These results fully reflect the impact of interaction between mid-latitude and low latitude on synoptic system.     

7月青藏高原大气热源空间型及其与东亚大气环流和降水的相关研究

对1958～1999年的7月份NCEP/NCAR再分析资料中青藏高原区域大气热源强度(整层气柱的总非绝热加热率)做旋转经验正交函数分析,结果表明该区域内大气热源强度的空间分布特征复杂,各地差异显著。前4个REOF型的加热中心位于高原东北部、高原西南部、克什米尔地区以及高原东南部地区上空。小波分析还表明各空间型都有2～4a的变化周期。文中计算了前4个RPC与东亚中、低空纬向风(U)、经向风(V)、纬向水汽通量(Q_u)、经向水汽通量(Q_v)的相关系数,并用这些相关系数构造矢量,进而分析其流场和水汽通量散度场,发现高原不同区域的大气加热异常所对应的东亚大气环流形势及降水也大不相同,由此表明,在研究高原加热对中国气候的影响时,应注意加热的空间分布特征。     

夏季青藏高原加热和北半球环流年际变化的相关分析

利用 195 8～ 1997年NCEP/NCAR再分析数据集中加热率和环流资料 ,采用相关分析和对比分析相结合的方法 ,诊断和分析了夏季青藏高原的非绝热加热与北半球环流系统的年际变化的联系。分析结果表明 :夏季青藏高原的加热强 (弱 )的年份 ,高原及邻近地区的上升运动、下层辐合及上层辐散均增强 (减弱 ) ,使高原加热对周边地区低层暖湿空气的抽吸效应和对高层大气向周边地区的排放作用加强 (减弱 )。从而影响着高原和周边地区的环流以及亚洲季风区大尺度环流系统。而且高原的加热强迫能够激发产生一支沿亚欧大陆东部海岸向东北方向传播的Rossby波列 ,其频散效应可影响到更远的东太平洋以至北美地区的大气环流     

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。     

NUMERICAL SIMULATIONS OF HEATING ANOMALY EFFECTS OF TIBETAN PLATEAU ON CIRCULATION IN SUMMER

A 5-layer numerical model with p-σ incorporated coordinate system and primitive equations is used to simulatethe effects of heating anomaly at and over the Tibetan(Qinghai-Xizang)Plateau on the circulations in East Asia in sum-mer,The model is described briefly in the text and the results are analysed in somewhat detail.Results show that the sur-face albedo,the drag coefficient,the evaporation rate and the ground temperature all have large influences on the circula-tion near the Plateau and in East Asia.When the heating at the surface increases,the Tibetan high in the uppertroposphere intensifies,too.Its area enlarges and its axis tilts to northwest.The upper tropical easterly increase andshifts to north.The southwesterly in the lower troposphere,in consistence,also increases.The cross-equatorial low-lev-el currents along Somali and South India are influenced to increase their speeds while those over North Australia de-crease.The land low over the Asian Continent deepens.Meanwhile the upward motions over the land of east China andover the Indo-China Peninsula intensify and therefore the precipitation over those areas increases.However,along thecoastal area of China the upward motions and therefore the precipitation decrease.Atmospheric heat source anomaly has large influence on the circulation,too.Simulated results indicate that heatsource anomaly in the lower atmosphere over the Plateau influences the intensity and the position of the monsoon circu-lation while that in the upper atmosphere only affects the intensity.The heating status over the Plateau has slight influ-ence on the westerly jet,north of the Plateau,while it has strong effect on the subtropical jet at the mid and low latitudes.     

ABRUPT SEASONAL CHANGE OF ZONAL CIRCULATION OVER THE TIBETAN PLATEAU

The abrupt changes of zonal circulation in the Tibetan Plateau (TP) region and their likely causes are derived from National Centers for Environmental Prediction and the National Center for Atmospheric Research reanalysis data. The zonal circulation over the TP abruptly changed in summer (31st pentad) and winter (59th pentad). The switch from summer to winter circulation is characterized by a sudden northward shift of the westerlies and the zero-velocity curve and disappearance of the westerly jet. The winter–summer switch is characterized by the reverse pattern. Therefore, the circulation conversion between summer and winter can be judged from the position of the zero-velocity curve. Curves located north of 20 °N indicate summer circulation over the TP and vice versa. The abrupt change of zonal circulation is mainly caused by the thermodynamic effect of the TP. In June, this effect causes a huge monsoon circulation cell extending from the TP to low latitudes. Consequently, the westerlies jump to the north as easterlies develop. This process, which is enhanced by the strong northerly in Coriolis, establishes the summer circulation. In October, the Hadley cell recurs as the thermal effects of the TP diminish, the westerlies rush southward, and the winter circulation is established.     

SENSITIVITY EXPERIMENTS ON INFLUENCES OF UPLIFT OF QINGHAI-XIZANG PLATEAU ON CIRCULATION IN SUMMER*

A coupled general circulation model in a zonal belt is used to simulate the variation of circulation features in the process of uplift of the Qinghai-Xizang Plateau.The results reveal that the heating rates of the Plateau increase with the rising of the Plateau topography,and the latent heating component in the heating field tends to be the most important heating factor.The uplift of the Plateau enhances the upward motion,intensifies the pressure systems in the high and low level atmosphere,reinforces Southeast Asia monsoon strength,increases precipitation and severely decreases the surface temperature over the Qinghai-Xizang Plateau.However.the basic structures of the general circulation do not vary much due to the uplift of the Qinghai-Xizang Plateau.and it is the land-sea distribution that is the decisive factor to form the present circulation pattern and monsoon.Therefore,to simulate the paleoclimate during the geological period people should consider more factors,especially the land-sea distribution.     

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.     

THE IMPACT OF MID-LATITUDE CIRCULATION OVER THE TIBETAN PLATEAU ON THE TRACK OF TYPHOON OVER THE NORTHWEST PACIFIC

There is a close relationship between variation of middle latitude synoptic system over theTibetan Plateau and movement of typhoon over the Northwest Pacific.This paper tries to discussthe mechanism of it through the numerical simulation and dynamic diagnosis.25-year statistical results during the period from 1970 to 1995 indicate that the relationshipbetween middle latitude circulation over the Plateau and the track of typhoon over the NorthwestPacific are as follows.When longwave trough in 500 hPa isobaric surface over the Plateau.it isfavourable to typhoon's westward movement:on the contrary,large scale anticyclone over thePlateau is favourable to typhoon's recurvature.We simulated the typhoon under these two flowpatterns with MM5 model.Numerical simulation results showed that:(a)Longwave trough on500 hPa surface over the Plateau makes subtropical high extended westward so as to typhoon southto high moves westward in easterly steering flow.(b)Anticyclone over the Plateau will bringabout the longwave trough developing along the east coast of Asia,it'will usually cause typhoonrecurring in front of the developed trough.In addition,the results of dynamic diagnosis show the physical mechanism on impact ofsynoptic system over the Plateau on the downstream synoptic system,that is,the transport ofdisturbance kinetic energy over the upstream Plateau makes the downstream trough developed.And then it affects the steering flow of typhoon.Because of existence of the high ridge over thePlateau.the energy transport channel moves to north.On the contrary,the trough exists over thePlateau,the energy transport channel moves to south.The southerly in front of trough over thePlateau and the southerly in the east part of typhoon transport low potential vorticity of lowlatitude into subtropical high.That is beneficial to development of subtropical high and affectstyphoon's movement.These results fully reflect the impact of interaction between mid-latitude andlow latitude on synoptic system.     

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.     

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 to understand the characteristics of spatial-temporal variation in surface albedo over the Tibetan(Qinghai-Xizang) Plateau. Under the condition without snow cover, the relatively regular annual variation cycle of the surface albedo can be expressed by an empirical formula. The effect of snow cover on the surface albedo in winter can be expressed by introducing two variables of snow forcing and sensitivity parameter. The existing satellite retrieved results of surface albedo may provide the digital grid data for describing the geographical distribution. However, some satellite retrieved surface albedos available over the Tibetan Plateau are obviously too low in winter. Taking the satellite derived results in summer as the background field representative of geographical distribution 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 of the underlying surface forcing of the Tibetan Plateau.     

青藏高原东南部夏季深对流加热研究

文章利用1948～1999年NCEP／NCAR全球深对流加热率再分析资料和1951～2001年全国160站月平均降水量研究了中国区域夏季深对流加热分布状况，青藏高原东南部夏季深对流加热较强，是潜热释放较多和深对流活动旺盛区域；青藏高原东南部深对流加热率与长江中下游地区降水的相关分析也表明，夏季青藏高原东南部深对流活动对长江中下游地区夏季降水偏多有重要影响，可以为长江中下游地区夏季降水提供水分和能量。     

VARIATIONS OF ATMOSPHERIC OSCILLATION FACTORS AND 500 HPA CIRCULATION PATTERNS AND THEIR RELATIONS*

Wavelet analysis,EOF and CCA are used to analyze the variations of atmospheric circulation factors,including the key atmospheric oscillations,W,C,E and subtropical high (SH),and their relationships.The results are shown that the variations of atmospheric circulation factors both in troposphere and lower atmosphere layer are different on different time scales.The most significant variation of atmospheric circulation factors is in the period less than 7 years.The correlation relations of atmospheric circulation factors are different on different time scales.Combination patterns of circulation factors have impact on the relationships of factors.There are lag correlations between combination patterns of circulation factors in former year and that of in next year.     

APPLICATION OF THE VARIATIONAL METHOD OF TOVS DATA OVER THE TIBETAN PLATEAU IN IMPROVEMENT OF THE INITIAL FIELD OF NUMERICAL MODELS*

In this paper,TOVS satellite data are used through variational method on the data-sparse plateau area.Diagnoses are carried out to find a way to solve the large error problem of model initial field.It is put forward that TOVS retrieval data can be used to improve the initial field of numerical prediction model on Tibetan Plateau area.Through variational method,TOVS data are processed and the liability of the initial information on the plateau is improved.Diagnostic results confirm further that the application of TOVS retrieval data can improve our capability to describe the dynamic system features on the plateau and the objectivity of related initial information such as the distribution of water vapor channel and stratification stability.     

1990年夏季青藏高原加热场与100hPa青藏高压

对青藏高原加热场与青藏高压发展和维持的影响程度进行了一些阐述，着重分析青藏高原的热量变化以及热量传递的主要方式，并对青藏高原加热场与青藏高压位置对应等方面进行了一些探讨