三次高原切变线过程演变特征及其对降水的影响

Analysis on Evolution Characteristics of Three Plateau Shearlines and Their Effect on Precipitation

为了揭示高原切变线的动、热力等特征,进一步认识高原切变线线演变机制,应用MICAPS资料、NCEP 1°×1°再分析资料和风云卫星红外亮温资料,选取出现在初夏(2008年5月19 22日)、盛夏(2007年7月1 3日)和夏末(2009年9月19 21日)的三次高原切变线个例,对夏季高原切变线不同时期、不同发展阶段的演变特征及其对降水影响进行了分析。结果表明:(1)当切变线两侧南北风速减弱,特别是北风风速减弱时,切变线过程趋于减弱。冷暖空气势力强弱影响切变线所处位置,初夏和盛夏切变线位置偏北,夏末切变线位置偏南。(2)切变线活动期间有正涡度、辐合上升运动与切变线配合。当切变线减弱消失,辐合带先于正涡度带减弱消失。切变线附近多正涡度中心和辐合中心,可能与低涡活动有关。盛夏和夏末切变线正涡度辐合中心东移特征明显,辐合上升区更为偏东且较强。(3)切变线附近通常有TBB<-20℃的带状或块状区域,切变线维持发展阶段,TBB进一步降低,盛夏切变线和形成初期的夏末切变线多TBB低值中心,对流活动比较旺盛。(4)由于地形的阻挡和加热,高原东坡和南坡是大气不稳定能量聚集地。盛夏在切变线附近近地层聚集的高温、高湿能量明显。初夏切变线引发的降水以稳定性降水为主,降水量小,呈零散分布,盛夏和夏末切变线引发降水其对流不稳定降水特征明显,带来的降水更强、范围更广,呈带状分布在切变线附近。(5)500 hPa切变线也是水汽聚集带,切变线附近上空的水汽和不稳定能量聚集,正涡度东传和对流发展是切变线引发强降水的重要机制。

In order to reveal the characteristics of the dynamics,thermal dynamics and the water vapor,cloud system and precipitation characteristics for plateau shear lines and to further understand the evolution mechanism of shear line,the evolution characteristics and effects on precipitation of the plateau shear line in different times and at different development stages have been analyzed by selecting three cases of plateau shear line that occurred on 19 22 May 2008,on 1 3 July 2007 and on 19 21 September 2009 respectively as well as adopting MICAPS data,NCEP 1° × 1° reanalysis data,FY-infrared temperature data,etc. The results show that:( 1) While winds on both sides of the shear line weaken,especially north wind,the shear line process tends to weaken. The strength of cold and warm air affects the position of shear line. Shear lines in early summer and midsummer are northerly situated while those in late summer are southerly situated.( 2) During shear line process,there is positive vorticity,convergence and ascent movement in coordination with the shear line. While the shear line is disappearing,the convergence belts weaken ahead of the positive vorticity belts. There are multi centres of positive vorticity and convergence around the shear line,which may be relative to the plateau vortex activities. The characteristics of positive vorticity centers moving eastwards are obvious for midsummer and late summer shear lines,and the convergence ascent areas are more eastern situated and the motion is stronger than that for early summer shear line.( 3) There are band areas or block areas of TBB <-20℃ nearby the shear line. During the shear line maintaining,TBB values further reduce. There are multi-centers of TBB low values for midsummer shear line at maintaining stage and late summer shear line at initial stage,which shows that convective activities are stronger.( 4) The east and south slopes of the plateau are areas with congregated unstable energies due to terrain blocking and heating. The characteristics of high temperature and high humidity energy are obvious in near-surface layer around the shear line in midsummer. Early summer shear line brings about stable and small precipitation which is in patch form while the characteristics of unstable convective precipitation are obvious for midsummer and late summer shear lines. The distribution of precipitation,which is in band form around the shear line,is larger and intensities are stronger.( 5) The shear lines on 500 hPa are also water vapour congregated band. The conglomeration of vapor and instability energies,and positive vorticity transmitting eastward and convection developing over the shear line are important mechanisms that lead to heavy precipitation.