我国低纬高原地区初夏强降水天气研究I·2001年5月印缅槽维持期间云南暴雨及其中尺度特征

2001年5月云南除东北部的昭通地区外,先后出现了1949年以来罕见的大雨、暴雨和连阴雨天气过程,其中以5月30日1200 UTC~6月2日1200 UTC的暴雨过程最强。本文采用常规资料、加密的降水和卫星资料对其进行了分析。结果表明,此次强降水过程是在有利的环流背景下,由中尺度系统造成的。云南有着特殊的地理位置和气候条件,其降水过程与我国的东部及华南沿海大不相同,主要结果如下:1)印缅槽与东亚冷槽的相互作用,有利于西南地区暴雨的发生。2)低空急流的产生和加强与暴雨之间存在一定的关系,它不但为暴雨提供了丰富的水汽,还有可能造成位势不稳定层结,且急流上扰动可诱使对流不稳定发展,致使强降水的发生。3)在有利的大尺度背景下产生的近地层中尺度辐合线以及它们之间的相互作用是产生此次强降水的重要系统,这类辐合线与我国东部的降水系统有很大的不同,与云南的地形特点密切相关。4)云南及其周边的特殊地形为此次强降水的产生提供了帮助。雨团大部分在原地生消,移动较少,形成了两个少动的雨强中心,中尺度对流云团的产生和发展与中尺度辐合线相交区关系密切。5)对降水区三维结构的分析表明,中尺度对流系统强烈发展区的低层为强辐合、正相对涡度,高层为强辐散、负相对涡度;存在整体的上升气柱,并在其左右两侧为下沉气流,且此气柱是高湿、低层存在对流性不稳定。6)对水汽来源和收支分析表明,这次云南强降水的水汽可能主要来自于孟加拉湾。

Study of Strong Heavy Rainfall in Low Latitude Plateau Area in China during Early Summer.Part I:A Case Study of Heavy Rainfall in Yunnan under Influencing of Indian-Myanmar Trough

The climate is complicated in Yunnan Province,China due to its special position in the lower latitude area,its heavy rainfall process is quite different from those in East and South China.Most of heavy rainfalls and floods occurred with the local characteristics.In May 2001,a record heavy rainfalls since 1949 appeared in Yunnan except its northeast part.During 3 days,from 1200 UTC 30 May to 1200 UTC 2 June,the strongest precipitation appeared in Yunnan province.In this paper,analysis has been done by using the conventional data,intensive precipitation data and hourly satellite data.The results show that the heavy rainfall was caused by mesoscale systems under the favorable large scale environment: 1) The interaction between Indian-Myanmar trough and East Asia colder trough made instability energy releasing and reappearance of convection.At the same time,the maintenance of Indian-Myanmar trough was very favorable to the water vapor transportation to heavy rainfall area.2)There may existed the certain close relationship between lower level jet and heavy rainfall.The lower level jet not only provided abundant water vapor,but also produce instability energy.3) Under the favorable large scale environment,the interaction between mesoscale convergence lines near the ground was probably the important triggering mechanism for the occurrence of heavy rainfall.And the convergence lines,closely relating to the terrain of Yunnan Province,were very different from that of South China.4) The special topography in Yunnan Province and its adjoining area also played the important role in the triggering of the heavy rainfalls.Most of rainy clusters produced and disappeared in same area,forming two maxima precipitationl centers and mesoscale convective cloud cluster related with mesoscale convergence lines closely.5) The analysis for the three-dimensional structure of precipitation systems shows,in the lower troposphere,mesoscale convective systems developed,the convergence was strong,the relative vorticity was positive;in the upper troposphere,the divergence stronger,the relative vorticity was negative.There existed a whole air column with strong upward motion,and downward motion in both sides of it.In this convective cloud column,there existed rich water vapor as well as instability energy.6) The analysis for water vapor budget indicates,the source of water vapor for heavy rainfall may be mainly in the Bay of Bengal.