低纬高原两次冬季南支槽强降水的对比分析

Comparison Analysis of Two Heavy Rain Processes on the Plateau at Low Latitude Caused by the Southern Branch Trough in Winter

利用常规高空观测资料,对发生在云南低纬高原2007年1月31日—2月1日和2008年1月26—27日的两次南支槽强降水过程进行对比分析。结果表明,两次南支槽降水的环流形势及影响系统均不同,前者为高原南支槽前西南暖湿气流与低层切变、冷锋共同影响,属典型的"槽潮"天气;后者无明显强冷空气配合,因中低层西南急流的建立并长时间维持,新生南支槽东移补充、西南涡及静止锋的共同影响造成。物理机制上,湿位涡的CSI机制分析表明,两次过程均属于对称不稳定降水;在2007年的强降水过程中,θse密集带在地面锋区附近形成,冷暖气流形成次级垂直环流圈,导致强降水强降温并伴有大范围降雪的寒潮天气出现。后者无次级垂直环流圈形成,但有两个θse密集带锋区长久维持,滇中以西持续的上升气流不断增强且深厚,滇中以东中低层形成有弱的气流辐合带,故强降水以液体降水为主,降温不明显;两次强降水过程均有锋生,前者锋生是由切变增强西移,低层冷高压促使冷锋增强,后者锋生为东移西南涡与稳定持续的强劲西南风低空急流触发而成。

By using the routine observational data, two heavy rain processes on the plateau at low latitude in Yunnan from 31 January to 1 February 2007 and from 26--27 January 2008 caused by southern branch trough are compared. The results show that the two processes happened with different circulation backgrounds： the former is a typical Cold Trough-Cold Wave Weather caused by the warm and humid southwesterly, the low level shear and cold front while the later is caused by the establishment and maintenance of the low level southwesterly jet, new southern branch trough, Southwest China vortex and the stationary front. In terms of physical mechanism, conditional symmetric instability occurred in both precipitation processes. In the former process, a secondary circulation was formed by the cold and warm airstream, and a zone with large horizontal gradient of the θsc was formed near the surface front, which led to the strong precipitation, an obvious fall of temperature and snowfall over a large area. During the latter process, there was no secondary circulation but the maintenance of two zones with large horizontal gradient of the θsc. In the west of Yunnan the ascendant current was strong and continuously strengthened while in the east a weak convergency belt is formed. Therefore, the precipitation was main-ly in the liquid form and there was no obvious fall of temperature. Analysis shows that there were frontogeneses in each of these two heavy precipitation processes. The former was caused by a westward moving low-level shear and a cold anticyclone which promoted the cold front to become stronger. A eastward moving cold vortex and the maintenance of a southwesterly jet were the causes of the latter one.