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

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 θse 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 θse.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 mainly 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.