The Climate Characteristics Of Southwest Vortex And Its Relationship With Heavy Rainfall In South China

Using observational data from MICAPS system supplied by CMA, the ERA-Interim reanalysis data of four times a day and 753 stations daily precipitation data in China, by means of statistical methods, the climatic characteristics of southwest vortex and its relation to heavy rain in South China from March to August in 20 years (1991-2010) are analyzed. The results show that:(1)The number of the southwest vortex generation has significant inter-monthly variation, inter-annual variation and inter-decadal variation. There are 535 southwest vortexes generated during the 20 years. Meanwhile,2009 is the largest number year and 2005 is the least, respectively. There is an abrupt change in 1998 with an evident decreasing trend in the southwest vortex generation. Reverse change occurs in 2007. The mean generation number of the southwest vortex is more in April, May and June than that in other months, and June is the most among these months. There are two main regions for the generation of the southwest vortex, one is the Kowloon located at the southeast of the Tibetan Plateau, and the other is the east of Sichuan Basin.(2) The southwest vortex influenced South China can be divided into four types according to its pathway. The differences in 500 hPa circulation field,700 hPa temperature advection field and 850 hPa divergence field, leading to the different moving direction of the southwest vortex. According to the different moving paths, southwest vortex influenced South China can be classified into four types:eastward, southeastward, southward and stagnated vortex. Comparing different moving paths of southwest vortex and corresponding large scale circulations, it can be concluded that when southwest vortex is moving (stagnated), the troughs and ridges in middle-upper troposphere is (isn’t) clearly seen with a stronger (weaker) divergence movement, also warm (cold) advection and wind convergence (divergence) in downstream of the cradle in the lower troposphere, which is (not) favorable to the development and movement of southwest vortex influenced South China.(3) Different movement type of the southwest vortex corresponds to different low level jet, water vapor transport and vertical movement, leading to the significant differences in the precipitation intensity and precipitation area. Stagnation type of the southwest vortex corresponds to weaker low level jet, water vapor transport and vertical movement, leading to smaller precipitation intensity and precipitation area. To each moving type of the southwest vortex, there is a strong low-level jet in the southeast of precipitation area. The vapor flux increases accompanied by the southward shift of the moving path. The westward movement of the large value area of both the water vapor flux and vertical motion, leading to the westward shift of the rainfall area.