Feature Analysis Of The Southwest Vortex Activities And Flood-producing Vortex In Summer

Southwest Vortex is one of the most powerful rainstorm systems in China,which needs more attention.Considering that the previous investigations on Southwest Vortex has not formulate its occurrence and development from different origination.Based on the ERA-INTERIM reanalysis data with 2.50x2.50 resolution of the geopotential height and wind with 6 hours' time resolution during June to August from 1979 to 2016.According to the different origination,southwest vortexes(SWVs)were subdivided into Jiulong area vortexes,Sichuan basin vortexes and Xiaojin area vortexes.The characteristics of the Southwest vortex were statistically analyzed after defining the closed cyclone circulation and setting up the statistic method of the vortex movement routine according to wind field,then we quantify and analyze the climate characteristics in SWVs anomaly years emphatically and the characteristics of precipitation contributed by the SWVs from 1979-2016 in summer(June to August).The results suggest that the annual frequency of occurrence of SWVs was 11.6 a-1,the maximum number of vortex generated in Sichuan Basin(9.3 a-1),followed by Jiulong area(1.9 a-1),at least in Xiaojin area(0.4 a-1).For purposes of moving rate,Sichuan basin vortexes reached 44.2%(the maximum),then 30.8%in Xiaojin vortexes,at least in Jiulong vortexes just 29.73%.There were total 140 high-impact SWVs generating during the 38 years in summer,and only 105 cases moved out of the birthplaces(among them,Sichuan basin vortexes,Jiulong vortexes and Xiaojin vortexes were 84,17 and 4 cases,respectively).The migratory path of the high-impact Sichuan basin vortexes and high-impact Jiulong vortexes is based on eastward-migrating path and northeastward-migrating path respectively.To research the relationship between migratory SWVs and precipitation,draw the distribution map of rain belt and strong precipitation center,and thus concludes the high-impact SWVs persist for more than 36 hours will bring precipitation,and over 88%of the probability will cause heavy rainfall and the above precipitation.The longer the lifespan of the SWVs is,the more rainstorm process will be brought to the downstream area.The probability of causing heavy rainfall and the above precipitation for high-impact Jiulong area vortexes and Sichuan basin vortexes are 83%and 91%respectively,which have significantly higher ratios than Xiaojin area vortexes.Meanwhile,the rain belt caused by SWVs is consistent with migratory path of SWVs.And this study also shows that:when the high-impact Sichuan basin vortexes moved northeastward,the rain belt appeared in the areas of Hubei,Anhui and Shandong,and some of them also appeared in the northeast area.The heavy rainfall centers were mainly distributed in the junction of Hubei and Anhui.When the high-impact Sichuan basin vortexes moved out along the eastward path,the heavy rainfall center was concentrated in the middle and lower reaches of the Yangtze River,which has the highest occurrence frequency.When the high-impact Sichuan basin vortexes moved out along the southeast path,there were two main rain belts corresponding to it,one in Hunan,Jiangxi and Fujian area and the other in Guizhou and Guangxi,the latter belt appears more frequently.The heavy rainfall center was mainly occurred at the junction of Guizhou and Guangxi.The analyses of Flood-producing vrtex showed that the wind flow aloft divergence is dominant,and convergence is dominant in lower layer,anti-clockwise rotation is mainly on 700hPa.The analyses of v wind suggested that heavy rains area was nearly the line jointed the maximum north and south wind currents,or north to the v wind intensive contours line.