Analysis Of The Characteristics Of Precipitation Formed By The Southwest Vortex In The Sichuan Basin During The Summer Of 2004-2017

Based on the CFS reanalysis data,we conduct a census of the Southwest Vortex(SWV)from May to October 2004-2017.According to the precipitation distribution shown by TMPA V7 data,we classified SWV.The number of the precipitation in different directions is counted,and the precipitation characteristics of the four most types are analyzed,including the range of rainstorm and the intensity of precipitation.The circulation differences as well as vapor characteristics of the four representative cases are analyzed.By using WRF model,the formation mechanism differences of different types of SWV is analyzed.The main conclusions obtained are as follows:(1)By statistically analyzing the SWV and its precipitation orientation in the summer half of 2004-2017,we found that the SWV precipitation frequently occurred in the northeast,east,southeast and central parts of the SWV successively,some southwest vortices have precipitation in two or more directions or have no precipitation.Analysis of precipitation characteristics of four SWV with the largest frequency,it is found that the average maximum rainfall of the four types reached the level of heavy rain.The maximum precipitation of 6 hours is 275.1 mm.(2)Through the analysis of circulation situation on representative case,we found that,both in the type of central part and northeast part precipitation,cold and warm currents meet in the northern part of the basin.The difference is that central part precipitation occurs when there is a wider range of cold air intrusion at the low level and the southwest airflow is stronger compared with the type of northeast part precipitation.These two branches form a circular flow field in the basin which is different from confrontation state happened in the type of northeast part precipitation.The type of east and southeast part precipitation happens when there is no cold air invasion.The last one happens when the southwest current turn westward in front of Daba Mountains.However,the former one happens when the current flows eastward compared with type of southeast part precipitation and finally flows across Daba Mountains.(3)The analysis of atmospheric vertical motion shows that there is a significant vertical circulation in the vicinity of the SWV,which is usually composed of the updraft in the SWV and the downdraft on the North side,while the precipitation area usually corresponds to the location of the circulation.By comparing the precipitation characteristics,it is found that the precipitation location is more south than that of SWV in the absence of cold air intrusion.(4)For the SWV at 12:00,the meridional variation of the equivalent potential temperature caused by the valley wind effect will occur.If the low-level jet stretches westward and there is no cold air intrusion.The valley wind effect causes instability in the lower level,which is strengthened by the westward extension of the low-level jet.The positive vorticity of the leeward slope and the divergence of the upper troposphere jointly strengthen the convection.When the low-level jet is located eastward,the SWV produces two updrafts.One is due to the invasion of cold air in the northwest side of the basin;the other is due to the heating effect of the basin margin to the surrounding atmosphere,which strengthens the potential instability caused by the low-level jet,thus causing the precipitation to extend eastward.