Modal Of Mesoscale Convective System In A Continuous Rainstorm In Jiangnan Area

Based on intensive surface observational data,sounding data,NCEP reanalysis data,Doppler radar data,and high resolution data produced by WRF model,the evolution process,organizational structure and modal transformation mechanism of the mesoscale convective system(MCS)during the process of a continuous heavy rainstorm in the Jiangnan area from 19 June to 22 June 2014 were studied.During the heavy rain,the stable stability of the subtropical high,the shear line,the Meiyu front and the blocking high pressure provided a stable large-scale circulation background for the storm,the intersection of the warm and humid airflow carried by low-level southwest jet and constantly invading northeastern cold airflow in the southern region,a vertical circulation formed by the low vortex convergence and high-level divergence coupling,and the constantly shifting short slot,provide adequate water vapor,power and thermal conditions for the heavy rain,which is conducive to the formation of persistent rainstorm in Jiangnan area.The period can be divided into three heavy rain process,the first process belongs to the front of the warm zone,which is mainly impacted by the first convective system(MCS1);the second process belongs to the Meiyu front rainstorm,which is mainly affected by the second convective system(MCS2),the third process also belongs to the Meiyu front rainstorm,during which is influenced by five continued MCS.MCS1 belongs to the PS MCS.In the course of the development and evolution of the convective system,affected by the mesoscale convergence,the lower level is the relative inflow of the vertical convection line,and the middle and upper layers show the relative inflow of the parallel convection line.Organizational modal transformation occurs during MCS2 evolution.In the eastward development,convection forms TS type MCS.Driven by the vortex,MCS2 gradually changed to PS MCS.In the environmental wind field,the relative inflow of TS MCS is vertical convection line,and the cyclone is generated on the north side of the arcuate echo.The circulation distribution of PS MCS is consistent with MCS1.In TS MCS,the strong convection zone is located in the positive disturbance zone,forming vertical force acting on the convection line and resulting in a relative inflow perpendicular to the convection line.But in the parallel convection line direction,disturbance pressure change is small.So the parallel relative airflow is weak,and the overall presentation of the relative airflow is a vertical convection line.In the PS MCS,in addition to the strong negative pressure disturbances in the middle of the disturbance zone,the overall performance of the distribution characteristics is the southwest positive pressure disturbance and northeast negative pressure disturbance.Under the action of pressure gradient force,the relative inflow of parallel convection lines is formed,resulting in a modal transformation.