A Diagnostic Analysis And Numerical Simulation Study Of A Southwest Vortex Rainstorm Process

In this paper,a Southwest Vortex rainstorm process on 16-19 August 2015 is analyzed by the use of the combination of numerical simulation method and diagnosis method such as moist potential vorticity(MPV),z-helicity and relative helicity.Then,six boundary layer parameterization schemes from WRF model are used to compare and analyze the the effects of different boundary layer schemes on the structure of the southwest vortex,precipitation and physical quantity fields,and based on the experimental results with better simulation results,four Sensitivity experiments are designed by changing the height of the terrain around the Sichuan Basin and closing Condensation Latent Heat or surface heat flux.The effects of topography around the basin,condensation latent heat and surface heat flux on the formation and development of the southwest vortex and rain are studied.The main conclusions obtained are as follows:(1)In this weather process,the rainfall area moves along with the movement of the southwest vortex,and the distribution of the convergence region of moisture flux divergence can be taken as a good indicator of rainfall area.The spatial distribution of the 700 h Pa moist potential vorticity(MPV)has a good correspondence with rainfall area and its center location in the corresponding period,the strong rainfall center is near the junction of the negative value area and the positive value area of MPV1,while the rainfall in the negative value area of MPV1 is weak.The overlapping area of the negative value of MPV1 and the positive value of MPV2 in the mid-low troposphere is a warning area for the storm.Stratification instability,upward movement of intensive convergence and the transfer of southwest warm and humid air in the mid-low troposphere are important mechanisms for the rainstorm.The distribution of the positive value area of z-helicity can give good indication to rainfall area in the corresponding period,the center position of the positive value area of relative helicity is highly consistent with the next 6h precipitation center,and the precipitation center is close to the densest isolines by the side of the positive value center of relative helicity.(2)The six boundary layer schemes have obvious influence on simulating precipitation,moving path and intensity of the Southwest Vortex.The ACM2 can preferably reflect the change of track of the Southwest Vortex and the center position,intensity,and dispersibility of the precipitation.Different PBL schemes made significant differences in simulating geopotential height and wind,relative vorticity,vertical velocity and equivalent potential temperature field.Compared with other schemes,the positive vorticity of the Southwest Vortex simulated by YSU and QNSE can extend from the lower troposphere to 200 h Pa,and the result of ACM2 was closer to the facts than others.Six PBL schemes performed better in the PBL height,surface sensible heat and latent heat fluxes,which the diurnal change is evident.Among these schemes,the QNSE simulated larger PBL height and surface heat flux,while the a smaller value is simulated by ACM2.It can be seen that different PBL schemes in WRF model have great influence on the simulation of different physical quantity field of the Southwest Vortex rainstorm process,choosing the appropriate PBL scheme can improve the forecasting accuracy of the Southwest Vortex weather process.(3)The Qinling-Bashan Mountains impacts the development of the southwest vortex by blocking the south airflow and the friction with the lower atmosphere,and reducing the height of Qinling-Bashan Mountains is not conducive to water vapor convergence and the vertical motion of the atmosphere over the Sichuan Basin,and seriously affects the formation and development of the southwest vortex and precipitation.Wuling Mountain and Dalou Mountain has an important influence on the development of the southwest vortex and the formation of precipitation by blocking the west warm and wet airflow and forced uplift of landform.Condensation latent heat has a significant effect on the development of convection in the process of the southwest vortex and the vertical growth of the positive vorticity of the low vortex.Closing condensation latent heat weakens convergence upward movement over the Sichuan Basin and affects the condensation of water vapor,which also affects the formation of precipitation.Turning off the surface heat flux weakens the development of the Southwest Vortex,precipitation intensity and the convection activity of the whole layer.The surface heat flux has a strong effect on the vertical development of positive vorticity and convergence upward movement of the airflow over the rainstorm area.