Numerical Study of a Severe Wind-producing Thunderstorm in South Zhejiang Province Based on Assimilation of Radar Data

The cloud-resolving scale detection and analysis of mesoscale dynamics and hydrometeors based on the assimilation of radar data is the key to the simulation and prediction of the severe thunderstorms, as well as to the analysis of their formation and evolution mechanisms. In this paper, based on the rapid updating and assimilation of S-band weather radar data, we reproduced a squall line accompanied with severe convective winds over mountain areas of southern Zhejiang Province, and discussed its mechanisms of mesoscale dynamical and microphysical processes. The in-depth study shows that: 1) rapid updating and assimilation of radar radial winds was the key to reproducing the organization progress of the squall line. The evolution and adjustment of mesoscale dynamics determined the development of the severe wind-producing convection system. 2) Cloud-resolved radar radial wind assimilation strengthened the mid-layer rear inflow and convection outflows and its convergence in the lower tropospheric, which facilitated the organization of squall line. 3) Assimilation of radar radial winds and reflectivity could significantly change the characteristics of hydrometeors and enhanced the cooling processes of graupel melting below the 0℃ layer height in the stratiform and the evaporation of low-level raindrops. Consequently, the rear inflow declined aslope toward the front of the convection as invading into the stratiform behind. This phenomenon played a key role on momentum transmission from mid- and high-level to the ground. 4) The rapid development of mesoscale dynamics and hydrometeors emerged adiabatic warming process generated by the downdrafts of rear inflow, which was favorable to the evaporation of the low-level raindrops and the strengthening of the cold pool. The contributions of high-level large momentum transmission downwards and strong horizontal pressure gradient force near the surface greatly promoted the formation of the severe winds.