Observational Study On A Squall Line Process Occurred Over The Western Part Of South China On19thjune2010.

Based on a variety of meteorological data (including the high spatial and temporal surfaceobservational data, sounding data, Doppler radar echo, TBB data, retrieval wind data of radarobservation and NCEP reanalysis data), the evolution and structure characteristics of a squallline occurred over the western part of South China on19thJune2010are investigated. Themajor conclusions are as follows:(1) The squall line is caused under the following situations:200hPa high-level divergence,rearward of500hPa trough, warm and moist area ahead of850hPa trough. The convectionlocates on the surface convergence line and cold front, instead of the warm section before thefront. In the region which the strong convection forms, the atmosphere appears a typicalunstable stratification with the cold air over the warm.(2) The squall line initially comes from a quasi-linear convective system along the surfaceconvergence line embedded in the cold front, subsequently combines with the newly triggeredquasi-linear convective system over the warming zone ahead of the cold front, and finallydevelops into the mature quasi east-west squall line system. At its mature stage, the leadingconvective line develops into a bowing echo, with a trailing stratiform region containing radarbright band at the mid-level and transition zone between them. Meanwhile, the thunderstormhigh and wake low form at the surface.(3) The convection cell merger process along the northern linear convection system ismainly caused by the convergence uplifting between downdraft in the adjacent cells. However,the merger process between the two convection lines is mainly due to the strong updraft in thesouthern convection cell, which triggering downdraft in the north with partial upper-leveldivergence airflow. By the action of the vertical circulation, the southern convection cellstrengthens while the northern one weakens, and gradually mergers with each other.(4) There are meso-γ bookend vortices along the convective bowing echo, including acyclonic vortex on the eastern and western flanks of the bowed segment and an anticyclonicvortex on the middle part of the bowing echo. Meanwhile, there is a larger scale cyclonic vortex in the rearward of the stratiform cloud. The squall line develops with updraft in theleading convective line and upper-levels of the stratiform region, as well as downdraft in thetransitional region and mid-to-lower levels of the stratiform cloud.(5) Vorticity budget analyses show that the positive vorticity along the convective regionforms mainly through the stretching of vertical vorticity, and the bookend vortices and thepositive vorticity in the rearward of the stratiform cloud form mainly through the tilting ofhorizontal vorticity.