The Mechanism Of Transition Of Convective Mode Of A Squall Line In South China

Based on intensive surface observational data, sounding data, NCEP reanalysis data, Doppler radar data, and high resolution data produced by WRF model, the severe squall line event that occurred over Guangxi and Guangdong Provinces on 15 May 2013 is analyzed. The paper analyzes precipitation distribution, synoptic background, evolution process of the squall line, and then mainly focuses on the process and mechanism of transition of convective mode from PS mode to TS mode, and the organization mechanism of bow echo:(1) The squall line formed in the region which is under the divergence area at 200-hPa, ahead of the eastward trough at 500-hPa, between the northwestern shear line and the southeastern low-level jet at 850-hPa, near the surface cold front and in the area of surface inverted trough.(2) The initial convections trigger in the north of surface inverted trough. The western convection organizes into PS mode owing to the surface cold pool and divergence, but no thunderstorm high is seen in the mature stage. The eastern weakening convection strengthens again when moving to Guangdong Province, and then combines the western PS MCS to convert to TS mode with thunderstorm high, wake low, and cold pool.(3) The simulation results show that, at the middle level of convective regions, pressure perturbation is negative both in PS and TS mode. But in PS mode, the pressure perturbation is positive in the southeast of convective regions, which results in line-parallel pressure gradient force, acceleration and relative line-parallel stream. In TS mode, line-parallel pressure gradient force and acceleration is very weak, but line-perpendicular pressure gradient force and acceleration is quite strong. It is the decrease of line-parallel acceleration and the increase of line-perpendicular acceleration results in the transition from PS mode to TS mode.(4)The low-level rear inflow is stronger than upper-middle rear inflow. In the early period, only the eastern cyclonic circulation is seen in low level. But in the latter period, western anticyclonic circulation develops fast, so that both western anticyclonic circulation and eastern cyclonic circulation are seen. Vorticity budget calculating shows up significant increase of negative vertical vorticity in the west area and weak increase of positive vertical vorticity in the east area.