Based on high spatial and temporal resolution 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 mainly focuses on the transition process and mechanisms of mesoscale convective system(MCS) mode from PS mode to TS mode.
The facts of observation showed that the squall line formed in the region which was 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.The initial convection was triggered in the north of the surface inverted trough.The western convection was organized into PS mode owing to the surface cold pool and strong divergence,but no thunderstorm high was seen in the mature stage.The eastern weakening convection strengthened again when moving to Guangdong Province,and then combined the western PS MCS to convert to typical TS mode with thunderstorm high,wake low,and cold pool.
In order to analyze the mechanisms of the transition,WRF model was used.The results of simulation were almost in accordance with the observation facts in the isochrones of the squall line,the 12 h simulated cumulative precipitation,and the transition process from PS mode to TS mode.The main results could be summarized as follows:
(1)The initial convection was triggered in the center of Guangxi Province.There were two convections in the beginning.One of them was the line convection in the northeast of Laibinand the other was the convection in the west of Laibin.The latter developed to northwest-southeast PS mode later.The former developed again when moving eastward to combine with the western PS MCS to typical TS mode with bow echo and stratiform cloud in the north.
(2)In the northwest-southeast PS mode,the pressure perturbation was negative in the convective regions.The pressure perturbation was positive in the southwest of the convective regions,which resulted in line-perpendicular pressure gradient force.So that the line-perpendicular pressure gradient force also resulted in line-perpendicular acceleration and stream.But in the southeast of the convective region,the pressure perturbation was positive,which resulted in great line-parallel pressure gradient force,acceleration and relative line-parallel stream.The result that PS mode had deep line-parallel stream which matches with the conclusion of Parker(2007a,2007b).
(3)When the squall line developed into TS mode,the pressure perturbation in the convective regions was still negative,while the pressure perturbation in the southwest of the line was still positive,thus there was still great line-perpendicular pressure gradient force,acceleration and relative line-perpendicular stream as PS mode.However,the positive pressure perturbation in the southeast of convective regions in the PS mode was converted to be negative,which made the line-parallel pressure gradient force,acceleration and relative line-parallel stream disappear.
Generally speaking,in PS mode there was strong line-parallel stream for line-parallel pressure gradient force and acceleration.But the stream was converted to be line-perpendicular in TS mode for the disappearance of line-parallel pressure gradient force and acceleration.It was this change which led PS mode to TS mode.