Mesoscale analysis on a squall line in the warm-sector heavy rainfall over Southern China

Based on the intensive surface observational data, sounding data, and Doppler radar echo and wind retrievals, the evolution and three-dimensional structure of a squall line leading to the heavy rainfall over Southern China during 6-7 May of 2010 were investigated. The results are as follows. (1) The squall line occurred in a broad region which is under the divergence area at 200 hPa, behind the upper trough at 500 hPa and in front of the quasi-stationary front. A shear line and a low-level jet existed, respectively, on the north and southeast of the squall line at 850 hPa. There was a moderate-intensity vertical wind shear at middle-low level of the squall line. (2) The sporadic convection cells in the squall line system were triggered by the blocking effect of the Dayao Mountain on the westerly wind. New convection cells were constantly generated at the border of Guangdong and Guangxi province. They moved eastward while developing and ultimately incorporated into the former eastern convection belt rapidly. Both the evolution of the convective cells and the formation of convective belt had close relationship with the middle-scale convergence line near the surface. (3) During the evolution of this squall line, the convection belt and cells were occluded by the interaction of the mesoscale convergence lines near the surface and the meso-cyclone in the middle level; the eastern part of the convective line developed into a bowing echo at the mature stage. The downdraft and the inflow of cold air at middle level were enhanced by the heavy rain drag and the water evaporation effects, which further intensified the cold pool and the divergence in the rear of the squall line, thus favored the development of the bow echo. (4) At the mature stage, bow echo, cold pool and weak stratiform cloud were evident. The three-dimensional structure of this squall line was similar to that of the classic ones except for the facts that the bookend vortices were failed to appear and the thunderstorm high was weak.