Analysis Of A Squall Line With Warm-sector Heavy Rainfall Over South China During6-7May2010

Using the intensive surface observational data, sounding data, Doppler radar echo,retrieval wind data of radar observations and NCEP reanalysis data, the formation, evolutionand three-dimensional structure of a squall line with warm-sector heavy rainfall, whichoccurred over southern China during6-7May2010, are investigated. The roles of low-leveljet and the rear inflow in the development of squall line are discussed by analyzing the WRFmodel outputs. The results and conclusions are as follows.(1)The squall line formed in a broad region which is under the divergence area at200-hPa,after the trough at500-hPa, on the southern side of shear line at850-hPa and the northwestside of the low-level jet and ahead of the quasi-stationary front. There is a moderate-intensityvertical wind shear at middle-low level of the squall line.(2)The back building squall line is composed of sporadic convection cells, which aretriggered by the block of the westerly wind by Dayao Mountain. During the period of thedevelopment of the squall line, new convection cells are generating in the junction betweenGuangdong and Guangxi province, and they are incorporated into the former easternconvection belt rapidly.(3) The evolution process of this squall line consisted of the combination of theconvection belt and cells, which closely related to the mesoscale convergence lines appear onthe surface and mesocyclone in the middle level. The heavy rain drug and the waterevaporation strengthen the downdraft and inflow of cold air at middle level. Meanwhile thesurface cold pool and divergence rearward of the squall line caused by downdraft and middlelevel cold air promotes the development of the bow echo. At the mature stage, bow echo, coldpool and weak stratiform cloud are obvious in the squall line system. The vertical structure ofthis squall line is roughly similar to the classical. There is a downdraft behind the squall lineafter the backward updrafts ahead of it merging with the dry cold inflow at the middle-level. But no bookend vortices could be found and the thunderstorm high is weak.(4) At the initial stage of the evolution of the squall line, a strong unstable stratification isfound in the convective region. The southwest low-level jet provides sufficient moisture forthe convection region which is advantageous to instability stratification strength. Thehorizontal vorticity has an increase with the low-level jet in the developing stage of the squallline. Then the strengthened vorticity affect the vertical circulation and the release of instabilityenergy, which is vital to the variation of the squall line. At the mature stage of bow echo,low-level jet begins to weaken and the rear inflow, which is dominated by midlevel negativebuoyancy, lower northwest airflow rear and anticyclonic vortex begin to appear. Theinfluences of rear inflow to the squall line appear in the balance between real inflow andenvironmental vertical shear, which promotes the wind convergence in non-stable region andstimulates the convection development. The thermal effects featured by the sink of rear dry airinflow, which speed up the evaporation of rain drops and the release of the latent heat ofcondensation. Then a negative potential temperature perturbation cold pool center occurred tomaintain the development of the squall line.