Comparative analysis on two squall line events in the same season and similar areas of southern China

Based on conventional observations, data from regional automatic weather stations (AWSs) over Guangdong, Doppler weather radar data, wind profile radar data, satellite cloud images and NCEP/NCAR reanalysis with 1°×1° resolution, we have conducted a comprehensive analysis for the environmental conditions and the difference between two squall line events occurred in the southern china on 13 and 17 April 2016 (hereinafter referred to as "4.13" event and "4.17" event, respectively), especially discussed the difference in the disaster situation between two squall line events in the same season and similar areas. Results are as follows. (1) Two squall line events, which occur in the similar circulation background, impacted on Guangdong with varying degrees in the later period. The "4.13" event is characterized by a smaller affected area and a greater intensity, and the squall line develops into the bow-shaped echo when it entered Guangdong. While the "4.17" event is characterized by a widely affected area and a weaker intensity, and the squall line gradually weakened when it entered Guangdong. (2) There is a forward-tilling trough structure and a strong vertical wind shear which is propitious to the occurrence and reinforcement of squall line in the "4.13" event. However, cold airs intruding from the downstream areas existed in the "4.13" event, which is not favorable to the occurrence of severe weather. (3) In the "4.13" event, the low-level warm advection and the sinking of middle-level dry and cold airs causeed a strong unstable atmospheric stratification, and then a cyclonic surface mesoscale convergence line was formed near Guangzhou, which is advantageous to the reinforcement of squall line in this region. (4) The cause of resultant surface gale is different between two squall line events. The "4.13" event is originated by downburst under the top of bow-shaped echo, with the very strong rear inflow and the high centroid height of strong echo. On the contrary, the "4.17" event is originated by general super cell in which the intensity of surface thunderstorm gale is relatively weak because of the low echo centroid height and the small gravity drag effect of precipitation particles.