Comparison of Convective Triggering Mechanisms of Typical Rainstorm Events in the Hami Area of East Tianshan Mountains

In this study, multiple data sources were used to conduct an in-depth analysis of two extreme rainfall events that occurred in the south-eastern Hami area on 31 July 2018 (hereinafter referred to as the “7.31” heavy rainstorm) and in the northern Hami area (Xinjiang, China) on 8 August 2016 (hereinafter referred to as the “8.8” heavy rainstorm). Based on the calculation of the frontogenesis function using the NCEP/NCAR FNL reanalysis (0.25°×0.25°) , the differences among the mesoscale convection trigger factors were compared and analyzed between the two heavy rainfall events. The following results were observed: (1) During the “7.31” heavy rainstorm, the 500-hPa western Pacific subtropical high was anomalously northerly and was westerly during the “8.8” heavy rainstorm; several mesoscale cloud clusters generated in front of the 700-hPa Hexi corridor jet stream during the “7.31” heavy rainstorm, and generated in front of the 700-hPa southwesterly jet stream during the “8.8” heavy rainstorm. (2) In “7.31” heavy rainstorm, the atmosphere was conditionally unstable over the region and had unstable convective energy triggered by warm frontogenesis at the low troposphere. Warm frontal frontogenesis was determined by the horizontal divergence and tilt term during convective initiation and by deformation and tilt terms while convection was mature. (3) During the “8.8” heavy rainstorm, the convective cloud clusters were triggered by cold frontogenesis at the low troposphere and then merged and developed along the 500-hPa steering flow. The atmosphere was conditionally unstable over the region and had unstable convective energy. Additionally, the maintenance and intensification of the 700-hPa shear line were in favor of the cold frontal frontogenesis. It was also the main responsibility for the long period of the mesoscale convective system. Cold frontal frontogenesis was determined by the horizontal divergence term during convective initiation and by tilt terms while convection was mature.