A case study on convection initiation mechanisms of an extreme rainstorm in Hotan, Xinjiang, NW China

Convection initiation (CI) mechanisms of an extreme rainstorm (ER) event that occurred in June 2021 in Hotan, Xinjiang, Northwest China, are investigated using the observational data of automatic weather stations, Fengyun-4 A satellite data, reanalysis data, and simulation data from the Weather Research and Forecasting (WRF) model. The results show that continuous CI sites occurred near a boundary layer convergence line (BLCL), which was generated due to the merger of a cold pool and a low-level easterly jet (LLEJ). Most of the initiated convective cells developed rapidly and gradually merged with other convective systems, resulting in a mesoscale convective system (MCS) that was responsible for the heavy precipitation in the ER event. Diagnostic analysis of vertical momentum budget along the Lagrangian backward trajectory of air parcels within a convective cell initiated near the BLCL indicates that the dynamic acceleration (a_d) was responsible for the vertical acceleration of the air parcels during the rapid-ascending stage of the CI. Further decomposition of a_d reveals that the Extension term played a decisive role, while the Twisting term provided negative contributions and the Curvature term was negligibly small. Further investigations revealed that the Extension term was attributed to the low-level convergence within the BLCL, resulting from the merger of the cold pool and LLEJ.