Comparative Analysis of Two Extreme Rainstormsin the Arid Area of Western South Xinjiang

This study examines the characteristics of environmental conditions and formation mechanisms of the two extreme rainstorms in the west of southern Xinjiang. Conventional observation data, NCEP reanalysis data, and ERA5 reanalysis data were used to provide a thorough understanding of the occurrence of strong precipitation in this arid area. The two processes occurred in spring and summer with the upper-level circulation presenting quite a different pattern. South Asian highs are located at different latitudes, but both formed favorable configurations with mid-level low troughs for extreme precipitation. The low-level 700–850 hPa easterly jet is one of the most important weather systems for extreme precipitation, transporting moisture, and forming strong convergence with the terrain to trigger the precipitation. The SMPV (second-order moist potential vorticity) quantity was used to diagnose the nonuniform features and the possible generation mechanisms of the two rainstorms. The results indicate that high-value areas of SMPV exhibit high correlations with the evolution of heavy rainstorms by the coupling of convective stability and vertical gradient of absolute vorticity. The process that occurred during 15–16 June 2021 presented extreme precipitation mainly along the Kunlun Mountains, which is associated with the strong low-level cyclonic rotation in the south of the Tarim Basin. This rotation promoted the concentration of water vapor. The vertical gradient of vertical vorticity, which was performed as a superposition of the negative vorticity over positive vorticity, was significant. Meanwhile, the condensation of water vapor during lifting warmed and moistened the mid-level atmosphere with the nonuniformity enhancement of convective stability. Both of these processes contributed to intensifying the upward vertical motion and thus formed extreme precipitation. The process that occurred during 17–24 April 2020 presented strong precipitation in the western trumpet-shaped topography of southern Xinjiang. The dry and cold air flow aroused by the stepped-distributed troughs encountered the easterly warm, moist air flow when crossing the mountain. This formed large vertical vorticity belts and intensified the vertical motions, which was the main mechanism for extreme precipitation.