Abstract:

On April 2, 2021, a catastrophic snowstorm, which exceeded the historical extreme value, occurred, causing large financial losses in Baicheng County, the northern edge of the Tarim Basin, Xinjiang, China. The hourly European Center for Medium-Range Weather Forecasts fifth-generation (ERA5) high-resolution reanalysis data (0.25º×0.25º), meteorological observation data, and FY-2G satellite data were used to fully analyze the atmospheric circulation anomalies, multi-scale atmospheric circulation characteristics, and physical mechanisms. The following results were obtained: (1) The atmospheric circulation of the snowstorm was typical of a South Xinjiang snowstorm circulation: upper-level South Asian high, subtropical trough, and subtropical westerly jet, mid-level Central Asian vortex, and low-level easterly jet and converging lines combined with the cold high pressure and converging lines at the surface. (2) Extreme snowstorm was caused by the interplay of different-scale atmospheric circulation anomalies. The upper-level anomalies of Iran subtropical high and anomalous easterly airflow at low latitudes led to the anomalous combination of the Central Asian vortex and the plateau vortex at 500 hPa and also generated anomalous easterly airflow at 700 hPa, guiding warm and humid air from the South China Sea and the Bay of Bengal along the Hexi Corridor to the central Tarim Basin, causing water vapor convergence and enhancing vertical upward movement, with the triggering of the surface convergence line, resulting in an extreme snowstorm. A stable maintenance of surface high-pressure systems and cold front produce continuous cooling in the Tarim Basin. Moreover, Baicheng County is located in a shallow mountainous area with an altitude above 1000 m, and the combination produced extreme snowstorm in April. (3) For Baicheng County, the key roles are the upward motion in the middle layer of the troposphere and the vertical wind shear at 300-500 hPa. The vertical profile shows that the level of upward motion of the snowstorm is located at 500-700 hPa, showing mesoscale symmetric instability characteristics. Using the frontogenetic function equation and the moist potential vorticity equation, we found that the intensity of the vertical potential temperature gradient and westerly wind anomalies lead to the development of baroclinic instability, causing surface frontogenesis, generating 300 hPa and surface moist potential vorticity anomalies, affecting the development of snowstorm through upward motion changes. (4) The consistency of the moving and propagation directions determines the evolutionary characteristics of mesoscale clouds production and extinction. In turn, the continuous development of mesoscale clouds moving northeastward through Baicheng County increases the duration and intensity of the snowstorm. The above findings can deepen the knowledge of extreme snowstorms in the Tarim Basin and provide scientific support for accurate forecasting and precise services.

Key words: extreme snowstorm, atmospheric circulation anomalies, moist potential vorticity, mesoscale clouds, Tarim Basin