关键词: 暴雪, 低空急流, 不稳定机制, 倾斜对流, 南疆西部

Abstract:

Based on the high radiosonde observation data, National ground weather station observation data, National Center for Environmental Prediction (NCEP, 0.25°×0.25°) reanalysis data, FY2E satellite data, and products of Kashgar Doppler weather radar station, extreme snowstorm weather in the west of southern Xinjiang of China from March 3 to 6, 2017 was comprehensively analyzed. According to the findings, the 500 hPa low vortex cutoff in Central Asia is the primary influence system of the extreme snowstorm. In this snowstorm, three main branches of water vapor emerge from the west and south of the middle and high layers, as well as from the east of the middle and low layers. The long-term maintenance of east jet formed between 850-700 hPa is not only critical in water vapor transportation (the water vapor transports from the eastern part of Tarim Basin in southern Xinjiang to the west and converges in the west of southern Xinjiang forming a convergence center, increasing the efficiency of water vapor convergence), but it also lifts the relatively warm and humid air in the west of southern Xinjiang to a certain height and acts as a “cold pad”. In this snowfall process, the development and maintenance of meso-β scale convective cloud clusters with black body temperature (TBB) <-65 °C and scales between 80-200 km in the cloud belt are the main reasons for the high snowfall intensity. Kashgar station’s Doppler radar echo appears to be a mixed echo accompanied by stratiform and block cloud echo. The reflectivity factor of stratiform cloud echo is typically 15-25 dBZ, with a large horizontal scale, long time duration, and a small variation range, resulting in long-term stable snowfall. The strongest reflectivity factor of block echo is >40 dBZ, and the top height is >5 km, indicating convective echo characteristics, resulting in strong hourly snow over 2 mm·h^-1. Further study shows that the extreme snowstorm process shows part of the characteristics of elevated convection in the cold season. The latitude-altitude profiles of geostrophic absolute momentum (Mg) and pseudo equivalent potential temperature (θse), together with the Wuqia station, the largest snowfall center, show that during the snowstorm process, the slope of θse is greater than that of Mg between 700-550 hPa, satisfying the conditional symmetric instability criterion. It is preliminarily judged that the extreme snowfall is caused by the inclined convection caused by the conditional symmetric instability.

Key words: snowstorm, low level jet, instability mechanism, oblique convection, the west of southern Xinjiang