Abstract:

Using conventional ground and upper-air observations, NCEP 1°×1° reanalysis data, and FY satellite data, this study focuses on three common types of snowstorm weather systems in Urumqi, Xinjiang, China from 1990 to the present, and considers snowstorms with high and low altitude circulation and weather system configuration, unstable conditions, water vapor, dynamic mechanisms, and black body temperature (TBB). The results show: (1) Three snowstorms occurred during the southeast recession of the European high-pressure ridge, which pushed the West Siberian trough eastward and southward and, combined with the mid-latitude short-wave trough, the weather system at high and low altitude showed a “backward trough” structure. Urumqi was in the area where the northwest jet, at 925-600 hPa, and the strong southwest jet, at 600-200 hPa, overlapped. Forced elevation uplift caused by the Tianshan Mountains contributed to maintaining and strengthening the snowstorm. (2) There are southeasterly winds from 850 hPa to 700 hPa before the snowstorm. The slight advection was beneficial to the generation and strengthening of advective inversion, and this led to a continuous accumulation of energy. In the later stage, cold air entered, a cold front was generated, and stratification developed unstably, resulting in heat conditions for the snowstorm. The longer southeasterly wind and advective inversions are maintained, the more energy is stored and, as a result, the stronger the snowfall. (3) There are two water vapor transport channels in the snowstorm area: the southwest and the west paths. Water vapor transport in the middle layer is very important for snowstorms in Urumqi. There is a strong water vapor convergence at 850-600 hPa, with 700 hPa being the strongest. Together, water vapor transport, convergence intensity and duration determine the intensity of the snowstorm. (4) There is a correlation between TBB and snowfall intensity. The lower the TBB, the higher the cloud top height, and the more vigorous the development of mesoscale cloud clusters, the stronger the snowfall. The first rapid decrease (increase) of TBB (cloud top height) before a snowfall begins indicates the start of the snowfall. The decrease of TBB during the snowfall corresponds to the increase in snowfall intensity, and the greater the TBB drop, the longer the maintenance time of the low TBB value and, as a result, the stronger the snowfall.

Key words: typical snowstorm, weather system configuration, weather mechanism, cause analysis, Urumqi