乌鲁木齐典型暴雪天气机理及成因分析

利用常规地面、高空观测资料、NCEP/NCAR 1°×1°再分析和FY卫星资料,针对乌鲁木齐1990年以来的3场典型暴雪天气,从高低空环流和天气系统配置、不稳定条件、水汽、动力及黑体亮温（TBB）变化等方面综合对比分析暴雪成因。结果表明：（1） 3场暴雪均发生在欧洲高压脊东南衰退,推动西西伯利亚低槽东移南下,与中纬度短波槽结合的环流形势下,高低空系统呈“后倾槽”结构,乌鲁木齐处在925~600 hPa西北急流与600~200 hPa强西南急流叠置区,且天山山脉的地形强迫抬升有利于暴雪的维持和加强。（2） 暴雪前850~700 hPa均有东南风存在,微差平流作用有利于平流逆温生成和加强,使得能量不断积聚,后期冷空气进入,冷锋锋生,层结不稳定发展,为暴雪天气提供热力条件,东南风和平流逆温维持时间越长,储存能量越多,降雪越强。（3） 暴雪区存在西南和偏西2条水汽输送通道,中层水汽输送对乌鲁木齐暴雪至关重要,850~600 hPa存在较强的水汽辐合,且700 hPa最强。水汽输送、辐合强度及持续时间共同决定暴雪强度。（4） TBB与降雪强弱有一定的对应关系,TBB越低,云顶高度越高,中尺度云团发展越旺盛,降雪越强,降雪前TBB（云顶高度）的第一次迅速降低（升高）预示降雪开始,降雪过程中TBB降低对应降雪强度增强,且TBB降幅越大、低TBB值维持时间越长,降雪越强。

Mechanisms and causes of typical snowstorms in Urumqi

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.