关键词: 暴雪, 相态转换, 多普勒雷达产品, 平均基本反射率因子, 定量估测

Abstract:  Based on conventional observation data and Doppler weather radar products from Shanxi Datong station, two types of blizzard weather processes in Shanxi (the cold front inverted trough type and the occluded front type, hereinafter referred to as typeⅠand typeⅡ, respectively) had been compared and analyzed. The results show that there were similarities between two types of blizzard weathers: (1) The lowlevel zero velocity line showed obvious “S”shaped curve when precipitation phase was rain or sleet. The change of the zerovelocity line shape and the position of the 0 ℃ layer bright band had obvious indications to phase transition. (2) The mean basic reflectivity factor profiles of the typeⅠand typeⅡsnowstorm drawn by using the 9point average method showed similar characteristics, that was, the intensities of mean basic reflectivity factor profiles ranged from 30 dBZ to 40 dBZ, which could be used to estimate snowfall level and judge the 0 ℃ layer bright band. However, there were obvious differences between the two types of blizzards: (1) For the typeⅠblizzard, it was cold cloud precipitation with deep cold pad. The water vapor channel was cut off by the strong lowerlevel cold air, however, the dynamic uplift caused by the radial convergence of the midlevel southward jet and the northwest air made the snowfall maintain and reach the magnitude of blizzard. While for the typeⅡblizzard, it was warm cloud precipitation and triggered by the warm shear Line. The longterm maintenance of occluded front and south wind jet was the main cause of formation of the typeⅡ blizzard. (2) During the typeⅠblizzard, a clear bright band of 0 ℃ layer appeared and remained for a long time, while for the typeⅡit had no clear bright band of 0 ℃ layer. (3) During the process of the typeⅠblizzard, when the “S” zerovelocity line reversed to a northsouth straight line, it indicated that the lowlevel warm advection was weakened and an easterly damp cold pad was formed. At this time, the precipitation phase changed from rain to snow. During the typeⅡ blizzard process, when the zero velocity line changed from “S” to reverse “S”, it indicated that the cold advection intrusion was obvious. At this time, the precipitation phase changed from sleet to snow. The zerospeed line turned significantly forward again, indicating that the weakening of blizzard was coming to an end.

Key words:  heavy snow, phase transition, Doppler radar products, average basic reflectance factor, quantitative estimation