山西两类暴雪过程的雷达产品特征比较及降雪量估测

Comparison of Radar Products Characteristics and Snowfall Estimation of Two Types of Snowstorms in Shanxi Province

利用山西大同站常规观测资料和多普勒天气雷达基数据,对山西两类暴雪天气过程(倒槽冷锋型和锢囚锋型,分别简称为“Ⅰ类”和“Ⅱ类”)进行对比分析。结果表明,两类暴雪天气存在相似点:(1)当降水相态为雨或雨夹雪时,低层零速度线均呈现较明显的“S”型弯曲特征,零速度线形状的变化和0℃层亮带位置变化对相态转换具有明显指示意义;(2)由9点平均法绘制的两类暴雪的平均基本反射率因子垂直廓线也具有相似特征,即平均基本反射率因子垂直廓线强度在30~40 dBZ之间,因此可利用平均基本反射率因子垂直廓线定量估测降雪量级,并可判断0℃层亮带。但两类暴雪天气也存在明显差异:(1)Ⅰ类暴雪属于冷云降水,冷垫深厚,虽然低层强冷空气将水汽通道切断,但由中层偏南急流和西北气流径向辐合产生的动力抬升作用使得降雪维持并达到暴雪量级;Ⅱ类暴雪属于暖云降水,由暖切变线触发,冷垫不明显,锢囚锋和南风急流长时间维持是造成暴雪的主要原因。(2)Ⅰ类暴雪过程中出现明显的0℃层亮带且长时间维持,而Ⅱ类则没有明显的0℃层亮带。(3)Ⅰ类暴雪过程中,“S”型零速度线逆转成南北分布的直线时,表明低层暖平流减弱的同时有东风湿冷垫形成,此时降水相态由雨转雪;Ⅱ类暴雪过程中,零速度线由“S”型转为反“S”型时,表示冷平流入侵明显,此时降水相态由雨夹雪转为雪,零速度线再次明显顺转则指示降雪减弱趋于结束。

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 typeⅡblizzard.(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 typeⅡblizzard 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.