西安地区一次罕见秋季冷锋后暴雨过程分析

Analysis of a Rare Autumn Rainstorm behind Cold Front in Xi’an

利用常规地面高空观测、西安多普勒天气雷达观测、欧洲中心细网格模式预报等资料对2017年9月27日西安暴雨天气过程进行诊断分析。结果表明,暴雨发生在秋季连阴雨结束前,距地面冷锋后部300~400 km,暴雨发生前地面气温较低,不利于高温高湿能量的积累,西安地区850 hPa、700 hPa均为偏北风,无法为其带来水汽,但500 hPa西风槽为西安地区暴雨提供了有利的天气形势;通过诊断饱和相当位温、地转绝对动量表明西安暴雨的不稳定机制是条件性对称不稳定。冷锋锋面自南向北逐渐倾斜,陕南地区西太平洋副热带高压外围700 hPa有一支偏南风带来暖湿气流,暖湿气流被锋面强迫爬升至西安条件性对称不稳定区域,产生倾斜对流。大气有着较强的斜压性,中等强度的垂直风切变有利于地转绝对动量维持较小的坡度,中高层暖湿气流使得中高层饱和相当位温有着较大的坡度,从而使中高层形成条件性对称不稳定。降水回波呈现出平行带状,与0~6 km风切变矢量西西南风平行。条件性对称不稳定区域与倾斜上升运动及回波高度有着较好的对应关系。

Conventional observation data,radar data and EC-thin data(025°×025°)are used to analyze an autumn rainstorm behind a cold front before the ending of a consecutive rain on 27 September 2017 in Xi’an.Results show that the rainstorm locates 300~400 km back from a cold front on surface,where the temperature is rather low,unfavorable for the accumulation of warm and moist energy.Meanwhile,the northerly wind on 700 hPa and 850 hPa brings no water vapor.However,westerly trough on 500 hPa provides a favorable background circulation for the rainstorm.Through analyzing saturated pseudo equivalent potential temperature and geostrophic absolute momentum,the mechanism of this rainstorm is considered conditional symmetric instability.The cold front slopes gradually from south to north.A southerly wind on 700 hPa brings warm moisture from the periphery of the Western Pacific Subtropical High over to the south of Shaanxi.The airflow is forced upward by the cold front to the conditional symmetric unstable area above Xi’an,which results in inclined convection.In the strongly baroclinic atmosphere,a moderate vertical wind shear is favorable to the geostrophic absolute momentum with a small slope.On middle and upper level,the warm and humid airflow makes the saturation pseudo-equivalent potential temperature have a large slope,thus conditional symmetric instability are formed.The precipitation emerges as paralleling belt echoes on radar,which is parallel with vertical wind shear vector from 0 to 6 km.There is a good correspondence among the conditional symmetric unstable area,the upward sloping motion and the echo height.