低层温度平流对华北雨雪天气过程的降水相态影响分析

摘要

利用常规地面、探空观测资料、NCEP FNL和GFS分析资料,通过对2012年11月3—4日华北地区雨雪天气过程的降水相态特征进行分析,发现涡旋外围的雨雪分界线基本与925 hPa的0℃等温线和925 hPa偏北与偏南风的流线辐合线相吻合;而涡旋中心附近的雨雪分界线则存在从涡旋西北象限向涡旋中心逆时针旋转的特征.气旋发展初期,降雪主要集中在850 hPa低涡的东北偏北象限到西北象限之间,低涡的东南象限为降雨.随着气旋强度增强,低层冷平流导致低涡西南象限温度下降,降雪落区逐渐沿气旋西侧的流场向南发展,最终呈现出气旋形状的分布特征.雨雪相态的转变取决于整个对流层低层(850~950 hPa)的温度平流状况.当900~850 hPa或者950~900 hPa出现较强暖平流时,即使其他层次存在明显冷平流,降水相态仍然可能以雨为主.低层涡旋西侧的西北冷平流是造成降雪的最重要原因,当低层气流转变为偏东风后,冷平流消失,降水相态转变为雨.%By using the data of conventional surface and sounding observations as well as the associated NCEP FNL and GFS reanalysis data,the features of precipitation type of heavy rain-snow process in North China during 3-4 November 2012 are studied in this paper.It is found that in the periphery of low-level vortex,the rain and snow separatrix is identical with the 0℃ isotherm and streamline convergence zone between northerly and southerly wind at 925 hPa.However,near the center of low-level vortex,the rain and snow separatrix takes on a feature of anticlockwise revolving from the northwest quadrant to the center of the vortex.In the early stage of cyclone developing,snow is located at the north-northeast quadarant to northwest quadrant of the 850 hPa vortex while the precipitation type at southeast quadrant is rain.As the extratropical cyclone intensifies,temperature at the southwest quadrant of low-level vortex drops quickly due to the low-level cold advection.Therefore,the snow area gradually expands southward along the wind flow of the west side of low-level vortex and takes on a cyclonic-shaped distribution ultimately.The tem-poral evolution of precipitation type depends on the temperature advection of the whole low-level tropo-sphere (850-950 hPa).When there is warm temperature advection in either 900-850 hPa or 950-900 hPa level,the precipitation type may still be rain even if cold advection exists in other levels.The coldtemperature advection of northwest wind at the west side of low-level vortex is very important to snow. When the low-level wind transforms to eastly wind,the low-level cold advection disappears,and finally, the precipitation type involves to rain.