一次回流型降雪过程的成因和相态判据分析

Causes and Phase Criteria of a Return-Flow Snowfall

利用常规高空和地面观测资料、天津铁塔和雷达资料、全球资料同化系统(GDAS)分析资料、雷达变分同化分析系统资料、EC和NCEP再分析资料对2016年11月20—21日天津初雪天气进行成因分析,结果表明:本次过程是在高空槽和回流冷空气共同作用下产生的,主要水汽来源为对流层中低层槽前西南暖湿气流和回流东风,回流东风经渤海低空运行时吸收水汽由"干冷"变为"湿冷";动力条件主要来自回流冷垫的动力抬升作用,降水期间回流东风层厚度由1.5km增加至2km;锋面上的非地转次级环流可将回流东风水汽向上输送成为降水原料,同时可加强其上暖湿空气的垂直上升运动;高空云水粒子向云冰粒子的转换和边界层回流冷空气加强对本次雨雪相态转换是不可或缺的,回流冷空气北风分量风速和厚度陡增、800～950hPa出现均温层、云冰粒子陡增并向低空延伸、700～850hPa与850～1000hPa厚度的变化特征对雨雪相态的判别均有较好的指示作用。

Based on the conventional meteorological data,observation data of the Tianjin Observation Tower,radar data,VDRAS data(Variation Doppler Radar Analysis System),and NCEP reanalysis data(ECWMF and National Centre of Environmental Prediction),the causes of the first snow weather process on 20 and 21 November 2016 in Tianjin are analyzed.The results show:the mixed rain-snow storm was produced in the interaction of the upper trough and return-flow cold air;the main sources of water vapor were the southwest warm and humid airflow in front of the trough in the middle and lower troposphere and the returning east winds.The returning east wind absorbed moisture when it passed through the low surface of the Bohai Sea and became wet and cold;the dynamic condition was mainly from the dynamic lifting of the cold return flow,the thickness of the returning easterly increased from 1.5 km to 2 km during precipitation.The non-geostrophic secondary circulation transported the returning water vapor upwards into a source for precipitation,and strengthened the vertical ascending motion of the warm and humid air to provide dynamic conditions.The north wind component wind speed and thickness of the cold air increased sharply.Simultaneously,the conversion of high-altitude cloud water particles to cloud ice particles and the reflow of cold air at the boundary layer were indispensable for the transformation from rain to snow,and the uniform temperature layer and cloud ice particles of 800 to 950 hPa increased greatly and extended to low altitudes.The variation characteristics of the potential thickness of 700 to 850 and 850 to 1000 hPa had good indication value for the discrimination of rain and snow phases.