伴随对流层中低层气温持续下降的雪转雨过程分析

Analysis of a Snow-to-Rain Transformation Process Accompanying Continuous Temperature Decrease at lower and middle levels

利用欧洲中期天气预报中心(ECMWF)0.25°×0.25°分辨率细网格模式产品、探空观测资料和风廓线雷达等资料,对2014年2月18日浙江嘉兴雨雪天气过程中降水相态先由雨转雪、再由雪转雨的变化条件进行了分析,并对ECMWF细网格模式产品进行了预报性能检验,结果表明:模式形势预报准确,但未能预报出雪转雨过程。在对流层中低层气温持续降低的情况下,水汽凝结高度不同是造成两次相态转换的主要原因。上午垂直运动加强,水汽充沛,降水粒子的凝结高度高,足以形成大的雪花,在较低的零度层高度以下降落时不至于融化;下午垂直运动减弱,水汽集中在低层,尽管这一高度层的气温在-3^-2℃,但是不足以凝结成固态降水,同时地面气温受海上暖平流影响而回升,因此降水相态由雪转雨。

Base on the ECMWF analysis and forecast data（0.25°×0.25°）,wind profile data,and radiosonde data,the transformation process of rain-to-snow and then snow-to-rain in Jiaxing on 18 February 2014is analyzed.The result shows that the temperature at 850 hPa and 925 hPa declined during the daytime,but the transformation from rain to snow occurred in the morning and that from snow to rain in the afternoon.In the morning,abundant water vapor was lifted to the higher and colder height,and then condensed,forming dense snowflakes;when snowflakes fell,the temperature at lower levels was not high enough for them to melt.While in the afternoon,water vapor mainly existed at the low levels,where temperature was only between-2 ℃ and-3 ℃.Super-cooled water droplets instead of snowflakes formed in this environment,and at the same time surface temperature increased up to over 0 ℃ by the effects of warm advection near the ground from the sea,so the precipitation phase transformed from snow to rain.