许昌两次初春回流类雨转雪过程对比分析

应用micaps3天气图资料、卫星云图,对许昌市2011年2月25—28日和2014年2月16-18日两次雨转雪天气过程进行诊断分析和对比分析,对环流形势、水汽条件、动力、热力特征的分析结果表明：（1）两次过程的共同点为前期主要是500hPa高空槽和700hPa切变线影响的稳定性降水,后期是850hPa回流冷空气不断补充堆积形成深厚的冷垫,低层偏东气流与中高层槽前的西南暖湿急流交汇形成回流降水形势。另外,高低空的有力配合,冷暖空气的交汇为两场降水提供了较好的动力条件。（2）不同点为两次回流降水时段、持续时间、降水强度、降水落区不同。过程一水汽含量更高,辐合偏河南省北中部而且湿层更深厚,强度更强,水汽通量较过程二大一个量级,降水量较过程二也大一个量级。过程二辐合落区在河南省南部,且持续时间较短,系统配置较弱。（3）冷空气侵入时段、气温下降速度、降水时间、850hPa温度的临界值,都是是初春降水性质发生改变的关键点。其中850hPa温度低于-4℃作为雨转雪指标外,气温的下降速度可作为雨转雪的补充预报指标。

Comparison analysis of two rain turning to snow processes of refluxes in Spring of Xuchang

Based on micaps3 weather map data, satellite cloud picture, diagnosis analysis and contrast analysis of two rain turning to snow weather processes in 2011 February 25-28 and 2014 February 16-18 were done. Analysis results of the circulation, water vapor condition, dynamic, thermal characteristics show that：（1） the common points two processes, in the early stage one of them is stable precipitation affected by 500 hPa trough and 700 hPa the shear line, the other one is the deep cold pad formed by the accumulation of continuous supplying of 850 hPareturn cold air and a reflux rainfall situation formed by the intersection between the low-level easterlies and the top slot in the southwest jet stream in the late stage. In addition, cooperation between high and low altitude and the intersection of cold and warm air provide preferable dynamic conditions for two precipitation processes.（2） The differences are in reflux duration, precipitation, rainfall intensity and rainfall area aspects. In the frist process, the moisture content is higher; the convergence is towards North Central of Henan province with deeper, stronger wet layer and bigger water vapor flux（over two magnitudes than process two）, more precipitation（over one magnitude than process two）.On the other hand, in the second process, the convergence area is in southern of Henan Province with short duration and weak system configuration.（3） During the cold air invading, temperature declining rate, precipitation time and critical value of 850 hPa temperature are key points of precipitation character changes in early spring. Among these factors, the 850 hPatemperature below-4 ℃ is chosen as rain turning snow index, while the temperature declining rate can be used as replenishment forecast index for rain turning to snow forecast.