我国冬季冻雨和冰粒天气的形成机制及预报着眼点

利用探空和地面观测资料，通过对2001年冬季至2010年冬季我国不同区域（分为4个区域：北方、江南、华南、西南）的冻雨和冰粒天气形成的物理过程进行分析发现：（1）除北方区域外，我国其他区域的冻雨主要以暖雨机制为主。北方区域的融化类冻雨比例也仅为39％，但纬度越高，出现融化类冻雨的几率高于上述比例。暖层出现是冻雨天气的重要特征，但暖层作用主要是输送水汽和维持锋面系统，以保证降水的发生和持续，低层及地面气温普遍低于0℃可能是最重要的原因。（2）我国冰粒天气的形成机制主要以融化机制为主。冰粒天气的云顶高度普遍高于冻雨天气。冰粒天气的暖层厚度和强度均小于冻雨天气，这主要是由于弱暖层只是部分融化冰晶和雪花，使其重新冻结成为可能。冰粒天气的700hPa风速值普遍小于冻雨天气，这一方面说明冰粒天气对水汽输送条件要低一些，另一方面也反映了冰粒天气暖层较弱的特点。（3）云顶高度、暖层强度和厚度、低层冷层温度露点差、700hPa风速以及地面气温是甄别冻雨和冰粒天气的特征量，但不同区域，这些特征量的有效性不一样。西南区域冻雨和冰粒天气的主要差别在地面气温，其他特征量或差别不明显，或代表性不足，只可以作为辅助判断的因子。

Formation Mechanism and Forecast on Freezing Rain and Ice Pellet in Winter of China

Based on upper-air sounding and surface observations in winter of 2001—2010,the formation physical process of freezing rain and ice pellet at different regions are investigated.It is found that:(1) Except northern China region,the formation mechanism of freezing rain in China is mostly due to a warmrain process.In northern China region,freezing rain with melting process only accounts for 39%,but the higher latitudes,the higher ratio of melting process freezing rain.Warm layer aloft is an important characteristic of freezing rain weather.And its main functions are water vapor supply and frontal system maintaining, thus help to produce precipitation.Negative temperatures at low levels and surface could be the upmost factor for freezing rain.(2) Melting process is the main formation mechanism of ice pellet in China. Generally,cloud top of ice pellet is higher than that of freezing rain.Warm layer’s thickness and intensity of ice pellet are weaker than those of freezing rain.That is because weaker warm layer only partly makes ice crystal and snow aloft melt,thus ensure their refreezing within low levels.Wind speed at 700 hPa of ice pellet is commonly smaller than that of freezing rain,which,on the one hand,reflects the lower requirement of water vapor supply for ice pellet,and weaker warm layer aloft on the other hand.Cloud top, warm layer thickness and intensity,low-level dew point depression,wind speed at 700 hPa and surface temperature are good factors to discriminate freezing rain from ice pellet,but in different regions,the effectiveness of these factors is different.The main difference of freezing rain and ice pellet in Southwest China is the surface temperature,and other factors play a secondary role due to unobvious difference of them.