一种浙江省冻雨落区的推算方法

A New Method of Calculating Falling Zone of Freezing Rain in Zhejiang

2008年初浙江省出现全省性的大范围强冻雨天气,在输电线路上形成很厚的覆冰,致使浙江电网遭受巨大的损失。然而,浙江省却仅有三分之一的气象站观测到冻雨,持续时间也很短。本文利用全球再分析资料ERA-Interim结合浙江电网覆冰灾情资料,分析了2008和2013年的两次强冻雨过程。研究表明浙江省强冻雨发生时具备冷暖冷的层结结构,且中间暖层气温>0℃,但相比湖南省,浙江省的暖层中心气温稍低,下层冷层厚度略厚,暖层中的液态水进人到下层冷层后易被冻结,落到低海拔地面为冰粒,或者低海拔地面层气温高于0℃,冻雨落到地面为降雨,所以冻雨期间浙江省绝大多数气象站(海拔在200 m以下)观测不到雨凇,观测到的多是冰粒或降雨;而在海拔较高的山区,冷层厚度变薄,液态水被冻结的概率大大降低,而且山区地面气温多低于0℃,有利于冻雨落在山区地面形成雨凇,因此浙江省冻雨多出现在浙中海拔400 m以上和浙南海拔600 m以上的山区。根据浙江省冻雨的特点,采用全球再分析资料进行冻雨落区推算,结果与浙江电网实际的覆冰灾情吻合得较好。本研究利用输电线路覆冰厚度确定冻雨强弱和分布,采用再分析资料推算冻雨落区,为地形起伏度较大的省份开展冻雨研究,进行冻雨监测和预报提供一条新的思路。

In 2008, the thick ice-coating caused by the severe freezing rain covered power transmission line in Zhejiang, leading to enormous damages to Zhejiang power grid. However, only one third of the weather stations observed the glaze with a short duration. In this paper, two severe freezing rain processes which occurred in 2008 and 2013 were analyzed using the global reanalysis data ERA-Interim and the disaster re- cords of ice-coating on Zhejiang power grid. The results show that there are three layers of cold-warm-cold with the warm layer＇s temperature above 0℃ during the heavy freezing rain process in Zhejiang Province. In Zhejiang, the liquid water falling from the warm layer would be frozen into ice particles because the temperature in warm layer is lower and the thickness of the bottom ice layer is thicker. Sometimes, the ground temperature is above 0℃, so rain would be observed at weather stations during freezing rain. However, the thickness of bottom ice layer become thinner at high elevation, then tiny liquid water can be frozen into ice particles after it passes through the bottom ice layer. The freezing rain can be found mostly above 400 m elevation of central Zhejiang and above 600 m elevation of southern Zhejiang. According to the freezing rain characteristics in Zhejiang, the falling zone of freezing rain can be calculated based on the global reanalysis data, and the results match the disaster records of ice-coating well. In general, this is a new method for freezing rain monitoring and forecasting, especially for some mountainous provinces like Zhejiang.