采用西南地区 (25°N～35°N, 95°E～110°E) 96个常规气象观测站1961年1月1日～2009年12月31日逐日气象要素资料, 利用同时满足地面日最低气温在-10～1 ℃、 相对湿度大于80%、 日照时数小于等于2个小时这三个条件, 计算1月中国西南地区覆冰日数。利用NCEP/NCAR再分析格点资料, 分析大气环流特征。结果表明: 强覆冰年时欧亚地区500 hPa高度距平场为 “北高南低”、 中高纬地区 “西高东低”、 中低纬地区“西低东高” 的形势, 乌拉尔山高压脊、 里海东部低压槽、 副热带高压均偏强, 有利于冷暖空气在西南地区强烈交汇, 是西南地区形成覆冰的基本条件; 冬季中东急流强, 则有利于西南地区覆冰的形成, 相关最好的区域位于四川—云南—贵州三省交界处、 贵州省大部分地区、 云南省北部与西藏东南部交界处、 陕西省西南部以及川西二郎山附近, 这些均是最易发生严重覆冰的区域; 500 hPa中亚低槽活动, 将中东急流变化与西南地区覆冰强弱紧密的联系了起来; 前期12月北大西洋百慕大群岛附近海表温度的异常偏低是1月中东急流异常偏强的重要影响因素。
Calculating the number of frozen days in January in southwestern China when satisfying the conditions with daily minimum temperature in the context of -10℃ and 1 ℃, relative humidity greater than 80%, and sunshine duration less than or equal to 2 hours at the same time, by using daily meteorological elements data for the period of 1961－2009 from 96 stations in southwestern China. Using NCEP/NCAR reanalysis grid data, the circulation features in the icing and no-icing years were analyzed. The results show that, in icing years in southwestern China, the 500-hPa height anomaly field are “north-higher and south-lower” in Eurasia, “west-higher and east-lower” in the high latitudes, and “west-lower and east-higher” in the low latitudes, indicating that the high pressure around the Ural Mountains, depression around the Caspian Sea, and the subtropical high are all strong, in favour of cold and warm air confluence in southwestern China, which is the basic condition of icing there; the best related areas of the Middle East jet stream and the icing in southwestern China are the boundary of Sichuan－Yunnan－Guizhou provinces, most of Guizhou Province, the boundary between northern Yunnan Province and southeastern Tibet, the southwest of Shaanxi Province, and about Erlang Mountain, which are the most areas of strong icing on average. Further studies indicate that 500-hPa trough activity around Central Asia is the bond between the Middle East jet stream and the icing in southwestern China. If the sea surface temperature near Bermuda in the north Atlantic is colder in the prior December, the Middle East jet stream will be much stronger in January.