北京盛夏一次罕见的大雹事件分析

Analysis on Infrequent Big Hail Event in Beijing Area

分析了1969年8月29日出现在北京的罕见大雹事件。冰雹发生在蒙古低涡低槽大尺度环流背景下,500hPa的冷空气叠置在低层暖空气上,为冰雹等强对流天气的出现提供了有利的条件。分析表明,华北东部的降雹区位于高能带中,北京在高能带的中心并且对流有效位能(CAPE)和抬升指数(LI)均达到了一个较高的水平,特别是在-10～-30℃层内有高的CAPE值,使大冰雹的出现成为可能。此外,对流层中下层较强的环境风垂直切变有利于多单体风暴或超级单体等强风暴云的发展,从而增加了冰雹出现的几率。而且,合适的冻结层高度以及冷暖云厚度比也为降雹创造了条件。

An infrequent big hail event occurring on 29th, Aug. 1969 is analyzed. A trough from Mongolia at 500hPa plays a triggering role. And the upper level cold air accompanying with the trough superposes the low level warm air, which leads to some instability. The calculation of CAPE and LI indicates that the hails fall in the instable region and Beijing is in the center of this zone with unusual great CAPE and LI. In addition, the strong wind shear in the low troposphere and the value of CAPE in the layer between - 10℃ and - 30℃ are both favorable to the big hail event. Finally, the suitable freezing level height and the ratio of cold and warm cloud thickness are also favorable to the hail shooting.