暖季两次高架雷暴的不稳定性及触发机制

Instability and Triggering Mechanism of Two Elevated Thunderstorms in Warm Season

摘要:

利用高空、地面气象观测资料和欧洲中期天气预报中心（European Centre for Medium-Range Weather Forecasts，ECMWF）第五代大气再分析资料（ERA5），分析了2008年6月25日和2017年7月21日发生在京津冀南部和中部两次暖季高架雷暴天气，探寻对流发生时大气的不稳定性与触发机制。结果表明：(1)两次暖季高架雷暴发生在低层大气相对较冷的稳定层结状态下，强逆温之上为条件性不稳定时，对流强度更甚于弱逆温之上的稳定层结过程。(2)中高层的对流不稳定层结早于高架雷暴8 h建立，强逆温高架雷暴不稳定度相对更高，利于生成雷暴大风，而表现为强降水天气的对流不稳定性相对较弱；K指数对暖季高架雷暴有较强反映，SI和T85对雷暴大风的出现更为敏感，而LI对强降水预报的应用效果更好。(3) 两次高架雷暴分别发生于低涡东南象限和副热带高压控制的不同天气背景下，逆温层附近或之上大气表现出对称不稳定特点，有斜升气流发展。(4) 925～700 hPa的锋生，以及850 hPa的纬向风切变对强对流的形成有着显著的触发作用，锋生高度下降越快、强度越强，越有利于大风出现，而锋生持续时间长，则强降水持久，冷垫之上强烈垂直风切变的长时间维持则预示了强降水雷暴单体的发展与传播。

Abstract:

Using multiple sources such as high-altitude, ground-based meteorological observatories and the European Centre for Medium-Range Weather Forecasts (ECMWF) fifth-generation atmospheric reanalysis data ERA5, two warm season elevated thunderstorms were analyzed in the southern and central parts of Beijing-Tianjin-Hebei on 25 June, 2008 and 21 July, 2017. The stability and triggering mechanism of the atmosphere when convection occurred were discussed. The results are as follows. (1) In warm season, elevated convection occurred in the relatively cold and stable stratification state of the lower atmosphere. In the case of conditioned instability over strong ininversion, the convective intensity was stronger than that over weak ininversion. (2) The convective instability layer in the middle and upper atmosphere was established 8 hours before the elevated thunderstorm. The instability of the elevated thunderstorm with strong inversion was higher, which was conducive to the generation of severe thunderstorm winds, while the convective instability for precipitation was relatively weak. K index had a stronger response to elevated thunderstorms in the warm season, the SI index and T85 were more sensitive to the occurrence of severe thunderstorm winds, and the LI index was better for the application of severe precipitation forecast. (3) The two elevated thunderstorms occurred under different weather backgrounds, which were in the southeast quadrant of the vortex and under the subtropical high. The atmosphere near or above the inversion layer showed the characteristics of symmetrical instability, it indicated slanting updraft development. (4) The frontogenesis between 925 and 700 hPa and the latitudinal wind shear at 850 hPa had a significant triggering effect on the formation of strong convection. The faster the height of frontogenesis falled and the stronger the intensity was, the more favorable the gale would appear. If the frontogenesis duration was long, the heavy precipitation would persist. If the vertical wind shear on the cold mat was maintained for a long time, the development and propagation of heavy precipitation thunderstorm cells were predicted.