山东地区冰雹云的闪电活动特征

Cloud-to-Ground Lightning Characteristics of Hail Clouds in Shandong Province

利用山东电力部门提供的雷电定位资料,对10次冰雹过程的地闪活动特征进行了分析。通过分析发现,雹暴中正地闪占总地闪的比例平均为57.39%,远高于当地正地闪比例的气候特征值13.48%。地面降雹区基本出现在正地闪密集（活跃）区或邻近区域。在雹云快速发展阶段,地闪频数存在明显的“跃增”;在减弱消散阶段,地闪频数显著减少,但正地闪比例有所提高。负地闪频数峰值的出现通常提前于降雹0～20 min,正地闪频数峰值的出现一般滞后于降雹发生时间。整个降雹阶段对应于正地闪的活跃阶段。另外,结合对卫星观测的总闪电资料分析,发现冰雹云的云闪与地闪的比值远高于一般的雷雨过程,其云闪密度也远高于雷雨过程。以上这些特征对于冰雹的识别和对冰雹的超短时预报具有指示意义。

There are usually intensive electrification and active discharges in severe hailstorms because of strong updraft and abundant ice particles and supercooled water content. Although a lot of researchers have analyzed the lightning activities in hailstorms and many useful results have been achieved, the evolution of lightning activities and its relations with hail-falling in hailstorms are not quite clear up to now. In order to help to understand the lightning characteristics in hailstorms and the application of lightning information to the warning of severe convective weather, the cloud-to-ground （CG） lightning activities of hailstorms occurring in Shandong Province are studied in this paper. The CG flashes were observed by Shandong Lightning Detection Network （SLDN）, which consists of 10 sensors covering all over the Shandong Province. Firstly, the characteristics of CG lightning polarity, spatial and temporal distribution of ten hailstorms are analyzed statistically. The results show that the mean proportion of positive CG lightning accounting for total CG lightning is 57.39%, which is much higher than the local climatic normal rate by 13.48%. The hail-falling is often reported in the region of dense positive CG lightning. Sometimes hailstone appears slightly at right flank of the dense CG lightning region. There is a distinct intensive increase in CG flash rate in the quickly developing period of hail cloud, while the rapid reduction in CG flash rate occurs at the dissipating stage. The hailstone falling corresponds to an active positive flash period, and the increase of positive CG flash rate is generally accompanied with a decrease of negative CG flash rate. The peak negative CG flash rate usually occurs 0 to 20 minutes earlier than hailstone falling, but the peak positive CG flash rate usually is late or simultaneous for the occurrence of hailstones. When the polarity of CG lightning changes, it often indicates the advent of severe weather such as hail, strong wind and heavy precipitation. Secondly, using the advantage of TRMM LIS being able to detect total lightning, i.e. intracloud （IC） and CG lightning, the ratios of IC lightning to CG lightning in hailstorms are compared with those in common thunderstorms. The analysis discloses that both the ratio of IC to CG lightning and IC flash density in hailstorms are far bigger than those in the thunderstorms. This is mainly caused by elevated charge structures associated with strong updrafts. These features are likely useful for hailstorms forecasting. At last, the reasons of high percentage of positive CG lightning in hailstorms are discussed, and it is speculated that inverted tripole charge structure is likely leading to high ratio of positive CG lightning in hailstorms.