北京一次强飑线过程的闪电辐射源演变特征及其与对流区域和地面热力条件的关系
北京一次强飑线过程的闪电辐射源演变特征及其与对流区域和地面热力条件的关系
Evolution of Lightning Radiation Sources of a Strong Squall Line over Beijing Metropolitan Region and Its Relation to Convection Region and Surface Thermodynamic Condition
| 中文摘要: |
| 利用2015年夏季北京闪电综合探测（BLNET）总闪辐射源定位、多普勒天气雷达、地面自动气象站和探空资料等多种协同观测资料，详细分析了2015年8月7日北京一次强飑线过程不同阶段的闪电特征，并探讨了闪电与对流区域和地面热力条件之间的关系。飑线过程整体上以云闪为主，根据雷达回波和闪电频数可以将飑线过程分为发展、增强及减弱三个阶段。发展阶段表现为多个孤立的γ中尺度对流降水单体，随着北京城区降水单体的迅速发展，强回波顶高延伸到-20℃温度层高度，闪电辐射源高度也逐步增加，闪电明显增多，但总闪电频数整体低于80次/min。增强阶段单体合并，闪电频数快速增长，0℃层以上及以下的强回波（＞40 dBZ）体积明显增大，飑线形成后，总闪和地闪均达到峰值，分别约248次/min和18次/min，负地闪占总地闪比例为90%，辐射源主要分布在线状对流降水区内，辐射源数量峰值出现在5～9 km高度层。减弱阶段飑线主体下降到0℃以下并迅速衰减，辐射源分布明显向后部层云降水区倾斜。95%的闪电发生在对流线附近10 km范围内，即对流云区和过渡区。在系统发展和增强阶段，对流云区与层云区辐射源的活跃时段基本一致；系统减弱阶段，对流降水云区辐射源数量迅速减少。在系统的不同发展阶段，闪电活跃区域对应于冷池出流同平原暖湿气流在近地面形成的相当位温强梯度带内。 |
| Abstract: |
| Based on the data obtained from the 2015 summer campaign in Beijing area, including the total lightning location data from Beijing Lightning Network (BLNET), S-band Doppler radar data, ground-based automatic weather stations observations and radiosonde data, the evolution of lightning activities during a severe squall line process that occurred over Beijing metropolitan region on 7 August 2015 was analyzed. Its relation to convection region and surface thermodynamic condition was also discussed. According to radar echoes and lightning occurrence frequency, the whole squall line process can be divided into three stages (developing, intensifying and weakening), and the intra-cloud (IC) lightning flashes predominated during all the three stages in general. In the developing stage, several isolated γ mesoscale convective cells rapidly developed. With the echo top of the storm cell over Beijing metropolitan region extending to -20℃ level, lightning activities significantly increased, and the lightning radiation sources gradually spread to upper altitudes, but lightning rate was still less than 80 flashes/min for the whole system. In the intensifying stage, the flash rate increased rapidly, which was associated with the merging process of the cells. When the squall line formed, the volume of strong radar echoes (＞40 dBZ) increased significantly for both above and below 0℃ levels, and the total flash and cloud-to-ground (CG) flash peaked with rates of 248 flashes/min and 18 flashes/min, respectively. Negative CG flashes accounted for 90% of the total CG flashes. The lightning radiation sources were mainly detected in the linear convection area, and the number of radiation sources peaked within the layer of 5-9 km. In the weakening stage, the core of the squall line dropped below 0℃ level and quickly decayed, with the radiation sources obviously sloping backward to the area of stratiform clouds. About 95% of total flashes occurred within 10 km of the convective line, namely the convection and transition region. During intensifying and weakening stages, radiation sources reached active period simultaneously in the convection and stratiform region, while during the weakening stage, radiation sources in the convection region declined abruptly in the number. Lightning flashes mainly occurred over regions with strong surface equivalent potential temperature gradient induced by the outflow of convective cold pool and the relatively warm moist airmass from the plain. |
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