Effects of Convective Mergers on the Evolution of Microphysical and Electrical Activity in a Severe Squall Line Simulated by WRF Coupled With Explicit Electrification Scheme

To investigate the process of convective merger (CM) and its effect on thunderstorms evolution and corresponding electrical activity, a severe squall line that took place on 27 July 2015 over the Beijing Metropolitan Region (BMR) is simulated and studied using the Weather Research and Forecasting (WRF) model coupled with an explicit electrification lightning scheme (E-WRF). The model-simulated radar reflectivity reasonably captured the whole squall line evolution, including the merging of individual cells and storms. The variation of normalized flash frequency simulated by E-WRF is highly consistent with the occurrence of a prominent flash rate surge during the CM both in the observation and the simulation. Cloud bridge is one of the regions where lightning events increase most. The upstream anvil and the sinking outflow led to the rapid connection and formation of new convective cells between the two older main storms. Subsequently, the mass of graupel and snow increased substantially at the middle level, and the mass of upper-level ice was horizontally advected from upstream. During the merger, the in-cloud charge distribution evolved from a staggered charge pocket structure into a vertically stratified five-layer structure. This study supports the hypothesized role of CM in enhancing lightning activity, and further expands our understanding of CM effects on microphysics and charge-redistribution.