Lightning activity and microphysical structure characteristics during the convective cell mergers in an extreme mesoscale convective system

This study examined the evolution of storm and lightning activity in an extreme mesoscale convective system (MCS) over the Hubei Province region using 3D lightning location instruments, S-band Doppler weather radars, dual-polarization radar, and other meteorological datas. Peak lightning activity occurred mainly during the bow echo (BE) and bow-and-arrow echo (BAA) stages, and the number of total lightning flashes (TLF) after the merger was nearly twice that at the beginning. Combined with the established hydrometeors model, the cold pool produced in the convective cloud of the BE stage was shallower and weaker, the formation of lightning flashes was related to the vertical transport of ice crystals and snow. However, the cold pool of the BAA stage was deeper and stronger, and the convective merger between rear convective and frontal cells by persistent merging. This mode may extend these pre-existing subregions and may be related to horizontal transport. The positive vertical velocity was driving convective cell development during the BAA stage, and the connecting bands between convective cells consisted of graupel and snow as well as raindrops. These may be the main reasons for the differences in lightning activity produced by the BE and BAA stages. Overall, this study advances our understanding of the effects of convective merger on the formation of lightning flashes. •The effects of convective merger on lightning flashes in an extreme MCS are investigated.•The number of total ground flashes (TCG) after the merger is nearly twice that at the beginning.•The outflow boundary (OB) provides favorable triggering conditions for lightning activity.•The main reasons for the differences in lightning activity are how convective merger and the transport mode.