Lightning Activity During Convective Cell Mergers In A Squall Line And Corresponding Dynamical And Thermodynamical Characteristics

The convective cell mergers and their impact on lightning activity during an extreme severe squall line system that occurred over the Beijing Metropolitan Region (BMR) have been investigated by using observations from the Beijing Broadband Lightning Network (BLNET), S-band Doppler weather radar, and other meteorological data. During the merger, the external and local multi-cells were connected by triggering new cells between them, and subsequently, cloud bridges formed at 1?5 km AGL. The total flash rate (TFR) of the whole storm decreased slightly at the beginning of the merger, increased sharply later, and peaked when the merger was completed. The TFR of the rear cell reduced to around zero while the TFR of the front cells increased in different degrees, which showed that the merger process exerted different effects on individual cells. Also, the vertical profiles of radar reflectivity illustrated that the merger process tended to weaken the rear cell and strengthen the front cells in spatial scale and intensity, presented as a rear-cell feeding merger. The results from the Variational Doppler Radar Analysis System (VDRAS) are used to derive the corresponding dynamical and thermodynamical processes. The rear-inflow jet (RIJ) penetrated the trailing-stratiform region and extended into the lower layer of the front cells of the storm during the merger period. The resulting strong convergence and updraft of front cells brought water vapor from the bottom to the upper layers, which is conducive to the formation of ice particles, the possibility of noninductive electrification, and the subsequent lightning flashes.