Vertical reflectivity structures near lightning flashes in the stratiform regions of mesoscale convective systems

Using an S-band Doppler radar and a very high frequency (VHF) radiation source location system, 254 lightning flashes occurring in stratiform regions (stratiform lightning flashes) of 14 mesoscale convective systems (MCSs) in Chongqing, China, were observed and used to analyze the structure of vertical reflectivity in the area around the lightning channel. In most stratiform lightning flashes, the vertical reflectivity structures can be grouped into three general categories: single-layer, double-layer, and triple-layer. The single-layer structure flashes are the most numerous (65.0%), followed by the double-layer (26.8%) and triple-layer (1.2%) flashes. The regions with the single-layer structure and the double-layer structure are inferred to have mesoscale updrafts of different intensity in the middle of stratiform region. Around 62.3% of the single-layer structure flashes initiate within 0-4 km above the vertical reflectivity core, and the main initiation range of these flashes widens with increasing value of the vertical reflectivity core. Around 49.2% and 12.7% of the double-layer structure flashes initiate within 3-6 km and 1-2 km above the vertical reflectivity core, respectively. The probability of lightning initiation at a greater distance to the vertical reflectivity core is higher than that at a shorter distance. This may imply that some electrifications occurring during the melting process are not as strong as expected and a non-riming non-inductive charging process occurring in a weak but broad mesoscale updraft in the middle of stratifom region may be more important for initiation of stratiform lightning flashes.