The Lightning Characteristics Of The Strong Squall Line Process With The Phenomenon Of Convective Merger And Its Relationship With The Dynamic Field

Based on the data from Beijing Lightning Network(BLNet),S-band Doppler radar data and radiosonde data,the total lightning activity characteristics in different convection area of two severe squall lines with cell merging process were analyzed in detail,and then characteristics of lightning activities before and after cell merging were also discussed.Results are as follows:(1)Lightning activities of two squall line cases were mainly intra-cloud(IC)lightning flashes;the cloud-to-ground(CG)lightning flashes were dominated by negative CG(NCG);the flash rate increased sharply when the convective cells were merging,while the ratio of positive CG to CG(PCG/CG)increased obviously in the dissipation stage.In both of cases,about 85% and 95% of lightning flashes were distributed within the 10 km range of the convection line respectively,while flashes occurring in the stratiform region were relatively less.The radiation sources were mainly distributed in the convection area,and sloped along the convective line to the stratiform behind.There were obvious bright bands in the stratiform,but no lightning occurred near the bright bands.Comparing the two cases,the Black-Body Temperature(TBB)in the case of July 27 th 2015 is obviously lower than that of August 8th 2017,and the number of lightning was 3 times higher than that of August 8th 2017,with a higher ratio of IC to CG and a larger flash number in per unit area of convection.(2)There were three merging processes in the two squall lines.It can be known that lightning frequencies in merged regions reached to the peak within 12-30 minutes after the three merging processes.According to the radar and the dynamic field retrieved by Variational Doppler Radar Analysis System(VDRAS),there was a large amount of water vapor concentration during the convection merging process,and the peak of lightning frequency was 24 min lagging behind the peak of vertical integral liquid water content(VIL).Concentration of water vapor in the convection process provided material conditions for the formation of charged particles.After the convection cloud merged,convection would develop,clouds would rise,and charged particles would be lifted to a higher position,which was more conducive to the occurrence of cloud flash.(3)According to the wind field retrieved from the VDRAS,the merging of convective clouds mainly occurred in the lower convergence zone.After cells merged,both the extent and intensity of updraft became intense,and lightning activity became more active.Lightning mainly concentrated in the region with strong vertical wind shear,meanwhile a small fraction of flashes occurred in the rear of the convection zone with downdrafts.