Precipitation Structure of a Convective Cell Involving a Lot of Cloud-to-Ground Flashes

We have been observing thunderstorms in Kansai region, Japan, with a use of 3D lightning mapper, called Broadband Observation network for Lightning and Thunderstorm (BOLT) and two phased array weather radars (PAWR) for further understanding of relationship between electrical characteristics of storms and storm kinetics. BOLT is a LF sensor network to locate LF emissions associated with both intra-cloud (IC) and cloud-to-ground (CG) flashes. PAWR is an X-band weather radar that employs electrical and mechanical scanning, respectively, in elevation and azimuth direction, resulting in observing the whole sky in as short as 10 seconds.

In this presentation, lightning activity and storm kinetics of convective systems that occurred on 30 July 2015 are analyzed in detail. One convective cell developed almost over PAWR and, after that, the convective cell was split into two convective cells; one of the two developed southwards (Cell A) and the other developed southeastwards (Cell B). Both two convective cells had about 25 minutes of electrical active phase. Cells A and B, respectively, involved 142 and 172 of total (IC and CG) flashes estimated by BOLT in the 25 minutes. Although Cell A had smaller number of total lightning flashes, Cell A involved substantially larger number of CG flash than Cell B had. Cells A and B, respectively, had 50 and 13 CG flashes, and the IC to CG lightning ratio, which is often termed Z, in Cells A and B are 1.8 and 12.2, respectively.

We focus on precipitation structure of Cell A that produced more CG flashes. In Cell A, IC flash rate had a steep increase and its peak of 10 min^-1, and then CG flash rate had a peak of 5 min^-1 7 minutes after the IC flash rate peak. During the increase phase of IC flash rate, the radar observation indicates that the echo top height and updraft echo volume in the upper level increased. On the contrary, IC flash rate decreased when the updraft in the high altitudes weakened. Cell B also shows similar relationship between IC flash rate and updraft in the high altitudes.

CG flash rate in Cell A shows an increase after the updraft in the high altitudes weakened. CG flash rate peaked when radar reflectivity region, probably involving large precipitation particles such as graupel, descended from high altitudes to around the -10 degree isotherm level. The updraft in the lower level existed below the descent of radar reflectivity region when CG flash rate peaked. Most of main negative charges estimated by BOLT are located in and around areas where the descent of precipitation particles arrived. It seems that the graupel provided from high altitudes might have intensified the main negative charge region, resulting in facilitating to initiation of CG flashes.