Positive Charge Region in Lower Part of Thunderstorm and Preliminary Breakdown Process of Negative Cloud-to-Ground Lightning

摘要

A new lightning locating technology,called Lightning Mapping Array(LMA),has been developed.The system takes advantage of GPS technology to measure the times of arrival(TOA) of lightning impulsive very high frequency(VHF) radiation events at each remote location.The spatiotemporal development processes of lightning are described in three-dimension by measurement of the system with high time resolution(50 ns) and space precision(50-100 m).The charge structures in thunderstorm and their relationship with lightning discharge processes are revealed.The temporal and spatial characteristics of preliminary breakdown process involved in negative cloud-to-ground(CG) lightning discharges are analyzed based on the data of lightning VHF radiation events.The effect of positive charge region in lower part of thunderstorm on the occurrence of negative CG lightning discharge is discussed.The results indicate that the preliminary breakdown process with longer duration in negative CG lightning discharges is an intracloud discharge process.It occurs between negative and positive charge regions located in middle and lower parts of thunderstorm respectively. It initiates from the negative charge region and propagates downward.After propagating into the positive charge region,the lightning channel develops horizontally.The characteristics of the preliminary breakdown process are consistent with that of intracloud lightning discharges.The stepped leaders are initiated by the K type breakdown which occurs in the last stage of the preliminary breakdown process and develops downward through the positive charge region.The existence of positive charge region in lower part of thunderstorm results in the occurrence of preliminary breakdown process with longer duration before the return stroke of negative CG lightning discharges.