A Distinct Class of Negative Cloud-to-ground Flashes Observed by a Broadband Interferometer

Two-dimensional and three-dimensional VHF source mapping systems of electromagnetic (EM) waves emitted by lightning discharge progression have been established by a unique technique based on _gthe broadband digital interferometry_h. Lightning Research Group of Osaka University (LRGOU) has been conducting lightning observations in Darwin during summer thunderstorm seasons. The main objective of the Darwin campaign is the investigation of thunderstorm activity in inter tropical convection zone (ITCZ) from the aspect of VHF observations. Through these observations real time monitoring in terms of two-dimensional are realized and three-dimensional images of lightning channels by the post processing are obtained. We pay our attention to leader propagation channels of negative cloud-to-ground (CG) flashes especially their initiations, and 2 categories are identified. One is characterized that negative leaders begin at an altitude of around 8 kilometers and go down to the ground for several tens milliseconds. The other, in contrast, has some intermittent VHF radiations at 10 kilometers agl about 1 millisecond before continuous leader development which one has. Narrow bipolar pulses (NBPs) are clearly noticed in the electrical field changes in synchronization with VHF radiations at high altitude. The observations with cross-polarized radar show that the main precipitation particles are wet graupel at an altitude of 8 kilometers and the dray snow is dominant above 10 kilometers high. Assuming the tri-pole electric charge structure based on the riming electrification mechanism, it is considered that the former is triggered at lower positively charged region or between lower positive and dominant negative charge region, and the latter is triggered at higher positively charged region or between higher positive and dominant negative charge region. Additionally, the ignitions at high altitude could be powerful events. We speculate that VHF radiations at high altitude relate to transionospheric pulse pairs (TIPPs) and/or compact intracloud discharges (CIDs). This work was supported by grant of Tropical Rainfall Measuring Mission (TRMM) 3rd Research Announcement of NASDA, Japan, and the Ministry of Education, Science, Sports and Culture, Grant-in-Aid for Scientific Research (A), 14254001, 2002. The authors thank them for their support.