Three-dimensional imaging of upward positive leaders in triggered lightning using VHF broadband digital interferometers

Upward positive leaders (UPLs) in two artificially-initiated lightning flashes were imaged in three dimensions using VHF broadband digital interferometers and a highspeed video camera with time-synchronized channel-base current measurements. Locatable VHF sources of the two UPLs began at 1.1 km and 1.5 km, a few milliseconds after the UPL inception, and ascended to 2.4 km and 3.7 km, respectively, with average 3-D speeds on the order of 10(6) ms(-1). The initial stage currents for both flashes were unusually large and had peak values of 6 kA and 18 kA. VHF sources associated with positive leader propagation were located when the average current was higher than 3 kA and had significant pulse activity. The source altitudes and channel-base currents suggest that there might have been a region of significant negative charge at altitudes from 2 to 4 km, which is below the freezing level of typical thunderstorms in Florida. Citation: Yoshida, S., C. J. Biagi, V. A. Rakov, J. D. Hill, M. V. Stapleton, D. M. Jordan, M. A. Uman, T. Morimoto, T. Ushio, and Z.-I. Kawasaki (2010), Three-dimensional imaging of upward positive leaders in triggered lightning using VHF broadband digital interferometers, Geophys. Res. Lett., 37, L05805, doi:10.1029/2009GL042065.