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 high‐speed 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.