Three-dimensional mapping of two coincident flashes - An upward positive flash triggered by the in-cloud activity of a downward negative flash

Abstract Lightning mapping with radio frequency location systems plays a more and more important role in understanding the mechanism of lightning discharges during thunderstorms. Here we present a comprehensive analysis of the electromagnetic field measurements and 3-dimensional (3D) mapping of two coincident lightning flashes: an upward positive flash (+CG) with long preceding upward negative leader (UNL), which was triggered by the in-cloud (IC) activity of a nearby downward negative flash (−CG). The flashes were mapped with a low frequency (LF) mapping array deployed near Duke University and during a late-spring thunderstorm passing a plain area. The 3D LF maps show that the IC activity related to the −CG moved negative charge away from the location overhead the UNL. The UNL developed firstly upward for about 2 km and then extended horizontally for about 8 km at that height with a mean speed of 6.2 × 105 m/s. The channel line charge density of the UNL was estimated to range from −542 μC/m to −9.21 mC/m with a mean value of −3.46 mC/m. The total positive charge transferred from the cloud to ground by the +CG flash was estimated to be about 5.8C. From the 3D maps, we inferred that the IC activity relating to the −CG flash which moved negative charge away from overhead region of the UNL could be responsible for the UNL initiation. And the existence of extensive lower positive charges further favored the long horizontal propagation of the UNL inside the cloud.