Intracloud Lightning Flashes Initiated at High Altitudes and Dominated by Downward Positive Leaders

Abstract

Intracloud (IC) lightning flashes are normally initiated below 10 km and start with upward negative leaders. In this paper, we report a special type of IC flash called “downward positive IC (+IC) flash” which is initiated at high altitudes (mainly above 12 km) and whose initial negative leaders do not propagate upward. Three-dimensional location results of three downward +IC flashes are described in detail. It is demonstrated that downward +IC flashes start with positive leaders propagating downward with speeds on the order of 104 m/s and negative leaders propagating horizontally for only a short distance. Downward +IC flashes are produced in thunderstorms with deep convective updrafts (radar echoes of cloud tops typically higher than 14 km). The charge structure responsible for downward +IC flashes is inferred to be a positive dipole including a negative charge region at a normal altitude (near the −10 °C isotherm) and an upper positive charge region at a relatively high altitude (usually above the −50 °C isotherm), with downward +IC flashes likely initiated from the upper positive charge region. Further, lightning flashes in a thunderstorm producing a large number of downward +IC flashes are analyzed. Results show that normal IC flashes in this thunderstorm are also initiated at altitudes closer to the upper positive charge region and usually consist of downward positive leaders propagating for longer distances than upward negative leaders. Based on these results, we propose a relationship between the altitude of the upper positive charge region and initiation locations of IC flashes.