Positive lightning: An overview, new observations, and inferences

Abstract

We examine the various conceptual cloud charge configurations and scenarios leading to production of positive lightning with a view toward an explanation of its observed properties. Data for 52 positive cloud-to-ground flashes containing 63 strokes recorded in Gainesville, Florida, in 2007-2008 are presented and discussed. The U.S. National Lightning Detection Network (NLDN) located 51 (96%) of the positive strokes at distances of 7.8 to 157km from the field-measuring station. In 42 (81%) of the 52 flashes, the first (or the only) leader-return stroke sequence was not preceded by in-cloud discharge activity. The average number of strokes per flash is 1.2. Single-stroke flashes constitute 81%. In three (38%) of eight two-stroke flashes, the second stroke likely followed the channel of the first stroke, and in five (62%) flashes it likely created a new termination on ground. Eight (15%) of the 52 positive flashes had detectable preliminary breakdown pulse trains in their electric field records. Electric field waveforms of 14 (27%) first strokes (also of one third stroke) are preceded by pronounced pulses, apparently indicative of leader stepping. NLDN-reported peak currents for 48 positive strokes in 40 flashes ranged from 20 to 234 kA with a geometric mean value of 75 kA. The median charge transfers in 1, 2, and 40ms after the beginning of return stroke electric field change for an assumed height of 12km were estimated to be 13, 18, and 34C, respectively. Two bipolar lightning discharges are additionally examined. Copyright 2012 by the American Geophysical Union.