Lightning flashes with high origins

Abstract

We studied detailed, time‐resolved maps of 21 cloud flashes that began at heights above 7.4 km above mean sea level by mapping sources of radio noise emitted by the flashes at 355 MHz. The electric field changes caused by these flashes were also recorded. All 21 flashes began as negative flashes, but one changed polarity while its stepped leader was in progress. Stepped leaders all emitted pulses at rates near one pulse per millisecond. Stepped leaders extended at speeds whose median value was 7.5×104 m/s. Their line charge densities ranged from 0.7 to 8.7 C/km, and the median value was 3.3 C/km. Stepped leader currents range from 37 A to 630 A; the median of 13 was 130 A. Most flashes (17 of 21) produced more than one stepped leader; 10 of 21 path‐lengths were longer than 17 km. Of 93 stepped leader channels, 53 were mainly horizontal, 26 were directed mostly upward, and 14 were mainly downward. The overall behavior of these cloud flashes was similar to that reported previously by Proctor et al. [1988] for ground flashes. Stepped leader paths extended in “forward” directions while emitting pulses and registering large, slow changes in electric field at ground level. The combined action of many rapid and intermittent Q streamers caused the flashes to extend slowly in “reverse” directions. A model for high cloud flashes is proposed. Most of these flashes began where the radar reflectivities were 20 dBZ. Some of the stepped leader paths were influenced by 20‐dBZ contours. We concluded that the origins were regions of high electric field that bordered charged regions that were probably screening layers, and that positive screening layers were threaded by the flashes. Evidently, positively and negatively charged screening layers existed simultaneously at separate regions in the thunderclouds. All 21 flashes extended beyond the 20 dBZ surfaces; 19 protruded extensively. We concluded that these regions contained diffuse, positive charge.