Optical and electrical characteristics of in-cloud discharge activity and downward leaders in positive cloud-to-ground lightning flashes

The characteristics of five downward positive cloud-to-ground (CG) flashes were analyzed based on images from a high-speed video camera and electric field (E-field) changes from slow antenna and fast antenna systems. The flashes persisted for 740ms to 1250ms. The luminous durations of the leaders ranged from 4ms to 24ms. The average 2-D development speeds of the positive leaders ranged from 0.3×105m/s to 2.0×105m/s. The propagation speeds of the positive leaders increased as they approached the ground. Approximately 53.9% of the 89 observed positive CG flashes exhibited isolated monopolar pulses during the leader propagation and immediately prior to the return stroke. The average time interval between adjacent leader pulses was 15μs according to the E-field changes. Most leaders of positive CG flashes (approximately 67.4%) began in the presence of prolonged and intense in-cloud discharge (IC) activity that ranged from 100ms to 973ms. A cloud discharge may be conducive to the formation of positive CG flashes. Generally, a higher occurrence of positive CG flashes corresponded to a higher occurrence of cloud flash, which suggests that positive CG flash can be initiated by a cloud discharge. Four positive CG flashes exhibited one return stroke, and only one flash had two strokes.