Characteristics of downward leaders in a cloud-to-ground lightning strike on a lightning rod

Abstract

A natural downward negative cloud-to-ground (CG) lightning was observed at a close distance of 370m by using electric field change measurements and a high-speed camera at 5400frames per second (fps). Two subsequent leader-return strokes of the lightning hit a lightning rod installed on the top of a seven-story building in Beijing city, while the grounding point for the stepped leader-first return stroke was 12m away, on the roof of the building. The 2-D average speed of the downward stepped leader (L1) before the first return stroke (R1) was approximately 5.1×104m/s during its propagation over the 306m above the building, and those before the subsequent strokes (R2 and R3) ranged from 1.1×106m/s to 2.2×106m/s. An attempted leader (AL) occurred 201ms after R1 and 10ms before R2 reached approximately 99m above the roof and failed to connect to the ground. The 2-D average speed of the AL was approximately 7.4×104m/s. The luminosity at tip of the leader was brighter than the channel behind it. The leader inducing the R2 with an alteration of terminating point was a dart-stepped leader (DSL), which propagated through the channel of AL and continued to develop downward with new branches at about 17m above the roof. The 2-D speed of the DSL at the bottom 99m was 6.6×105m/s. The average time interval between the stepped pulses of the DSL was approximately 10μs, smaller than that of L1 with value of about 17μs. The average step lengths of the DSL were approximately 6.6m. The study shows that the stepped leader-first return stroke of lightning will not always hit the tip of a tall metal rod due to the significant branching property of the leader. However, under certain conditions, the subsequent return strokes may alter the grounding point to the tip of a tall metal rod. For the lightning rod, the protection against subsequent return strokes may be better than that against the first return stroke.