Observation And Analysis Of Characteristics Of Downward Stepped Leader In Cloud-to-ground Flashes

Recent years, using high-speed video camera observing development of natural downward leader has become one of the significant study methods. This paper mainly analyzed the developmental characteristics of downward-propagating stepped leader observed by Tall-Object Lightning Observatory in Guangzhou(TOLOG). Based on the records of high-speed video camera, ordinary video camera and electromagnetic field, three main conclusions were achieved:(1) The two-dimensional and three dimensional propagation characteristics of downward-propagating negative stepped leader: In all, we have observed 7 lightning cases that could clearly show the developmental characteristics of downward stepped leader since 2011; the three-dimensional and two-dimensional speeds of stepped leader ranged from 0.7 × 105 to 8.1 × 105 m/s(with height between 159-1,528 m) and ranged from 0.6 × 105 to 6.0 × 105 m/s, respectively. The ratio of the average three-dimensional speeds to two-dimensional speeds ranged from 1.2 to 1.6, with a mean of 1.4; for the connecting behaviors of the 7 lightning cases, the tip of downward leader to the tip of upward connecting leader(Type 1) accounts for 2/7, the downward leader’s tip to the lateral surface of upward connecting leader(Type 2) accounts for 4/7, and the combination of Type 1 and Type 2 accounts for 1/7.(2) The mechanism of stepping process: we present new observations of a natural negative lightning flash that occurred at a close distance of 350 m; nine pronounced field pulses, which we interpreted as being due to individual leader steps, were recorded. Inter-pulse intervals ranged from 13.9 to 23.9 μs, with a mean of 17.4 μs; using high-speed video cameras(10,000 and 50,000 fps), a total of 23 luminous segments were imaged and were found to be similar to their counterparts in long laboratory sparks; based on our observations, we proposed a generalized leader step formation process in natural negative leader and three possible space stem/leader development scenarios:(A) the space stem/leader fails to make connection to the leader channel;(B) the space stem/leader connects to the existing leader channel, but may die off and be followed, tens of microseconds later, by a low luminosity streamer;(C) the space stem /leader connects to the existing channel and launches an upward-propagating luminosity wave.(3) Characteristics of downward-propagating positive leader: we also observed four positive cloud-to-ground lightning flashes in our experiments. The two-dimensional speeds of downward-propagating positive leaders ranged from 0.8 × 105 m/sto 3.2 × 106 m/s(with height between 110-2,150 m), with an average of 8.8 × 105 m/s. In one positive lightning flash case, the three-dimensional speeds of a downward-propagating positive leader ranged from 2.8 × 106 m/s to 8.4 × 106 m/s, with a mean of 4.4 × 106 m/s and height between 308-1,588 m. Furthermore, the positive leader also presented stepped characteristic like negative one. The time-interval ranged from 12.2 to 19.0 μs, with a mean of 15.6 μs.