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Optical and Electric Field Observations of Two Concurrent Upward Flashes Triggered by a Positive Cloud-to-ground Flash
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摘要: 广州高建筑物雷电观测站光电同步观测系统于2017年6月16日记录到一次峰值电流达+141 kA的单回击正地闪触发两个并发上行闪电过程。利用高速摄像、普通摄像和电场变化数据分析了触发型上行闪电的始发特征和机理。结果表明：正地闪回击后约0.8 ms内，在距正地闪接地点约3.9 km的广州塔（高600 m）和4.1 km的东塔（高530 m）分别有上行闪电始发。正地闪回击过程中和大量正电荷以及之后可能有云内负先导朝高塔方向快速伸展造成塔顶局部区域的电场发生突变是两个上行闪电激发的原因。两个上行闪电在353 ms内发生7次回击，其中6次在广州塔上，仅1次在东塔上，且广州塔回击峰值电流平均值（-21.4 kA）约为东塔回击峰值电流（-7.3 kA）的3倍，表明广州塔上行闪电通道可能比东塔上行闪电通道伸展至分布范围更广、电荷量（或电荷密度）更大的负电荷区。两个上行闪电先导的二维速率变化范围为9.4×104~1.8×106 m·s-1，平均值为6.9×105 m·s-1。Abstract: Tall objects are easy to trigger upward flashes. They also provide more opportunities for observation of upward flashes and a suitable platform for analyzing the intiation and development of upward flashes. Upward flashes are usually divided into two types, one is self-initiated and the other is triggered by environment, and the latter type is often related to the activity of positive cloud-to-ground (CG) flash. At present, development characteristics and triggering mechanism of the upward lightning channel are mainly studied based on the image of the extracloud lighting channel, the ground electric field change data and radar echo data. Few examples of mutiple upward flashes triggered by the same positive CG flash on different tall objects have been observed, and detailed characteristics of upward flashes are not well understood.On 16 June 2017, a positive CG flash (single stroke, peak current +141 kA) is recorded by the lightning photoelectric synchronous observation system of Tall Object Lightning Observatory in Guangzhou (TOLOG), triggering upward flashes of two nearby super-high-rise objects. Results show that within about 0.8 ms after the return stroke of the positive CG flash, two upward negative flashes intiate from the Canton Tower (600 m high) and the East Tower (530 m high), which are about 3.9 km and 4.1 km away from the positive CG flash, respectively. The initiation of two upward flashes could be caused by combined effects of the return stroke of positive CG flash, its associated continuing current, and the negative leader in the cloud approching to towers. A total of 7 strokes occur in about 353 ms, 6 strokes on the Canton Tower and 1 stroke on the East Tower, and the leader/return stroke sequence on the Canton Tower occur after the leader/return stroke sequence and the attempt leader on the East Tower, i.e., there is no overlapping between strokes of two upward flashes. The average peak current (-21.4 kA) of return strokes of upward flash initiating from the Canton Tower is about 3 times of the peak current (-7.3 kA) from the East Tower. It's supposed that the negative charge region in the upper cloud of the Canton Tower is wider than that in the upper cloud of the East Tower, and the charge amount is larger. The 2D velocity range of the positive leader of two upward flashes is 9.4×104 to 1.8×106 m·s-1, with an average of 6.9×105 m·s-1.
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Key words:
- tall object;
- positive cloud-to-groud flash;
- upward flash
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图 1 单反相机拍摄的闪电发光通道静止照片(曝光时间为1 s)
Fig. 1 The still image of lightning channel obtained using a digital SLR camera with an exposure time of 1 s
图 2 站点1、站点2及上行闪电和正地闪接地点相对位置
Fig. 2 The position of Station-1, Station-2, the initial position of upward flashes and the ground termination point of positive cloud-to-ground flash
图 3 站点1的高速摄像(HC-3，1000 fps)拍摄的4幅不连续图像(图像进行了反相和对比度增强处理)
(a)-44 ms，(b)-1 ms，(c)1 ms，(d)19 ms
Fig. 3 Four(not all consecutive) images obtained using the high-speed video camera(HC-3, 1000 fps) installed at Station-1(images are inverted and contrast-enhanced)
(a)-44 ms, (b)-1 ms, (c)1 ms, (d)19 ms
图 4 站点1高速摄像(HC-2，50000 fps)拍摄的4幅不连续图像(图 4a进行反相处理，图 4c和图 4d进行对比度增强处理)
(a)0 μs，(b)20 μs，(c)720 μs，(d) 800 μs
Fig. 4 Four(not all consecutive) images obtained using the high-speed camera 2(HC-2, 50000 fps) installed at Station-1 (Fig. 4a is inverted, Fig. 4c and Fig. 4d are contrast-enhanced)
(a)0 μs, (b)20 μs, (c)720 μs, (d) 800 μs
图 5 图像亮度(a)、快电场(FA)(b)和慢电场(SA)(c)同步变化波形
(竖线表示正地闪始发时间(约-74 ms)，R+CG为正地闪回击，RET表示东塔上行闪电的回击过程，R1~R6为广州塔上行闪电6次回击过程，LET和LCT分别为东塔和广州塔上行闪电中的企图先导)
Fig. 5 Waveforms of changes in channel brightness(a), fast electric field(FA)(b) and slow electric field(SA)(c)(the vertical line denotes the initial time of the positive cloud-to-ground(CG) flash
(approximately-74 ms), R+CG denotes the return stroke of positive cloud-to-ground flash, RET denotes the upward flash of the East Tower, R1-R6 denote six strokes on the Canton Tower, LET and LCT are attempted leaders of upward flashes on the East Tower and the Canton Tower)
图 6 正地闪和上行闪电放电过程示意图
(“+”代表正电荷，“-”代表负电荷; 箭头表示通道的传输方向; ▲为由闪电定位系统获取的正地闪接地点位置; 带圈数字代表不同放电过程的先后顺序)
Fig. 6 Possible scenario of the initiation of two upward flashes
("+" denotes positive charge, and "-" denotes negative charge; arrows denote directions of channels extension, ▲ denotes ground termination point of positive cloud-to-ground flash A reported by the GDLLS; circled numbers denote the sequence of different discharge processes)
图 7 两个上行闪电正先导二维速率随高度变化
Fig. 7 Changes of 2D speed with height of positive leaders of two upward flashes
表 1 上行负闪回击特征参数
Table 1 Parameters of two negative upward flashes
始发建筑物 回击次序 峰值电流/kA 回击间隔/ms 东塔 1 -7 广州塔 1 -16 2 -10 25.3 3 -25 34.4 4 -27 51.6 5 -21 51.9 6 -30 68.2 
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