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一次多回击自然闪电的高速摄像观测
Observation of a Natural Cloud-to-ground Lightning with Multiple Return Strokes
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摘要: 2006年8月1日在广东省从化市利用成像率为5000幅/s的高速摄像系统观测得到了一次包含有13个回击过程的自然负地闪, 其梯级先导的传播速度为106 m/s的量级; 一次企图先导的传播速度随高度的降低而减小; 一次直窜先导的传播速度随高度的降低而增加。有3次继后回击相对积分亮度的峰值大于首次回击相对积分亮度的峰值。研究发现:此次自然闪电的继后回击及紧跟其后的连续电流过程的发光总量与该次继后回击之前闪电通道的截止时间有关, 较大的发光总量对应于较长的截止时间, 较小的发光总量对应于较短的截止时间, 但两者没有固定的比例关系。Abstract: A field experiment on artificially rocket-triggered lightning and natural lightning observation is conducted in summer of 2006 in Conghua, Guangdong. The luminosity characteristics of lighting discharges are observed simultaneously by using a video camera and a Redlake MotionPro HS-4 high-speed camera which consists of a 512×512 pin photodiode array module. A cloud-to-ground (CG) lightning with 13 return strokes (RS) is observed by using the high-speed camera with a frame rate of 5000 frames per second at 16:52:51 Aug 1, 2006. The distance from the high-speed camera to the flash is about 2.1 km. The record of the electric field on the surface of the earth shows that this CG lightning discharge transports the negative charge in cloud to the ground. All of the 13 strokes propagate along the same main channel and have one ground termination.The duration of this flash is 822.4 ms. The peak luminance of the 2nd, 3rd, and 8th return strokes are brighter than that of the first one, being 25% of all 12 subsequent return strokes. Of the 13 strokes, 6 (46 %) present some luminosity persistence after the return stroke frame. The durations of 2 persistence luminosity are in excess of 40 ms (CC (continuing current) following 9th stroke:157.6 ms; CC follow ing 10th stroke:221 ms).There are 17 Mcomponents occurring during the 6 continuing current occurrences. The luminosity characteristics of 3 leaders are examined. The average speed of the stepped leader is 2.23×106 m/s. It is a β-type leader according to the previous results which the stepped leader emerges from the cloud base and develops with a high average speed of the order of 106 m/s. The speed of the attempted leader before the 4th return stroke exhibits the decrease as it develops down toward the ground.The speed of the dart-stepped leader before the 13th return stroke appears to increase as it approaches the ground. Cut-off time is the time when the lightning channel shows no luminosity between two adjacent return strokes. In other words, it is the sum of the time from the frame that the previous return stroke and the CC followed disappears to the frame that the next return stroke is observed. Total luminosity is the sum of luminance of the frames including the return stroke and the CC followed. It is found that the cut-off time before a subsequent return stroke exhibits a consistent trend with the total luminosity of this subsequent return stroke and the CC followed (although there is no fixed proportionality between them), especially there is a proportionality relationship between them after the 9th return stroke. The geometric average of 17 Mcomponents' lasting time is 1.28 ms, and that of 11 intervals between the consecutive M-components is 1.69 ms.
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Key words:
- CG lightning;
- multi-return stroke;
- high-speed camera
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图 1 闪电通道相对积分亮度随时间变化
Fig. 1 Relative integrated luminosity of the CG lightning channel versus time
图 2 -1.0~0.0 ms的高速摄像照片
(a)-1.0 ms, (b)-0.8 ms, (c)-0.6 ms, (d)-0.4 ms, (e)-0.2 ms, (f)0.0 ms
Fig. 2 Frame sequence of first leader/return stroke from-1.0 ms to 0.0 ms
(a)-1.0 ms, (b)-0.8 ms, (c)-0.6 ms, (d)-0.4 ms, (e)-0.2 ms, (f)0.0 ms
图 3 经处理的184.6~186.2 ms高速摄像照片
(a)184.6 ms, (b)184.8 ms, (c)185.0 ms, (d)185.2 ms, (e)185.4 ms, (f)185.6 ms, (g)185.8 ms, (h)186.0 ms, (i)186.2 ms
Fig. 3 Frame sequence processed from 184.6 ms to 186.2 ms
(a)184.6 ms, (b)184.8 ms, (c)185.0 ms, (d)185.2 ms, (e)185.4 ms, (f)185.6 ms, (g)185.8 ms, (h)186.0 ms, (i)186.2 ms
图 4 第4次回击前企图先导速度随高度变化
Fig. 4 Variation of speed of the attempted leader before the 4th RS with height
图 5 802.4~804.6 ms的高速摄像照片 (其中图b~f经过了亮化处理)
(a)802.4 ms, (b)802.6 ms, (c)802.8 ms, (d)803.0 ms, (e)803.2 ms, (f)803.4 ms, (g)803.6 ms, (h)803.8 ms, (i)804.0 ms, (j)804.2 ms, (k)804.4 ms, (l)804.6 ms
Fig. 5 Frame sequence from 802.4 ms to 804.6 ms (Fig. b-f have been enhanced in luminosity)
(a)802.4 ms, (b)802.6 ms, (c)802.8 ms, (d)803.0 ms, (e)803.2 ms, (f)803.4 ms, (g)803.6 ms, (h)803.8 ms, (i)804.0 ms, (j)804.2 ms, (k)804.4 ms, (l)804.6 ms
图 6 第13次回击前先导速度随高度变化
Fig. 6 Variation of speed of the dart-stepped leader before the 13th RS with height
图 7 回击前截止时间和回击及其后连续电流过程发光总量变化
Fig. 7 Variations of cut-off time and total luminosity with the order of return stroke
图 8 第9次 (a) 和第10次 (b) 回击后长连续电流过程的亮度变化波形
Fig. 8 Waveshapes of long continuing current with M-components following the 9th (a) and the 10th (b) return stroke
表 1 2006年8月1日地闪的一些物理特征
Table 1 List of some physical parameters of the CG lightning on August 1, 2006
表 2 此次自然闪电出现的17个M分量持续时间 (单位: ms)
Table 2 Characteristics of M-components involved in the lightning (unit:ms)
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