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3D Spatial-temporal Characteristics of Initial Breakdown Process in Lightning Observed by Broadband Interferometer
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摘要: 闪电的起始位置和起始阶段发展速度是闪电研究中的重要问题。2010年夏季，使用架设在广州市从化区的两套甚高频 (VHF) 宽带干涉仪对闪电的起始阶段放电过程进行三维定位观测。对观测数据给出的地闪和云闪的起始高度分布特征以及起始阶段击穿过程的时空发展特征进行统计和对比分析，结果表明：闪电的起始高度分布呈双峰值特征，分别在5.0 km和8.8 km有两个明显的分布峰值，符合雷暴云三极性总体电荷结构的描述。对起始阶段闪电放电发展速度的计算表明，云闪和地闪在起始阶段的前15 ms内的平均发展速度均在104~105 m·s-1量级之间；多数云闪、地闪起始阶段前15 ms内的平均发展速度表现出减速趋势，但云闪个例中起始阶段前10 ms存在减速趋势的比例更高，且其中在前15 ms一直保持减速趋势个例所占比例也大于地闪。云闪和地闪的起始阶段放电过程的发展方向有向上、向下和水平发展3种情形，可用于指示闪电始发位置的环境电场方向。Abstract: The location from where lightning initiate and the initial breakdown process of lightning are both important issues in research of lightning. In the summer of 2010, a series of 3D location data of lightning initial breakdown process are observed by two VHF broadband interferometers, which can provide the image of the development of lightning discharge with a time resolution of 5 μs and a space resolution better than 1 km. The initiation height distribution and 3D time-space characteristic of initial breakdown process in lightning are given by some statistical analysis on 80 intra-cloud lightning records and 61 negative cloud to ground lightning records. Results show that there are two obvious peak values, 5.0 km and 8.8 km above the ground, in the distribution of lightning initiation height. This two-peak feature can meet the picture described by the tripole gross charge structure of thunderstorms very well. The average velocities of 23 intra-cloud lightnings and 22 cloud-to-ground lightnings during the first 15 ms of initial breakdown process is calculated every 5 ms. The calculation indicates that the average velocities during the first 15 ms of initial breakdown process are in 104-105 order of magnitude both in intra-cloud flash and cloud to ground flash. Most intra-cloud flash and cloud to ground flash decelerated during the first 10-15 ms of the initial discharge. This result is a little different from a previous study in which all of the 24 intra-cloud flash records present a deceleration phase in the first 10-15 ms. It also demonstrates a possibility that runaway breakdown mechanism is not the only effective lightning initiation mechanism. Proportions of the intra-cloud flash which decelerated during the first 10 ms and 15 ms are both higher than those of cloud to ground flash in this observation. This may indicate that there is some difference between initiation processes of intra-cloud flash and cloud to ground flash. At last, it is found that the initial stage of intra-cloud lightning and cloud to ground lightning both have three possible development tendencies: Upward, downward and horizontal in vertical direction. The development direction of lighting initial stage is related to the environment electric field, and that can be used to give information about the direction of the electric field at the initiation location of lightning.
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图 1 数据处理标准示意图
(a) 一次云闪起始阶段的地面快电场变化记录，(b) VHF辐射源高度随时间变化，(c) 辐射源定位结果的x-z面投影，(d) 三维定位显示，(e) 辐射源定位结果的x-y面投影，(f) 辐射源定位结果的y-z面投影
Fig. 1 The standard of data processing
(a) a fast electric field change record of the initial stage of anintracloud flash, (b) the height of VHF radiation sources versus time for this stage, (c)x-z plane projection of 3D location results, (d)3D location results of this process, (e)x-y plane projection of 3D location results, (f)y-z plane projection of 3D location results
图 2 闪电起始高度分布
(a) 地闪，(b) 云闪，(c) 总闪
Fig. 2 Distributions of the starting altitude of lightning
(a) cloud to ground lightning, (b) intracloud lightning, (c) total lightning
图 3 23次云闪和22次地闪始发约0~5，5~10，10~15 ms时间段内的闪电放电平均发展速度
(分别对应依次减小、减小后增大、增大后减小和依次增大4种情况，N为闪电样本量)
Fig. 3 The average developing velocities of lightning initial discharge for time intervals of 0-5, 5-10, 10-15 ms roughly of 23 intracloud lightnings and 22 cloud to ground lightings
(records classified into decrease, decrease and then increase, increase and then decrease and increase 4 groups by the order of value for v5, v10 and v15; N stands for the number of records in each panel)
表 1 闪电起始阶段平均发展速度 (单位:105 m·s -1)
Table 1 The average developing velocities of lightning initial stage (unit:105 m·s-1)
统计参数 云闪 (23次) 地闪 (22次) 最小值 最大值 平均值 标准差 最小值 最大值 平均值 标准差 v2 1.0 9.5 3.7 2.1 2.0 7.3 3.9 1.8 v5 1.2 4.2 2.4 0.8 0.6 5.4 2.4 1.2 v10 0.7 3.8 1.8 0.9 0.8 5.7 2.5 1.2 v15 0.8 4.8 2.0 1.2 0.4 5.5 2.4 1.4 
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表 2 闪电起始阶段发展方向统计结果 (单位：%)
Table 2 Developing trends of lightning initial stage (unit:%)
发展方向 云闪 (23次) 地闪 (22次) 向上 92 4 向下 4 88 水平 4 8
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