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The 3D Spatial and Temporal Evolution of K Process in Intra-cloud Flash
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摘要: K过程是闪电放电过程中的一种放电事件。该文使用两套VHF宽带干涉仪2010年夏季在广州从化地区获取的3次云闪三维闪电辐射源定位数据，分析其中的K过程时空发展特征与地面电场特征。结果表明：宽带干涉仪观测到的K过程主要由快速发展的负极性放电事件组成，按定位结果的分布可划分为3个阶段：负极性反冲先导发生在云闪起始区域下方，沿正先导一端已有路径向闪电起始区域传播；部分反冲先导能进入之前负先导建立的通道并快速发展；反冲先导将原有通道激活后进一步促进负先导继续发展。同时使用辐射源三维定位数据计算了8次反冲先导的平均发展速度，反冲先导的发展速度为106~107 m·s-1量级，与负地闪中的负极性直窜先导相似，但均小于回击速度。Abstract: K process is a kind of discharge event in lightning. The study on evolution features of this event helps to increase understanding on the mechanism of lightning initiation and developing. 3D lightning imaging data of 3 intra-cloud flashes are used to describe and analyze spatial and temporal characteristics of K events and corresponding electric field's changing waveforms. These 3D location data are recorded by 2 VHF broadband interferometers at Conghua, Guangdong Province, in the summer of 2010, providing the developing image of lightning discharges with a temporal resolution of 5 μs and a spatial resolution better than 500 m. The VHF radiation of lightning is recorded by a high-speed oscilloscope with a sample rate of 1 GS·s-1. Both fast and slow filed change antennas are employed in two broadband interferometer systems. Their decade time constants are 1 ms and 8 s, respectively. Changing waveforms of the electric field are record by an A/D card working synchronously with the oscilloscope used to record the VHF signal of lightning.Results show that K process is a kind of fast negative breakdown discharge and can be divided into 3 stages according to the distribution of VHF radiation sources located by broadband interferometers. In the first stage, negative recoil leaders occur under the initiation position of intra-cloud lightning, progress along the path of pre-existing positive leader, heading to the initiation region of lightning. In the second stage, some negative recoil leader can progress fast in the channel established by the previous negative leader of lighting initiation stage and induce a relatively large variation of electric field on the ground. In the last stage, the negative recoil leader reactivates the channel of negative leader in lighting initiation stage and facilitates the negative breakdown at the end of existing path. Speeds of the new air breakdown processes happen at the end of existing path are generally reduced to an order of 104~105 m·s-1. The evolution speeds of 8 recoil leaders in the 3 intra-cloud lighting records are also calculated. The maximum, minimum and average value of the developing speeds of 8 recoil leaders are 3.1×107, 3.1×106 m·s-1 and 1.6×107 m·s-1, respectively. The range of K process speeds is similar with that of dart leader but slower than return stroke.
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Key words:
- K process;
- K change;
- recoil leader;
- broadband interferometer;
- lightning
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图 1 云闪个例1的地面电场变化波形和三维定位结果
(a)快电场变化波形，(b)慢电场变化波形，(c)VHF辐射源高度随时间的演变，(d)闪电辐射源在X-Z平面投影，(e)闪电辐射源三维显示，(f)辐射源X-Y平面投影，(g)辐射源Y-Z平面投影
(图 1f中用箭头曲线标出了图 1c中A时段4条分叉发展的放电路径；图中辐射源定位结果使用颜色代表时间，图中红、绿两菱形分别代表从化市气象局和人工引雷试验场观测站位置，地面电场波形为从化市气象局观测站记录)Fig. 1 The ground electric filed change waveforms and 3D location results of intra-cloud flash case 1
(a)fast ground filed change waveform, (b)slow ground filed change waveform, (c)heights of VHF radiation sources versus time, (d)the projection of 3D location result on X-Z plane, (e)3D layout of this cloud flash, (f)the projection on X-Y plane, (g)the projection on Y-Z plane
(4 paths of discharges in stage A are marked in Fig. 1f, color of dots denote time, red and green diamonds signify locations of two observation sites, filed change waveforms recorded at Conghua)
图 2 云闪个例1中B时段三维定位结果
(灰色圆点表示所有B时段前的辐射源定位结果)
(a)B时段放电事件三维定位结果在X-Z平面投影，(b)定位结果在X-Y平面投影，(c)定位结果在Y-Z平面投影Fig. 2 3D locations of stage B in intra-cloud case 1
(grey dots denote sources occurred before stage B)
(a)projections of 3D location results in stage B on X-Z plane, (b)projections on X-Y plane, (c)projections on Y-Z plane
图 3 云闪个例1在C时段闪电三维定位结果
(灰色圆点表示所有C时段以前的辐射源定位结果，图 3c中箭头指向对应事件起始位置)
(a)C时段放电事件三维定位结果在X-Z平面投影，(b)闪电辐射源三维显示，(c)定位结果在X-Y平面投影，(d)定位结果在Y-Z平面投影Fig. 3 Ground electric field changes and 3D locations of stage C in intra-cloud flash case 1
(grey dots denote sources occurred before stage C, the black arrow points to the initiation position of corresponding event)
(a)the projection of 3D location results in stage B on X-Z plane, (b)3D layout of lightning radiation sources, (c)the projection on X-Y plane, (d)the projection on Y-Z plane
图 5 同图 1，但为云闪个例2的地面电场变化波形和三维定位结果
(地面电场波形为从化市气象局观测站记录；图 5f中箭头曲线1，2标记出A时段2条分叉发展的路径，图中箭头指向对应事件起始位置)
Fig. 5 The same as in Fig. 1, but for the ground electric filed change waveforms and 3D locations of intra-cloud flash case 2
(filed change waveforms are recorded at Conghua, two paths of discharges in stage A are marked in Fig. 5f, black arrow points to the initiation position of corresponding event)
图 6 同图 1，但为云闪个例3的地面电场变化波形和三维定位结果
(地面电场波形为人工引雷试验场观测站记录，图 6f中用箭头曲线标出A时段4条发展路径)
Fig. 6 The same as in Fig. 1, but for ground electric filed change waveforms and 3D locations of intra-cloud flash case 3
(field change waveforms are recorded at triggered-lightning experiment site, four paths of discharges in stage A are marked in Fig. 6f)
图 7 K过程的3个发展阶段
(红色线和蓝色线分别表示闪电起始后已经发生过的正、负先导路径，黄色线表示新建立负极性击穿路径，黑色箭头为反冲先导的发生路径，灰色方框标示出闪电起始位置)
Fig. 7 Three stages of K process
(red and blue lines denote the pre-existing paths of positive and negative leaders occurred after the initiation of lightning, the yellow line denotes the path establishing by new negative breakdown, black arrows denote the path of recoil leader, the black box denotes the initiation position of lightning)
表 1 8次K过程反冲先导事件的持续时间和发展速度
Table 1 Durations and velocities of 8 recoil leaders
个例 编号 持续时间/μs 速度/(m·s-1) 1 C1路径1 800 3.1×106 1 C1路径2 450 2.6×107 1 D1 140 3.1×107 1 D4 291 2.2×107 2 D2 160 1.5×107 3 B1 100 1.9×107 3 C1 50 2.2×107 3 C2 270 8.2×106 
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