On the Transition From Initial Leader to Stepped Leader in Negative Cloud‐to‐Ground Lightning
On the Transition From Initial Leader to Stepped Leader in Negative Cloud‐to‐Ground Lightning
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DOI：
10.1029/2019jd031765
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发表时间：
2020-02
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影响因子：
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通讯作者：
M. Stolzenburg;T. Marshall;S. Karunarathne
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文献类型：
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作者：
M. Stolzenburg;T. Marshall;S. Karunarathne
高速视频和电场变化数据用于描述负云对地闪光的前5 ms。这些观测结果揭示了领导人的两个分支分别从最初的领导人过渡到传播作为一个负阶跃领导人（SL）的亮度和电场变化脉冲的性质的演变。这是第一次报道，有证据表明弱光度与初始事件一致，在第一个初始击穿（IB）脉冲之前60 μs的弱双极脉冲。在IB阶段，最初的先导以100-280 μs的间隔间歇性地前进，在单独的光爆发中，在几个20 μs的帧内是明亮的，并且与IB脉冲时间一致。在IB脉冲之间的间隔中，在最早的1.8 ms期间，初始先导是暗淡的或不可见的。在2 ms内，先导传播开始过渡到早期SL阶段，其中先导尖端在相对暗淡和短暂的步骤期间以40-80 μs的更规则的间隔前进，这些步骤与具有短持续时间、小幅度和典型单极波形的SL脉冲一致。这些数据表明，当下端后面的整个初始前导长度开始在突发之间保持照明时，传播模式从IB突发变为SL步骤，并且IB阶段结束。研究结果支持了一个假设，即早期初始先导发展发生在没有连续热通道的情况下，因此初始先导传播在物理上不同于自传播SL推进。
High‐speed video and electric field change data are used to describe the first 5 ms of a negative cloud‐to‐ground flash. These observations reveal an evolution in character of the luminosity and electric field change pulses as two branches of the leader separately transition from initial leader to propagating as a negative stepped leader (SL). For the first time reported, there is evidence of weak luminosity coincident with the initiating event, a weak bipolar pulse 60 μs prior to the first initial breakdown (IB) pulse. During the IB stage, the initial leader advances intermittently at intervals of 100–280 μs, in separate light bursts that are bright for a few 20‐μs frames and are time coincident with IB pulses. In the intervals between IB pulses, the initial leader is dim or invisible during the earliest 1.8 ms. Within 2 ms, the leader propagation begins transitioning to an early SL phase, in which the leader tip advances at more regular intervals of 40–80 μs during relatively dim and brief steps which are coincident with SL pulses having short duration, small amplitude, and typically unipolar waveform. These data indicate that when the entire initial leader length behind the lower end begins to remain illuminated between bursts, the propagation mode changes from IB bursts to SL steps, and the IB stage ends. The results support a hypothesis that the early initial leader development occurs in the absence of a continuously hot channel, thus the initial leader propagation is physically unlike the self‐propagating SL advance.