Characteristics of negative breakdowns in extinguished channels of a positive cloud-to-ground flash

The characteristics of a natural positive cloud-to-ground lightning flash which had only one return stroke are investigated in detail using synchronous detections of lightning very-high-frequency (VHF) interferometer, fast antenna, high-speed video and S-band Doppler weather radar. The flash initiated at 5.8 km above the ground and at the edge of an enhancing convective cell, where a positive bi-level charge structure was involved. The initial negative leader developed upward, forming multiple branches, while the positive leader was recognized 22.2 ms later. The breakdown with different polarities exhibited distinct properties. Three significant negative breakdowns initiating in the previously established but extinguished negative channels were well detected by the VHF interferometer, with a duration of 0.30 ms, 0.06 ms and 0.20 ms, and propagation speed of 1.9 × 106 m/s, 2.2 × 107 m/s and 1.9 × 107 m/s, respectively. The first one occurred 0.3 ms before the return stroke, its occurrence promoted the development of the downward positive leader that was getting close to the ground, leading to an obvious acceleration and an enhancement of the channel luminosity. The second and the third negative breakdowns occurred during the continuing current after the return stroke, acting as the causation of the positive M-components. Symmetric V-shaped and hooked pulses on the fast electric field change waveform and obvious increased luminosity of the grounded channel were well detected. The occurrence of negative breakdowns in the pre-conditioned channels served for producing a backward surged current wave that promote the discharge process through the grounding/grounded channel.