Characteristics of a bipolar cloud-to-ground lightning flash containing a positive stroke followed by three negative strokes

- A rare bipolar CG lightning flash with one first positive stroke followed by three subsequent negative strokes are reported.
- Radiation field of negative strokes in this bipolar flash is narrower and faster than usual negative subsequent strokes.
- Recoil leaders retrograded along the positive stroke path play a critical role in initiating the following negative strokes.

Using time-correlated high-speed video images at 3200 frames per second, broadband electric field change data and low-frequency magnetic fields, a natural bipolar cloud-to-ground (CG) lightning flash with one first positive stroke followed by three subsequent negative strokes is analyzed. All of these four strokes transferred electric charge to the ground through the same lower channel with a time interval of 328 ms between the positive stroke and the first negative stroke. The flash onset was followed by several positive leaders that extended below the cloud base, one of which descended to culminate in a positive stroke with a continuing current. Another positive leader extended horizontally to a distant negative cloud region and induced several recoil leaders that intermittently retrograded along the leader channel. Eventually, three recoil leaders successively traversed along the path of positive stroke to produce respective negative strokes, resulting in the polarity reversal of charge transferred to the ground. The average two-dimensional (2-D) speed of the positive leader was 1.1 × 105 m/s, while for the 3 negative leaders was 6.7 × 106 m/s. The zero-crossing time and rise time of the radiation field waveform for the 3 negative strokes are smaller than the typical negative subsequent strokes, making them hard to be recognized as return strokes by the CG lightning location network.