Examining charge transfer in an unusual rocket-triggered lightning flash with branched upward positive leaders

We have examined an unusual rocket-triggered lightning flash during the summer campaign of the SHAndong Triggered Lightning Experiment (SHATLE) in 2018. High-speed video camera observations and three-dimensional (3D) lightning mapping show that the upward positive leader split into two branched channels (referred to as branch A &amp; branch B, respectively) at a height of about 370 m, and then progressed into different charge regions of the thundercloud. Branch A initially developed upward before turning northwest from the trigger point; ten pronounced intermittent negative leaders were observed propagating downward along this branch channel, causing strong current pulses. Branch B propagated obliquely upward towards the northeast before continuing northward to a region of weak radar echo at 3 km altitude, resulting in a large-scale charge transfer of approximately –250 C (C = Coulomb) and generating a sustained, strong current exceeding 2 kA. Furthermore, downward dart leaders propagating along branch A connected to the active channel of branch B at the bifurcation point. This connection generated a surge of large current pulses (<italic>M</italic>-components) superimposed on the continuing current. Evidence from 3D lightning mapping and concurrent channel-base current measurements suggests that the 10^th negative dart leader split during its downward propagation, with one branch propagating to ground, while the other entered into a positive charge reservoir. This initiated a positive charge transfer to ground via the existing channel, ultimately triggering the final stroke which exhibited a bi-polarity current pulse.