Upward negative leaders in positive triggered lightning: Stepping and branching in the initial stage

By conducting comprehensive observation on the rarely triggered positive lightning flashes, two cases of upward negative leaders were quantitatively investigated with simultaneous data of current, optical, and electromagnetic field changes at fine time resolution. The impulsive channel current associated with the initial leader stepping involved pulse interval, peak, charge of 19.1 μs, 122.3 Å, 465.9 μC, and 20.3 μs, 58.4 Å, 267.2 μC, for the two cases, respectively. The negative charge of an individual step was nearly an order of magnitude larger than that of the positive leader step in triggered lightning. The upward negative stepped leader branches shortly after initiation, and consequently, the stepping‐caused current pulses became multipeaked, wider, and more frequent in waveforms. The main leader tip propagated at an average speed of 2.10 × 105 m/s, while its partial speed fluctuated a lot as was significantly influenced by the behaviors of subbranches development. Key Points First fine observation of upward negative leaders in rare positive rocket‐triggered lightning Charge transferred in each step of the upward negative leader was nearly an order of magnitude greater than the upward positive leader Initial extension of the main leader channel was influenced by neighboring branches and even paused