Step-like characteristics of an upward positive leader in triggered lightning

An upward positive leader in a rocket-triggered lightning flash was analysed based on high-speed video images with a time resolution of 10,000 fps and channel base current. The luminous intensity at the leader tip was stronger than that at the channel behind it. The average 2-D propagation speed of the leader was 1.0×10⁵ m/s during the early stage of its ascending, with partial speeds ranging from 2.0×10⁴ m/s to 1.8×10⁵ m/s between 130 m and 730 m above ground. The leader speed showed an accelerating tendency with height and time, however, an obvious irregularity of the speed was observed during the ascending of positive leader, which indicates a possible discontinuity of its extending process. Eleven current pulses, with peak values ranging from tens of amperes (A) to about 150 A, were clearly observed during the inception process of the upward positive leader. During the following ascending process, there were obvious fluctuations superimposed on the slow varying continuous current. Both the optical images and current waveforms indicate step-like properties of the upward positive leader.