Characteristics of downward leader in a positive cloud-to-ground lightning flash observed by high-speed video camera and electric field changes

Leader process in a positive single-stroke cloud-to-ground lightning flash was captured by a high-speed video camera with a time resolution of 1000 frames/s. The positive leader process and the intracloud discharge process before the return stroke, which lasted about 600 ms, are studied in detail by combining the data from fast and slow antenna. The channel branches seemed to develop horizontally with a speed of the order of 10(4) m/s during the initial stage just outside the thundercloud. The luminous duration of the leader was about 12 ms, and the luminous intensity at the tip was much stronger than the channel behind it, presented obviously a stepped-like characteristic. The 2-D propagation speed of the stepped-like leader ranged from 0.1 x 10(5) m/s to 3.8 x 10(5) m/s. The time interval between 26 leader pulses during the last 0.5 ms just before the stroke was about 17 mu s according to E-field changes.