Downward positive leader propagation modes and their effects in positive cloud-to-ground lightning characteristics

The return stroke in a cloud-to-ground (CG) flash is initiated when the downward leader either reaches the ground or connects with an upward leader from a grounded structure. Unlike negative downward leaders, which typically propagate in a highly branched manner, downward positive leaders can propagate either in a branched or unbranched fashion. High-speed camera recordings show that some downward positive leaders exhibit faint channels accompanied by multiple recoil leaders—small bipolar and bidirectional plasma channels—that repeatedly retrace these channels as the leader propagates toward the ground prior to the return stroke (RS). Other leaders, however, consist of a single bright channel with no occurrence of recoil leaders. This study compares the physical characteristics of these two types of downward positive leaders and examines how their propagation modes influence other properties of positive CG lightning. We show that leaders without recoil leaders propagate significantly faster and are followed by RS with much higher peak current values, on average about twice those associated with leaders accompanied by recoil leaders. The final speed of leaders without recoil leaders exhibits a very strong correlation (R = 0.94) with the RS peak current. The duration of the continuing current following the RS is similar for both cases, although there is a slight tendency for longer durations when recoil leaders are present. Analysis of multiple-stroke positive CG flashes with different ground terminations shows that leaders initiating successive strokes can propagate in markedly different ways within time intervals as short as 80 ms.