Initiation of Downward Positive Leader Beneath the Negative Leader Channel

Due to the weak radiation generated by positive leaders, our understanding of how positive leaders are initiated and reach the ground remains limited. This study investigated positive cloud-to-ground (CG) lightning induced after the long intracloud lightning based on high-speed video and fast antenna mapping results. Below the cloud base in the field of view, re-breakdowns of decayed negative branches occurred consecutively beneath the horizontal negative channel in the form of bidirectional leaders. These bidirectional leaders advanced along the same channel, and eventually, the third reached the ground, generating a positive return stroke (RS) with a peak current of 157 kA. Following the RS, new negative discharges emerge adjacent to the vertical return stroke channel, persisting and propagating to form a continuing current (CC) lasting over 200 milliseconds. Positive cloud-to-ground flash attracts more attention than its negative counterpart because it is more hazardous. Previous studies have confirmed the close relationship between intracloud (IC) lightning and positive cloud-to-ground (+CG) lightning. Downward positive leaders forming +CG can originate from branching of intracloud lightning or even from negative polarity leader channels. However, due to the weak radiation from positive leaders during their development within the cloud, common lightning detection devices, such as LF/VLF fast antennas and VHF antennas, find it very difficult to detect positive leaders. Consequently, our understanding of how positive leaders initiate within the cloud and reach the ground remains limited. In this study, we comprehensively utilized lightning channel mapping results and high-speed video to investigate a case of +CG. We found that the occurrence of downward positive leaders around the negative leader channel is facilitated through multiple side breakdowns, events that continue even after the return stroke. Therefore, we propose that the multiple positive side breakdowns beneath the reactivated negative leader channel are responsible for generating positive leaders from the negative lightning channel. These side discharges play a crucial role in the generation of +CG lightning. A positive cloud-to-ground lightning flash was analyzed based on synchronous high-speed video and lightning mapping results Downward positive leader initiated, reached ground via three bidirectional side discharges beneath horizontal negative leader channel Side negative breakdowns from vertical return stroke channel and reactivation of decayed negative branches were found