Observation of the First Stepping Process of Three Winter Lightning Discharges

Using three different lightning observation systems, we have observed the first stepping processes of three lightning discharges with unprecedented details. We found that each of the first stepping processes contains a clear upward fast negative breakdown (FNB) corresponding to the preliminary breakdown (PB) pulse peak. Prior to the FNB from the lightning initiation, most sources are located in a fan-shape area, and the FNB grows from this area forming a channel with a length from 45 to 120 m. During the period of about 100 microseconds immediately preceding the PB pulse, multiple 1–12 MHz radio bursts are located within or near the fan shape area. These radio bursts may indicate stem/space leaders. Eventually the FNB decayed and scattered in another fan shape area at the upper end of the channel. Based on these findings, we have proposed a complete picture of the first stepping process.