Locating Preliminary Breakdown Pulses in Positive Cloud-to-Ground Lightning

This paper provides the first concrete evidence that preliminary breakdown (PB) pulses of either polarity in positive cloud-to-ground (+CG) lightning are produced by negative leaders. Three-dimensional location results of PB pulses in 46 +CG flashes are analyzed. The majority (40) of the +CG flashes started with positive PB pulses (+PBPs), the same polarity as the positive return stroke. Location results showed that +PBPs were produced by leaders propagating upward, which were determined to be negative leaders based on PB pulse polarities. Similarly, for the negative PB pulses (-PBPs) found in six +CG flashes, location results showed that they were produced by leaders propagating downward, and we determined that these were also negative leaders. Upward negative leaders producing +PBPs in +CG lightning are very similar to those in intracloud lightning. They usually propagate upward before turning in a horizontal direction. Downward negative leaders producing -PBPs in +CG lightning are more complicated. They usually move back upward after a period of downward propagation. Positive leaders could not be detected, but their possible propagations are analyzed along with possible charge structures for different types of PB pulses. We also demonstrate that PB pulse studies based on single-site records are potentially unreliable. Plain Language Summary Lightning flashes usually start with a train of bipolar pulses called preliminary breakdown (PB) pulses. In negative cloud-to-ground (-CG) flashes, PB pulses usually have negative initial polarity, the same as the negative return stroke. However, in +CG flashes, PB pulses with either polarity have been observed by many studies. PB pulses in +CG flashes are usually explained by the movement of positive leaders because a positive leader connecting the ground is always necessary for a +CG flash. In this study, we provide the first concrete evidence that PB pulses of both polarities are produced by negative leaders. Specifically, positive PB pulses are produced by upward-propagating negative leaders and negative PB pulses are produced by downward-propagating negative leaders. Possible charge structures for different types of PB pulses in +CG flashes are also discussed. Key Points