Some Characteristics of Leader Pulses in Positive Cloud-to-Ground Flashes

Compared to negative ground flashes, not much is known about the characteristics of leaders in positive ground flashes. This paper presents some characteristics of the electric field pulses observed during leader propagation in positive ground flashes. We analyzed in detail the electric field changes produced by 50 positive ground flashes during 2014 summer thunderstorms in Uppsala, Sweden. Pronounced leader pulses were observed in 22% of the cases. They were observed to occur 1.4 ms or less before the first return stroke. Interpulse duration ranged from 13.3-50.3 mu s with a mean value of 24.7 mu s. The largest relative amplitude ranged from 2.7-17.8 % of the return stroke peak. No dependence of the pulses occurrence were observed with distance. However, the relative amplitude were found to decrease with distance. The presence of these pulses suggests that the leaders propagate in a stepped manner. One case of positive ground flashes preceded by negative polarity pulses just before the return stroke is also reported.