Characteristics of leader pulses in positive ground flashes in Sweden

•Analysis on the characteristics of leader pulses in 51 positive ground flashes observed during 2014 summer thunderstorms in Sweden.•A report on one case of positive return stroke preceded by opposite polarity leader pulses and discussion on the possible source of these pulses.•Estimation of the height above the ground when the leader begins to appear and estimation of the striking distance of the ground flash. This paper presents the characteristics of the electric field pulses observed during leader propagation in positive ground flashes. We analysed in detail the electric field changes occurring just before the first return stroke in 51 positive ground flashes during 2014 summer thunderstorms in Uppsala, Sweden. Pronounced leader pulses (having the same polarity as the return stroke) were observed in 22% of the cases. They were observed to occur within 1.4ms before the first return stroke. Interpulse duration ranged from 13.3 to 50.3μs with a mean value of 24.7μs. The peak amplitude of the leader pulses relative to the return stroke peak ranged from 2.7 to 17.8%. The presence of these pulses shows that the leaders propagate in a stepped manner. Based on the leader pulses’ time of initiation and average speed of the leader, the distance travelled by the leader was also estimated. One case of positive ground flash preceded by opposite polarity leader pulses just before the return stroke is also reported. To the best of our knowledge, this is the first time that such a case in positive ground flashes is reported. We suggest that these opposite polarity leader pulses are due to the negatively-charged leader branch of a bi-directional leader inside the cloud that propagates towards observation point.