Upward Negative Leaders in Positive Upward Lightning in Winter: Propagation Velocities, Electric Field Change Waveforms, and Triggering Mechanism

Abstract

AbstractUpward negative leaders (UNLs) in positive upward lightning are rarely observed. In this study, 24 UNLs observed by the Fast Antenna Lightning Mapping Array during winter thunderstorms in Japan are analyzed. Three‐dimensional velocities of UNLs are calculated, and it is found that velocities during the upward propagation stage range from 1.8 to 27.9 × 105 m/s with a mean value of 10.4 × 105 m/s, and they are always larger than velocities during the following horizontal propagation stage. UNLs produce distinctive electric field change waveforms with V‐shaped overall change trend, with small pulses at the beginning when UNLs start, large pulses during upward propagations, and small pulses again when UNLs turn into horizontal directions. Pulses produced by UNLs are mainly unipolar but sometimes also bipolar. Pulse interval, pulse width, rise time, and fall time are calculated for waveforms produced by five UNLs. Pulse intervals range from 13.7 to 18.9 μs. Pulse width, rise time, and fall time are on average 5.8, 3.3, and 2.4 μs, respectively. All UNLs are preceded by other discharges. Strong negative strokes including some special types of strokes usually precede initiations of UNLs by tens of milliseconds. These negative strokes are usually found only a few kilometers away from UNLs. It is speculated that these negative strokes along with following in‐cloud positive leaders contribute to the depletion of negative charges in thunderclouds, which result in positive electric field changes (atmospheric electricity sign convention) large enough for initiations of UNLs.