Electrostatic Conditions Favoring Fast Breakdown Initiation in Thunderclouds

Abstract:

Energetic intracloud discharges known as narrow bipolar events (NBEs) have been shown to be produced by streamer-based fast positive breakdown (FPB), and appear to be responsible for initiating most if not all lightning flashes in storms [Rison et al., Nat. Comms., 2016]. Subsequent observations revealed that NBEs can also start with fast negative streamer breakdown (FNB) propagating in the opposite direction to the thunderstorm electric field [Tilles et al., Nat. Comms., 2019], and furthermore that each polarity of fast breakdown (FB) typically initiates the opposite polarity breakdown back along its path as the FB dies out. Observations of discharges started by FNB rather than FPB challenge our current understanding of how electrical breakdown in air happens, since it is widely accepted that the electric field threshold for formation and propagation of negative streamers is about twice as large as for positive ones. Useful insights into the environmental conditions required for the onset of fast breakdown can be gained from electrostatic simulations of a system of streamers [Attanasio et al., JGR, 2019]. Our electrostatic, energy-based model is validated by matching observed fast-antenna NBE electric field change waveforms. Furthermore, we perform a parametric analysis to identity the minimal electric field magnitudes and extents that allow for fast breakdown of either polarity to develop. Finally, we show how simple electrostatic simulations can nicely illustrate the sequence of events giving rise to NBEs, which typically involve sets of opposite polarity streamer systems propagating in both forward and backward directions.