Initiation of Negative Streamers from Hydrometeors at Subbreakdown Field Conditions

Recent high-speed interferometer observations have indicated that the initiation of lightning may begin as compact intracloud discharges (CIDs) [Rison et al., Nat. Commun., 7, 10721, 2016], which are caused by a process termed fast positive breakdown. It is hypothesized that fast positive breakdown consists of a system of positive streamers [Rison et al., 2016]. A follow-up investigation by using the same interferometer has found that CIDs can also be caused by fast negative breakdown [Tilles et al., AE12A-03, AGU Fall Meeting, 2016]. If negative streamers are the underlying process driving fast negative breakdown, this suggests that negative streamers can be initiated and propagate through a significant distance of about one kilometer without detectable signals from positive streamers. However, initiation of negative streamers from isolated hydrometeors in subbreakdown fields has never been reproduced by modeling studies [Liu et al., Phys. Rev. Lett., 109, 025002, 2012; Sadighi et al., J. Geophys. Res. Atmos., 120, 3660, 2015; Shi et al., J. Geophys. Res. Atmos., 121, 7284, 2016]. In this talk, we will show that negative streamers can be successfully initiated from the tip of a cone-shape hydrometeor in an electric field well below conventional breakdown threshold field, suggesting that the shape of a hydrometeor plays an important role in streamer initiation. Formation of positive streamers may follow the initiation of negative streamers, and the characteristics of positive and negative streamers developing in the same subbreakdown field can be very different. For example, the peak field of the positive streamer head is almost two times larger than that of the negative head; the streamer channel density of the positive streamer is larger than that of the negative streamer; and the growth rate of the positive streamer is larger than that of the negative streamer. We will discuss the implications of our modeling results in the context of the observations of fast positive and negative breakdown.