Streamer formation and branching from model hydrometeors in subbreakdown conditions inside thunderclouds

Abstract

Electric field values measured inside thunderclouds have consistently been reported to be up to an order of magnitude lower than the value required for the conventional electrical breakdown of air. This result has made it difficult to explain how lightning frequently occurs in thunderclouds. A few different theories have been offered to explain the lightning initiation process, one of them being the theory of lightning initiation from hydrometeors. According to this theory, lightning can be initiated from electrical discharges originating around thundercloud water or ice particles in the measured thundercloud electric field. These particles, called hydrometeors, are believed to cause significant enhancement of the thundercloud electric field in their vicinity and then initiate streamers that are the precursor discharges for the hot lightning leader channel. Previously, Liu et al. (2012a) reported streamer formation from a model hydrometeor in an electric field value of half of the conventional breakdown threshold (E k ) for air. In this paper, we present modeling results for streamer formation in electric fields as low as one third of the breakdown threshold. According to our results, initiation of stable streamers from thundercloud hydrometeors in a 0.3E k electric field is possible, only if enhanced ambient ionization levels (e.g., the ionization created by corona discharges around the same or other nearby hydrometeors) are present ahead of the streamer. The magnitude and distribution of this ambient density may be a determining factor on whether the streamer branches, recovers after the prebranching stage, or continues propagating stably. We investigate the streamer branching behavior and characteristics and test a theory that has recently been proposed to explain this phenomenon. We find that the geometry of the streamer head plays an important role in the streamer branching phenomena. The fast radial movement of the maximum streamer head curvature, combined with the slow reduction of the maximum curvature value, eventually leads the streamer head to branching. Finally, we compare our modeling results with laboratory experiments and realistic thundercloud conditions and discuss the implications of this study to lightning initiation and other lightning-related phenomena.

Properties of relatively long streamers initiated from an isolated hydrometeor

“…Our simulations are first conducted at 0.5 E k ambient field, as pointed out by recent modeling and experimental studies of streamer initiation from isolated hydrometeors that 0.3–0.5 E k is required to initiate streamers from thundercloud hydrometeors [ Liu et al , ; Petersen et al , ; Sadighi et al , ]. The hydrometeor is modeled by an ionization column, 5 mm long, of equal numbers of electrons and positive ions with a peak density of 10 14 cm −3 and a radius of 0.1 mm.…”

Numerical simulation of positive streamer development in thundercloud field enhanced near raindrops

“…Unlike [12], we did not require enhanced pre-ionization and started the simulation with a condition fitting one-electron initiation. We used quite realistic drop sizes and charge unlike in [13].…”

“…As in earlier works [10][11][12][13], we have simulated the important "smallest" element seeding the streamer corona. However, the problem remains that the scale is so small and the required fields are larger than observed fields, that a second stage of larger scale is required to amplify the field in larger volume.…”

“…The first stage of lightning initiation was analyzed in [3][4][5][6][7][8][9][10][11][12][13]. Obviously, to initiate lightning, the field strength at least locally must be above the E br /P g , but the E/P g magnitudes measured in thunderclouds do not exceed 3-4 kV/cm/atm [14][15][16].…”

“…In both cases, a stable streamer discharge develops. Sadighi et al [12] study streamer formation in the field of 0.3E br from hydrometeor modeled by an ionized column. The authors conclude that the minimum column length required for the streamer initiation is from 5 to 8 mm with a background electron density of 10 15 m -3 , which is to be non-uniform; otherwise, the streamer branches.…”