Effects of polarity-dependent initiation criteria on lightning leader propagation dynamics

Polarity asymmetry in lightning leaders --- the stark differences in speed, brightness, current, and stepping behavior between positive and negative leaders --- is a well-documented yet poorly understood phenomenon. We extend a physics-based upward leader model to simulate leaders which emulate both polarities and suggest these differences may be traceable to the more demanding initiation requirements for negative leaders, which for the simulations included herein require a stability field three times stronger sustained over a streamer zone ten times longer than for positive leaders. The viability spectrum of positive leaders is consequently broad, spanning aborted discharges, conditionally viable leaders prone to cutoff and dart-leader formation, and absolutely viable leaders; negative leaders, by contrast, exhibit near-binary behavior and are almost invariably absolutely viable once initiated. The model reproduces the key aforementioned differences between leaders of both polarities and also agrees reasonably with the available rocket-triggered lightning data. The results of this study suggest that many of the observed polarity asymmetries may be an outcome of the more stringent initiation conditions by which negative leaders propagate. Stricter initiation conditions require favorable environments (high field, tall conducting object) which are naturally more inclined to produce faster and hotter leaders which deliver more current to ground.