Lightning initiation-conventional and runaway-breakdown hypotheses

We evaluate two candidate mechanisms for the onset of lightning in the relatively low electric fields measured inside thunderstorms. The first is conventional dielectric breakdown due to local enhancement of the electric field in the vicinity of hydrometeors. The second is runaway breakdown, due to extended acceleration of high-energy electrons (from cosmic rays or terrestrial sources of ionizing radiation) by the in-cloud electric field. We compare the electric fields required for lightning onset by each mechanism with those observed inside lightning-producing clouds, and we examine the sensitivity of the computed results to input parameters and assumptions. The conclusion of our analysis is that the conventional breakdown mechanism alone cannot trigger lightning while the runaway breakdown mechanism appears a more likely candidate. We identify the parameters on which each mechanism depends and emphasize the impact of observational uncertainties on our conclusions.