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 highenergy electrons (from cosmic rays or terrestrial sources of ionizing radiation) by the incloud 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.
SummaryThe maximum measured electric fields in thunderclouds are an order of magnitude less than the fields required for electric breakdown of the air. One explanation for lightning initiation in these low fields is that electric breakdown first occurs at the surfaces of raindrops where the ambient field is enhanced very locally due to the drop geometry . Laboratory experiments [Crabb & Latham, 1974] indicate that colliding raindrops which coalesce to form elongated water filaments can produce positive corona in ambient fields close to those measured in thunderclouds. We calculate the E-field distribution around a simulated coalesced drop pair and use a numerical model to study the positive corona mechanisms in detail. Our results give good agreement with the laboratory observations. At the altitudes (and thus low pressures) at which lightning initiation is observed, our results show that positive corona can occur at observed in-cloud E-fields.
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