A model study of corona emission from hydrometeors
  The 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. 
Vicki Schroeder, M.B. Baker and John Latham