Computed conditions of corona emission from raindrops

The altitude of corona emission from charged raindrops located in a vertical ambient electric field is calculated by coupling the corona experimental results obtained by Dawson (1969) with the raindrop model of Coquillat and Chauzy (1993). This model provides the drop shape and electric surface field necessary to calculate the corona occurrence altitude from a fitting of Dawson's data. The original results are presented in the form of vertical profiles of the critical field, which is the ambient field that causes disruption or a corona. These results are directly comparable with in situ measurements of electric field, raindrop size, and net charge. If we make the assumption that positive streamer propagation is of prime importance for lightning initiation, the critical field profiles allow us to determine the minimum net charge of a drop which could initiate a discharge in a given ambient field.