Effects of pressure and humidity on positive corona inception from thundercloud hydrometeors

This paper reports a study on the inception condition of positive corona discharges around thundercloud hydrometeors that are simulated as a spherical point electrode. The corona inception from the hydrometeor is investigated using a model developed by Naidis (2005), which suggests that the discharge becomes self-sustaining when the number of ionizing photons produced by all secondary electron avalanches is equal to that by a primary avalanche. For an isolated, charged hydrometeor in dry air of varying pressure, the results show that the onset of positive corona discharges from a large hydrometeor at high pressure requires a stronger avalanche multiplication than those from a small hydrometeor at low pressure. The onset voltage and surface electric field are obtained for thundercloud hydrometeors of different radii. For a hydrometeor of a radius of 1 mm at atmospheric pressure, the onset surface field reaches about 2.75 times the conventional breakdown threshold field. In humid air (a gas mixture of dry air and water vapor), it is found that a stronger avalanche is required for the corona onset, and the effect of humidity is more pronounced for large hydrometeors. The absorption of the ionizing photons by water vapor is the main reason why a stronger avalanche is required for the corona inception in humid air. As corona discharges from an isolated hydrometeor remove the charge on it, this discharging mechanism sets an upper limit on hydrometeor charge that agrees reasonably with the reported observations of maximum precipitation charge. Finally, we discuss the corona inception condition from a charged hydrometeor in non-zero ambient field of thundercloud condition. & 2012 Elsevier Ltd. All rights reserved.