A numerical model of cumculonimbus allowing a simulation of the growth of the electric field is described. The cloud model is three dimensional and includes a representation of the ice phase. The charge generating mechanism is assumed to arise from the collision of rimed hail particles with smaller ice crystals. Two possibilities are considered: an inductive method, in which hail particles are polarized in the local electric field, and a non-inductive ice-ice method, where charge transfer is attributed to a difference in surface potential of the ice particles in contact.
These mechanisms are compared for a test cloud displaying the rainfall characteristics of a typical, small maritime thunderstorm and their sensitivity to the assumed form of the hail size spectrum examined. A cloud of relatively small size is chosen to constitute a more severe criterion of the strength of the electrification mechanisms than is possible in large thunderstorms where weaker modes of charge transfer could also produce a lightning discharge. Hence the comparison of mechanisms presented is specifically for a small thunderstorm. The results indicate that the electric field can reach a breakdown threshold within half an hour of the appearance of precipitation with the inductive method, providing that the hail size spectrum represents relatively small particles and that the effect of multiple collisions of any ice crystal with more than one hail particle is discounted, i.e. when it is assumed that ice crystals are uncharged before impact. It is found that the non-inductive ice-ice mechanism can lead to lightning within a similar time providing that the product of ice particle concentration and charge separation per collision exceeds 7 pC per litre.