A Dynamic Model of Thundercloud Electric Fields

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Abstract
A dynamic interactive computer model of the electrical behavior of a thundercloud surrounded by the distributed atmosphere, earth, ionosphere circuit is described. The electrification mechanisms in the model are represented by current or voltage generators. The electrical breakdown is taken care of by switches that close when the voltages between nodes correspond to assumed breakdown electric fields and open again when the voltage fall below those necessary to sustain the arc. The model is thus able to simulate the very complicated interactive processes that occur in and around a thundercloud due to the linear conduction and displacement currents, as well as the nonlinear charge redistribution processes that occur during arc breakdown within and outside the cloud. The new models are intended to serve as an extension of the convective dynamic and microphysical models. They allow predictions to be made of the interrelation of the observable electrical parameters of the storm to the geometry and parameters of the electrical generating mechanisms. It is shown that there are major differences between the behavior of a fully interactive dynamic model and the simple monopole or dipole models on the one hand and models which neglect atmospheric conduction and displacement currents and electrical breakdown on the other. To demonstrate the possibilities of this type of model, several exploratory studies were carried out. The effects of variations in the height of the cloud above ground and of the vertical separation between the top and bottom of the cloud were investigated. Changing the cloud conductivity from one tenth to ten times the conductivity of the surrounding atmosphere was shown to have only a small effect on the external electric fields and on the current to the ionosphere. Several generator configurations were examined to study the effect of partial neutralization of the precipitation at the bottom of the cloud. The effect of neutralization between the cloud and ground was also examined. It was shown that only in cases when the cloud current was almost completely neutralized in the bottom of the cloud or when almost all the current flowed to the ground could cloud-to-ground strokes occur.

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