Abstract
Most of the precipitation related theories on charge generation in thunderstorms fall into one of two categories: the inductive or polarization mechanism initiated by the ambient fair-weather field, and the non-inductive mechanism connected with certain electrochemical or thermoelectric particle characteristics. Our numerical study addresses the question of which mechanism gives more realistic results with regard to charge distribution and hold strength and what effect a combination of the two processes produces. The investigation is a first attempt using a simplified model.
In this model the microphysical processes of particle growth and simultaneous electrification are embedded in a steady state two-dimensional vortex circulation with and without vertical wind shear. The net space charge and potential are obtained everywhere in the cloud and the resulting electric fields are calculated. Computations are made for the collisions of growing solid precipitation (graupel) particles with either supercooled droplets (ice-water case) or with ice crystals (ice-ice case).
The results indicate that the non-inductive mechanism produces a rapid growth of the electric field in the early stages but tends to level out at a stable value considerably below the breakdown field strength. The inductive mechanism in turn shows a slow initial field growth with quickly varying charge distributions of often inverted polarity; however, it will reach breakdown field strength eventually due to its quasi-exponential growth character. Only the combination of the two processes achieves realistic thunderstorm conditions. It appears that the non-inductive mechanism controls the charge distribution and its polarity, and the inductive mechanism the field strength. both ice-water and ice-ice collisions give similar results, the only difference being a higher elevation of the charge dipole. in the ice-ice case. The opposite precipitation and cloud charges are always strongly masked.
The results permit some interesting conclusions on the origin of the fair-weather field.
