Modelling of interaction processes between two raindrops in an electrical environment

The behaviour of two raindrops of opposite polarity interacting in an electric field has been investigated numerically with a view to eventually identifying the microphysics and electrical conditions favourable for the natural triggering of cloud discharges. In the past, a few studies have been carried out on the subject of distortion and disintegration of drop‐pairs but in none of them has the aerodynamic pressure featured in the equation for the pressure balance between the two drops. Consequently in what follows we have expanded and modified the earlier model for the distortion of an isolated raindrop (Coquillat and Chauzy 1993) in which the effect of the aerodynamic pressure is taken into account. the action of this pressure on each drop leads to oblate shapes, whereas in the earlier studies both drops became prolate. The mutual approach of two drops of opposite polarity and/or an increase in the ambient electric field do not always have the same effect on the distortion of each drop, so revealing the highly nonlinear character of the interaction processes between two hydrometeors. In fact the local electric fields do not necessarily increase (they can even decrease) as the distance, d , between them decreases and/or as the ambient field increases. Therefore it does not follow that the corona emission may be triggered more easily from a drop‐pair than from an isolated drop. We show that a short air gap between the two drops is not equivalent to a strong ambient field. In particular, a pair of closely separated drops may coexist in the weakest and in the strongest intensities of the ambient electric field, but not in the mid intensities. We finally discuss the consequences of cloud electrification.