BEHAVIOR OF PRECIPITATING WATER DROPS UNDER THE INFLUENCE OF ELECTRICAL AND AERODYNAMIC FORCES

The present work performs a realistic modeling of precipitating charged water drops under the influence of electrical and dynamical forces in the vertical and downward electric field of a thundercloud. The following factors which control the shape of an individual raindrop are taken into account: surface tension, internal hydrostatic pressure, aerodynamic pressure, and electrostatic pressure. Unlike a recent and notable work by Chuang and Beard (1990) in which this problem is approached by adjusting an empirical pressure distribution for the distortion, our model considers simple local pressure balance to determine the drop shape. This computation aims at characterizing drop distortion, falling speed modification, and disruption. The overall present results are similar to those of Chuang and Beard's more sophisticated model, and the predicted critical fields are even closer to wind tunnel measurements by Richards and Dawson (1971). The disruption of positively charged drops requires lower ambient fields than that of the negatively charged drops, and for highly charged and large drops they are of the order of those commonly measured within thunderclouds. At last, the terminal velocity is highly affected by net charge and ambient field. These processes are probably important in lightning initiation during drop disruption.