Rapid droplet coalescence produced by thunder

Abstract

This article considers a simple model to numerically study whether thunderclaps can produce droplet coalescence in natural clouds due to the relative velocities they induce between different-sized droplets. This mechanism is evaluated for a cloud that has both a relatively small liquid content and a narrow droplet size distribution. The cloud is idealized as a static environment, consisting only of water droplets and air, in which no changes occur except those produced by thunderclaps. All other coalescence mechanisms, such as gravitational effects and air turbulence, are disregarded. The thunderclap models we use mimic the profile of an actual clap far from a lightning discharge. Their acoustic power spectrum is obtained analytically and found to peak in the infrasonic frequency range, in agreement with some experimental results. The velocities of the droplets during the passage of a clap are obtained from their equations of motion using drag coefficients that apply in the conditions imposed by the clap. The possibility that droplet growth may occur as a result of the claps is studied by integration of the discretized coalescence equation. For the case considered, the results show that a thunder event, consisting of several consecutive claps, can produce in a few seconds significant changes in the mean droplet diameters. Such times are orders of magnitude shorter than those required by other coalescence mechanisms to produce similar effects.