Speaker
Description
Rain formation in clouds begins with collisions and coalescence among small droplets, which govern the evolution of the droplet size distribution and the onset of precipitation. Because cloud droplets often carry electric charges and experience atmospheric electric fields—from weak fair-weather fields to intense thundercloud fields—electrostatic forces can modify their collision dynamics. In this study, we examine how droplet charges and external electric fields alter pairwise trajectories and collision rates for droplets settling under gravity or interacting in laminar background flows. Our analysis accounts for exact hydrodynamic interactions, electrostatic forces, and van der Waals forces. Furthermore, we incorporate these effects within a Smoluchowski framework to predict the evolution of the droplet size distribution. We show that electrostatic interactions can substantially enhance collision rates, promoting faster droplet growth and potentially accelerating precipitation initiation, with important implications for cloud microphysics parameterizations and weather prediction models.