Electrostatic effects on droplet growth: evolution of droplet size distributions in electrified clouds

The collision rate between cloud droplets drives the evolution of the droplet size distribution (DSD); therefore, understanding the collisional dynamics of cloud droplets in air underlies parameterizations in cloud microphysics. In weakly electrified clouds, the accumulation of space charges on droplets is governed by the diffusion of atmospheric ions produced by the flux of cosmic rays. A downward fair-weather electric field always exists because of the potential difference between the Earth's surface and the upper atmosphere. In thunderclouds, the electric field intensifies rapidly due to charge separation driven by various charging mechanisms. Therefore, due to both surface charges and electric field effects, electrostatic forces can significantly alter the collision rate of cloud droplets. This study employs trajectory analysis to examine the collision efficiency of inertial droplets in quiescent air, considering hydrodynamic interactions, electrostatic forces, interfacial mobilities, and van der Waals forces. We solve the Smoluchowski equation to predict DSD evolution with electrostatic effects. Our findings show that electrostatic effects can significantly enhance collision rates, leading to faster droplet growth and potentially accelerating the onset of precipitation.