Preliminary evaluation of the effect of electro-coalescence with conducting sphere approximation on the formation of warm cumulus clouds using SCALE-SDM version 0.2.5-2.3.0

Abstract. The analytic expression for electro-coalescence with the accurate electrostatic force for a pair of droplets with opposite sign charges is established by treating the droplets as conducting spheres (CSs). Then, the weak electric effect on a cumulus cloud is investigated by size resolved cloud model with particle treatment of the super-droplet method. The results show that with CS treatment, the electrostatic force contributes a larger effect on cloud evolution than previous research. With a 3 % lower charge limit of the maximum charge amount of the droplet, the domain total precipitation with CS treatment for droplets with opposite signs is 52.5 % higher than that with the no charge (NC) setting. Compared with previous work by Khain et al. (2004), with the multi-image-dipole treatment of CS, the amount of precipitation is 5.42 % higher. It is found that the charged droplets could affect cloud formation even when the droplet charge is lower charge limit. High pollution levels result in greater sensitivity to electro-coalescence. The results show that when the charges ratio between two droplets is over 100, the short-range attractive electric force due to the multi-image dipole would also significantly enhance precipitation for the cumulus. It is indicated that although the accurate treatment of the electrostatic force with CS method would require 30 % longer computation time than before, it is worthwhile to include it in cloud, weather, and climate models.