Collision efficiencies of ice crystals at low-intermediate Reynolds numbers colliding with supercooled cloud droplets: A numerical study

Abstract

The efficiencies with which ice crystals at low-intermediate Reynolds numbers collide with supercooled cloud droplets are determined numerically. Three ice crystal habits are considered here: hexagonal ice plates, broad-branch crystals, and columnar ice crystals. Their Reynolds numbers range from 0.1 to slightly beyond 100. The size of cloud droplets range from a few to about 100 μm in radius. The collision efficiencies are determined by solving the equation of motion for a cloud droplet under the influence of the flow field of the falling ice crystal. The flow fields of the falling ice crystals were determined previously by numerically solving the unsteady Navier-Stokes equations. Features of these efficiencies are discussed. The computed efficiencies are compared with those obtained by previous investigators and improvements are indicated. New results fit better with the observed riming droplet sizes and cutoff riming ice crystal sizes.