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

The efficiencies with which ice crystals at low-intermediate Reynolds numbe rs collide with supercooled cloud droplets are determined numerically. Thre e ice crystal habits are considered here: hexagonal ice plates, broad-branc h 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 ab out 100 mu m in radius. The collision efficiencies are determined by solvin g the equation of motion for a cloud droplet under the influence of the flo w field of the falling ice crystal. The flow fields of the falling ice crys tals were determined previously by numerically solving the unsteady Navier- Stokes equations. Features of these efficiencies are discussed. The compute d efficiencies are compared with those obtained by previous investigators a nd improvements are indicated. New results fit better with the observed rim ing droplet sizes and cutoff riming ice crystal sizes.