Collisions of ice crystals with water droplets in turbulent flows

Riming, the process whereby ice crystals get coated by impacting supercooled liquid droplets, is one of the dominant processes leading to precipitation in mixed-phase clouds. The present numerical study aims at providing insight on how turbulence affects the riming of ice crystals, which we model as very small, narrow oblate ellipsoids. By neglecting the effect of fluid inertia on the motion of the crystals and droplets, and using direct numerical simulations of the Navier–Stokes equations in a moderately turbulent regime, with a kinetic energy dissipation, ε, in the range 1 cm² s^-3 ≤ ε ≤ 256 cm² s^-3, we determine the collision rate between disk-shaped ice crystals and very small liquid water droplets. Whereas differential settling plays the dominant role in determining the collision rate at small turbulence intensity, the role of turbulence becomes more important as ε increases, an effect that can be partly attributed to the role of inertia. The difference in the settling velocity of crystals and droplets induces a strong asymmetry in the probability of collision between the faces of the ellipsoids. Collisions occur with a large probability on the rim of the ellipsoids, a phenomenon that can be explained by kinematic considerations.