Some effects of cloud turbulence on water-ice and ice-ice collisions

Formation of relative velocities between low-density ice particles and the surrounding air within a turbulent medium is analyzed. Turbulence is assumed to be homogeneous and isotropic. Both the inertial and viscous turbulent ranges are taken into account when describing the turbulent flow characteristics. It is shown that the inertia of drops and ice particles falling within a turbulent flow leads to the formation of a significant relative velocity between particles, which can be greater (especially for those with small sedimentation velocity) than gravity-driven velocity difference. It is also shown that for ice particles, the turbulence effects related to particles' inertia are much more pronounced than for water drops. An increase in relative velocity between particles leads to an increase in the swept volume and the collision kernel. As a result, a marked increase in the rates of riming and ice-ice interactions is expected. (C) 1998 Elsevier Science B.V. All rights reserved.