Acceleration intermittency and enhanced collision kernels in turbulent clouds

Abstract

Scaling arguments suggest that in turbulent clouds, the droplet collision kernel is a fluctuating quantity with peak values at least an order of magnitude greater than the typically used kernel corresponding to droplet sedimentation in still air. These peaks are expected to occur in regions of intense fluid acceleration that are encountered in the fine scales of atmospheric turbulence, characterized by exceedingly large Reynolds numbers and strong energy-dissipation intermittency. The increase in the collision kernel is due to enhanced droplet relative velocities resulting from fluid accelerations and likely is further enhanced by increases in the droplet collision efficiency, which is a function of relative velocity. Velocity data obtained from a turbulent atmospheric flow are presented in order to support the general picture of acceleration intermittency and to estimate its peak values. Several independent estimates all point to the collision kernel as having peak magnitudes several times larger than the traditional kernel that accounts for gravitational acceleration alone.