Turbulence Effects of Collision Efficiency and Broadening of Droplet Size Distribution in Cumulus Clouds

AbstractThis paper aims at investigating and quantifying the turbulence effect on droplet collision efficiency and exploring the broadening mechanism of droplet size distribution (DSD) in cumulus clouds. The sophisticated model employed in this study individually traces droplet motions affected by gravity, droplet disturbance flows, and turbulence in a Lagrangian frame. Direct numerical simulation (DNS) techniques are implemented to resolve the small-scale turbulence. Collision statistics for cloud droplets of radius between 5-25μm at five different turbulence dissipation rates (20-500cm2s⁻3) are computed and compared with pure-gravity cases. The results show that the turbulence enhancement of collision efficiency highly depends on the r-ratio but is less sensitive to the size of the collector droplet investigated in this study. Particularly, the enhancement is strongest among comparable-sized collisions, indicating that turbulence can significantly broaden the narrow DSD resulting from condensational gro...