Drop Size Separation in Numerically Simulated Convective Clouds and Its Effect on Warm Rain Formation

Abstract

It is shown that, as a result of the flow diffluence at the upper levels of the cloud, droplets of different sizes move along different trajectories. The small droplets with negligible fall velocities will have higher probability of being carried away from the cloud and completely evaporating, while the droplets with larger fall velocities are more likely to be recycled in the cloud. During the recycling process droplets of different sizes reenter the cloud at different spatial locations and mix with the droplet-free air brought from the upper levels of the cloud. This results in the decrease of the total concentration, as well as in the concentration of larger droplets, thus facilitating rain formation through enhanced condensational growth. It is also shown that the notion of an air parcel as an entity containing various constituents (water vapor, aerosol particles, cloud droplets, etc.), all evolving under the same dynamical conditions, may be rather limited. Our results indicate that these elements may have quite different histories resulting in inhomogeneities in cloud microstructure.