MICROPHYSICAL RESPONSE OF CLOUD DROPLETS IN A FLUCTUATING UPDRAFT

摘要

The effect of a fluctuating updraft upon a distribution of cloud droplets is examined. Computations are performed for fourteen vertical velocity patterns;each allows a closed parcel of cloud air to undergo downward as well as upward motion. Droplet solution and curvature effects are included. It is found that with suitable conditions, adiabatic warming during downward motion causes small droplets to evaporate, and large, unactivated droplets to continue growing. Periods of long, slow, downward motion followed by uplifting can result in a broadening of the droplet size distribution in the lowest 100 meters of cloud height. The results depend upon the frequency and amplitude of the vertical velocity fluctua-tions.