A Cloud-Evaporation Parameterization for General Circulation Models

Abstract

Abstract An evaporation-zone (EZ) model for cloud evaporation is developed. In this model a cloud consists of I “cloudlets,” each comprising cloud droplets with radii from zero to rmax, the latter value depending on the drop size distribution (DSD). Evaporation occurs only within the EZ comprised of J ≤ I cloudlets. When the cloudlet at cloud edge evaporates, the EZ progresses one cloudlet into the cloud's interior. This eventually results in evaporation of the cloud in time tE = K(H/h)r2max(1 − Se)−1, where H is the cloud thickness h the EZ thickness, Se the environmental saturation ratio, and K a constant. Values of tE(1 − Se,) versus h are presented for eight observed DSDs. For use in atmospheric general circulation models (GCMs), the cloud evaporation process is represented by dm/dt = −(1 − Se)m/τ, where m is the cloud-water mixing ratio and τ = K(H/h)r2maxn−1. With parameter n chosen sufficiently large, a GCM cloud will evaporate virtually entirely in time tE, for example, 99.3% for n = 5. Values of ...