A CLOUD-EVAPORATION PARAMETERIZATION FOR GENERAL-CIRCULATION MODELS

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 r(max), the latter value depending on the drop size distribution (DSD). Evaporation occurs only within the EZ comprised of J less-than-or-equal-to 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 t(E) = K(H/h)r(max)2(1 - S(e))-1, where H is the cloud thickness, h the EZ thickness, S(e) the environmental saturation ratio, and K a constant. Values of t(E)(1 - S(e)) 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 - S(e))m/tau, where m is the cloud-water mixing ratio and tau = K(H/h)r(max)2 n-1. With parameter n chosen sufficiently large, a GCM cloud will evaporate virtually entirely in time t(E), for example, 99.3% for n = 5. Values of tau for use in our multilayer atmospheric GCM have been determined by performing ten perpetual-January simulations and ten perpetual-July simulations, each set of ten for prescribed pairs of tau values for stratiform (tau(s)) and cumuloform (tau(c)) clouds. An optimum choice of tau(s) and tau(c), based on minimizing the errors of the model's simulated cloudiness, planetary albedo, outgoing longwave radiation, and precipitation, is tau(s) = tau(c) = 3 min. This corresponds to t(E)(1 - S(e)) = 15 min for both stratiform and cumuloform clouds; hence, to an EZ thickness of about 0.6-0.8 m for stratus, stratocumulus, and altostratus clouds, 2-3 m for nimbostratus and cumulus clouds, and 17 m for cumulonimbus clouds.