A CLIMATOLOGICAL PARAMETERIZATION FOR CUMULUS CLOUDS

A research airplane was used to study the microphysical characteristics of ice-free, nonprecipitating summertime cumulus clouds in Montana. Each cloud was penetrated at a multiplicity of level encompassing, in general, a large fraction of the cloud depth. Similar studies covering a more limited altitude range were made in New Mexico. The clouds were substantially diluted by entrainment of environmental air, which produced great variability-at all levels and on all scales of measurement-in the liquid water content L, and droplet number concentration, N. The effective radius, r(eff), at any particular level was found to be essentially independent of L or N. Consideration of this result leads to the prediction that the parameter A = r(eff)/r(ad) almost-equal-to 1 everywhere within these clouds, where r(ad) = (3/4-pi-rho-w)1/3(L(ad)/N(ad))1/3, N(ad) and L(ad) are the "adiabatic" values of N and L, and rho-w is the density of water. Analysis of the airborne data (35 cloud penetrations) for the Montana cumuli reveals that A = 0.83 +/- 0.07, while for the New Mexico study (25 penetrations) A = 0.93 +/- 0.05. Thus, the foregoing prediction is confirmed to a reasonable degree of accuracy. Model calculations for both Montana and Hawaii cumulus consistently yield values of A close to 1.O. It is considered that the parameter A should be useful in climate modeling.