Inhomogeneities in the water properties of fogs and clouds

Abstract

ALTHOUGH there is good evidence that entrainment of undersaturated environmental air exercises a profound influence upon the development of the droplet spectra in clouds, the classical description of the mixing process¹ has failed to yield quantitative predictions of spectral evolution which are consistent with observation. In the classical model the mixing process is homogeneous, that is, all droplets at a given level within the cloud are, at any time, exposed to identical conditions of supersaturation or undersaturation. Recent laboratory experiments², however, have indicated that the mixing process in clouds may be highly inhomogeneous, with some droplets completely evaporated, or substantially reduced in size, and others negligibly influenced. This work² led to the formulation of a primitive model of inhomogeneous mixing in which it was predicted that sizeable fluctuations in the water properties of natural clouds should exist, on scales commensurate with those of the mixing process. In particular, it was proposed that there may exist closely adjacent regions in which the liquid water content was very different but the spectral shape essentially unchanged. Some recent field evidence supports this idea³ but few observational data are currently available. We describe here some preliminary results indicative of the highly inhomogeneous nature of the water properties of some clouds, on certain spatial scales.