A Quasi‐Analytic Model of Droplet and Ccn Lifetimes, Optical and Chemical Characteristics of Ice‐Free Cumulus Clouds

Abstract

AbstractWe have formulated a semi‐analytic model of the microphysical and passive chemical development within entraining ice‐free cumulus clouds growing in surroundings characteristic of Hawaii and Montana in summertime. Calculations were made of liquid‐water contents, age distributions of droplets and cloud condensation nuclei (CCN), optical parameters (effective radius and optical depth), the concentration of a passive chemical tracer in both cloud and rainwater; and the variability of the concentration of cloud‐water tracer over the droplet spectrum, all as functions of altitude within the clouds, entrainment scale, droplet number concentration and altitudinal variability in environmental tracer concentration.All of these microphysical and chemical parameters were found to be sensitive to the entrainment scale II. Droplets at any level above cloud base may have been formed on primary nuclei or entrained nuclei, either of which may already have activated droplets which subsequently evaporated as a consequence of entrainment. the average nuclear ages are greater (typically by about 50%) than that of the average droplets, the latter being in conformity with those inferred from observations. the effective radius at a given level was only mildly dependent on II, whereas the optical depth increased rapidly as II increased. the concentration of tracer in both cloud water and rainwater decreased markedly as the entrainment scale increased.