Microphysical Implications of Precipitation Formation in an Adiabatic Vertical Current: Aerosol Scavenging by Enhanced Nucleation

Abstract

The time-dependent effect of precipitation formation and development on the dynamics and microphysics of adiabatic vertical motion has been studied with the help of one-dimensional numerical models. Since the most important microphysical effect of precipitation formation is nucleation due to enhancement of supersaturation, efforts have been concentrated mainly on the elucidation of this effect. Essentially two time-dependent models have been considered: the first, having all its nucleation near the cloud base, and no additional nucleation as supersaturation develops; and the second, with nucleation proceeding in step with supersaturation development. The second model also examines the question of the activity of nuclei in that experiments have been performed separately with insoluble (but wettable) nuclei and soluble nuclei. The conclusions arrived at are the following: (i) in an updraft, with developing precipitation, a progressive development of supersaturation and the consequent activation of fresh nuclei is an essential requirement for maintaining the updraft in its approximately adiabatic state; (ii) a precipitating (adiabatic) cumulus can scavenge aerosol particles effectively by first activating them to form droplets and then removing the latter in the form of precipitation; (iii) the downdraft driven by precipitation is unsaturated; and (iv) within the restrictions of the model the downdraft is also warmer than the environment except very close to the ground where evaporative cooling causes a slight ?temperature break.? The limitations of the model are discussed, the most important being that the model is adiabatic. Consequently the study reported should he considered as the results obtained in an idealized numerical laboratory, having implications for atmospheric processes.