Aerosol effects on the lifetime of shallow cumulus

Abstract

We explore the effects of increases in aerosol concentration on cloud lifetime for warm convective clouds using a two-dimensional single cloud model and three-dimensional large eddy simulations (LES). The models include size-resolved treatment of drop size distributions and warm microphysical processes. It is shown using a variety of soundings representing marine trade cumulus, and continental convective clouds that contrary to expectation, an increase in aerosol concentration from very clean to very polluted does not increase cloud lifetime, even though precipitation is suppressed. Cloud lifetimes are statistically similar although individual clouds may experience decreases in lifetime of 10-40%. An evaporation-entrainment feedback that tends to dilute polluted clouds more than clean clouds is identified. It is proposed that the small changes in cloud lifetime are due to competing effects of precipitation suppression and enhanced evaporation, with the latter tending to dominate in these shallow clouds. Copyright 2006 by the American Geophysical Union.