Can aerosol decrease cloud lifetime?

Cloud responses to changes in aerosol remain a dominant uncertainty in the radiative forcing of climate. Two main constructs related to aerosol effects on clouds have been postulated: (i) the "albedo effect'' whereby anthropogenic aerosol results in increased droplet concentrations that generate increases in cloud albedo, all else (particularly cloud water) being equal; (ii) the "lifetime effect'' whereby anthropogenic aerosol suppresses precipitation and results in clouds with more liquid water, higher fractional cloudiness, and longer lifetimes. Based on new observations presented here, and supported by previous fine-scale modeling studies, we suggest that the balance of evidence shows that non-precipitating cumulus clouds can experience an evaporation-entrainment feedback, and respond to aerosol perturbations in a manner inconsistent with the traditional "lifetime effect.'' Because most cumulus clouds evaporate without producing significant precipitation, this is particularly relevant to estimates of aerosol indirect effects on climate. Citation: Small, J. D., P. Y. Chuang, G. Feingold, and H. Jiang (2009), Can aerosol decrease cloud lifetime?, Geophys. Res. Lett., 36, L16806, doi: 10.1029/2009GL038888.