MMM SEMINAR NCAR
"Cancellation of aerosol indirect
effects in marine stratocumulus
through cloud
thinning"
Robert Wood
There is wide support for the idea that increases in aerosol concentration in marine stratocumulus lead to a suppression of precipitation. However, there has been relatively little exploration of the consequences of this suppression other than in climate models that do not resolve stratocumulus well. In this talk, I describe perturbation experiments with Lilly's mixed layer model, which is a simple, but physically meaningful model of the cloud capped marine boundary layer. The simplicity permits a more complete examination of the forcing phase space than is possible with more sophisticated models. The results, which indeed support recent LES simulations, indicate that feedbacks involving enhanced entrainment are critical to understanding the response of stratocumulus to perturbations in cloud droplet concentration. Further, the results suggest that for a wide range of forcing conditions, particularly those that lead to mixed layers with cloud base heights of 400 m or more, the radiative forcing associated with the second indirect effect tends to partially cancel that from the Twomey effect. The results imply that aerosol indirect effects in marine stratocumulus may be weaker than previously supposed. Given the importance of small scale processes to this cancellation, it is reasonable to think that global climate models will not accurately resolve this cancellation and simple tests could and should be devised to assess this possibility.
Refreshments
NCAR-Foothills Laboratory
Bldg 2 Auditorium (Rm1022)