PNNL

Abstract

Abstract: Satellite-based estimates of aerosol indirect effect (AIE) are consistently smaller than the estimates from global aerosol models, and, partly as a result of this difference, estimates for this climate forcing include large uncertainties. Satellite estimates typically use the present day (PD) relationship between observed cloud drop number concentrations (Nc) and aerosol optical depths (AOD) to determine the pre-industrial (PI) values of Nc. These values are then used to determine the PD and PI cloud albedos and, thus, the effect of anthropogenic aerosols on TOA fluxes. Here, we use a model with realistic aerosol and cloud processes to show that empirical relationships for ln(Nc) versus ln(AOD) derived from PD results do not represent the atmospheric perturbation caused by the addition of anthropogenic aerosols to the pre-industrial atmosphere. As a result, the empirical satellite-derived estimates of the AIE are between a factor of 3 to more than a factor of 6 smaller than model estimates based on actual PD and PI values for Nc. Using ln(Nc) versus ln(AI) (Aerosol Index, or the optical depth times Ångström exponent) to estimate pre-industrial values for Nc provides estimates for Nc and forcing that are closer to the values predicted by the model. Nevertheless, the AIE using ln(Nc) versus ln(AI) may be substantially incorrect on a regional basis, and may underestimate or overestimate the global average forcing by of order 25 to 35%.