Effects of Aerosol and Horizontal Inhomogeneity on the Broadband Albedo of Marine Stratus: Numerical Simulations

Abstract

Recent estimates of the effect of increasing amounts of anthropogenic sulfate aerosol on the radiative forcing of the atmosphere have indicated that its impact may be comparable in magnitude to the effect from increases in CO2. Much of this impact is expected from the effects of the aerosol on cloud microphysics and the subsequent impact on cloud albedo. However, internal horizontal variations in cloud optical properties are also known to affect cloud albedo. A solar broadband version of a 2D radiative transfer model was used to quantify the impact of enhanced aerosol concentrations and horizontal inhomogeneity on the solar broadband albedo of marine stratus. The 2D cross sections of cloud physics data taken from a set of 3D RAMS/LES simulations of marine stratus provided realistic optical property data for radiative transfer simulations. A control run using typical marine CCN concentrations and a sensitivity study using enhanced concentrations of CCN were examined. The results of the radiative transfer calculations indicated that in unbroken marine stratus clouds the net horizontal transport of photons over a domain of a few kilometers was nearly zero, and the domain-average broadband albedo computed in a 2D cross section was nearly identical to the domain average calculated from a series of independent pixel approximation (IPA) calculations of the same cross section. However, the horizontal inhomogeneity does affect the cloud albedo compared to plane-parallel approximation (PPA) computations due to the nonlinear relationship between albedo and optical depth. The reduction in cloud albedo could be related to the variability of the distribution of log (cloud optical depth). These results extend the findings of Cahalan et al. to broadband solar albedos in a more realistic cloud model and suggest that accurate computation of domain-averaged broadband albedos in unbroken (or nearly unbroken) marine stratus can be made using IPA calculations with ID radiative transfer models. Computations of the mean albedo over portions of the 3D RAMS domain show the relative increase in cloud albedo due to a 67% increase in the boundary-layer average CCN concentration was between 6% and 9%. The effects of cloud inhomogeneity on the broadband albedo as measured from the PPA bias ranged from 3% to 5%.