INTERCOMPARISON OF MODELS REPRESENTING DIRECT SHORTWAVE RADIATIVE FORCING BY SULFATE AEROSOLS

The importance of aerosols as agents of climate change has recently be en highlighted. However, the magnitude of aerosol forcing by scatterin g of shortwave radiation (direct forcing) is still very uncertain even for the relatively well characterized sulfate aerosol. A potential so urce of uncertainty is in the model representation of aerosol optical properties and aerosol influences on radiative transfer in the atmosph ere. Although radiative transfer methods and codes have been compared in the past, these comparisons have not focused on aerosol forcing (ch ange in net radiative flux at the top of the atmosphere). Here we repo rt results of a project involving 12 groups using 15 models to examine radiative forcing by sulfate aerosol for a wide range of values of pa rticle radius, aerosol optical depth, surface albedo, and solar zenith angle. Among the models that were employed were high and low spectral resolution models incorporating a variety of radiative transfer appro ximations as well as a line-by-line model. The normalized forcings (fo rcing per sulfate column burden) obtained with the several radiative t ransfer models were examined, and the discrepancies were characterized . All models simulate forcings of comparable amplitude and exhibit a s imilar dependence on input parameters. As expected for a non-light-abs orbing aerosol, forcings were negative (cooling influence) except at h igh surface albedo combined with small solar zenith angle. The relativ e standard deviation of the zenith-angle-averaged normalized broadband forcing for 15 models was 8% for particle radius near the maximum in this forcing (similar to 0.2 mu m) and at low surface albedo. Somewhat greater model-to-model discrepancies were exhibited at specific solar zenith angles. Still greater discrepancies were exhibited at small pa rticle radii, and much greater discrepancies were exhibited at high su rface albedos, at which the forcing changes sign; in these situations, however, the normalized forcing is quite small. Discrepancies among t he models arise from inaccuracies in Mie calculations, differing treat ment of the angular scattering phase function, differing wavelength an d angular resolution, and differing treatment of multiple scattering. These results imply the need for standardized radiative transfer metho ds tailored to the direct aerosol forcing problem. However, the relati vely small spread in these results suggests that the uncertainty in fo rcing arising from the treatment of radiative forcing of a well-charac terized aerosol at well-specified surface albedo is smaller than some of the other sources of uncertainty in estimates of direct forcing by anthropogenic sulfate aerosols and anthropogenic aerosols generally.