This poster reports results of a project involving 15 models by 12 groups to examine radiative forcing by monodisperse sulfate aerosol for a wide range of values of particle radius, aerosol optical depth, surface albedo, and solar zenith angle. The models included high- and low-spectral resolution models, incorporating a variety of radiative transfer approximations, as well as a line-by-line model. The normalized forcings (forcing per sulfate column burden) obtained with the several radiative transfer models were examined and the differences characterized. All models simulate forcings of comparable amplitude and exhibit a similar dependence on input parameters. As expected for a non-light-absorbing aerosol, forcings were negative (cooling influence), except at high surface albedo combined with low solar zenith angle. The relative standard deviation of the zenith-angle-average normalized broadband forcing for 15 models was 8% for particle radius near the maximum in magnitude of this forcing (ca. 200 nm) and at low surface albedo. Somewhat greater model-to-model differences were exhibited at specific solar zenith angles. Still greater differences were exhibited at small particle radii, and much greater discrepancies at high surface albedos, at which the forcing changes sign; in these situations, however, the normalized forcing is quite small. Differences among the models arise mainly from differing treatment of the angular scattering phase function, differing wavelength and angular resolution, and differing treatment of multiple scattering. The relatively small spread in these results suggests that the uncertainty in forcing arising from treatment of radiative forcing of a well characterized aerosol at well specified surface albedo is a minor source of uncertainty compared to that from representing other processes influencing direct forcing by anthropogenic sulfate aerosols and anthropogenic aerosols generally.
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