An expression for a globally averaged value of direct radiative forcing by absorbing aerosols is derived and applied to the case of smokes produced by biomass burning. It is shown that the direct radiative forcing due to the biomass burning aerosols is a sensitive function of the size distribution of aerosol particles. For the range of measured size distributions of smoke aerosols the direct radiative forcing varies between −0.2 and −1.1 W/m².

[1991] showed that the distribution of sulfate aerosol could reflect a significant amount of shortwave radiation back to space and that this so-called direct effect of sulfate aerosol on the radiative budget had significant spatial variability. Kiehl and Briegleb [1993] showed that regionally, the direct effect of sulfate aerosols could offset the greenhouse forcing due to increases in CO 2, CH4, N20 , and CFCs. Thus sulfate aerosols alter the spatial pattern of net climate forcing due to anthropogenic activity.Jones et al. [1994] argued that the effect of sulfate aerosols on cloud albedo, the indirect albedo effect, could be as large as or larger than the direct radiative effect of sulfate aerosols. Jones and Slingo Model Description

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Abstract.The importance of aerosols as agents of climate change has recently been highlighted. However, the magnitude of aerosol forcing by scattering of shortwave radiation (direct forcing) is still very uncertain even for the relatively well characterized sulfate aerosol.A potential source of uncertainty is in the model representation of aerosol optical properties and aerosol influences on radiative transfer in the atmosphere. Although radiative transfer methods and codes have been compared in the past, these comparisons have not focused on aerosol forcing (change in net radiative flux at the top of the atmosphere). Here we report results of a project involving 12 groups using 15 models to examine radiative forcing by sulfate aerosol for a wide range of values of particle 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 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 discrepancies were 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 small solar zenith angle. The relative standard deviation of the zenith-angle-averaged normalized broadband forcing for 15 models was 8% for particle radius near the maximum in this forcing (.-•0.2 tzm) 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 particle radii, and much greater discrepancies were exhibited at high surface albedos, at which the forcing changes sign; in these situations, however, the normalized forcing is

The exact solution of the scattered electromagnetic field from a water droplet containing an arbitrarily located spherical black carbon particle is used to investigate the effect of black carbon on the absorption of solar radiation by clouds. When droplet absorption is averaged over all possible locations of black carbon within a droplet, the averaged absorption is close to the value calculated using the effective medium approximation. The preferential black carbon location on the top or close to the bottom of the droplet leads to an increased absorption. The estimated upper bound on the increased absorption of solar radiation (global and annual average) is 1-3 W/m 2 over the absorption of pure water clouds. 1. Black Carbon Atmospheric Loading: Estimate of Lower and Upper BoundsTo determine the amount of solar radiation absorbed by black carbon, the atmospheric black carbon concentrations must be known. Although there have been several measure-•Atmospheric Science Program,