No abstract

No abstract

Recent estimates of global or hemispheric average forcing of climate by anthropogenic sulfate aerosol caused by scattering of shortwave radiation (“direct” effect) are uncertain by somewhat more than a factor of 2. The principal sources of this uncertainty are atmospheric chemistry properties (yield, residence time), and microphysical properties (scattering efficiency, upscatter fraction, and the dependence of these properties on particle size, composition, and relative humidity, (RH)). This paper examines the sensitivity of forcing to these microphysical properties to identify and improve understanding of the properties required to reduce the uncertainty in the forcing. The relations between aerosol loading and forcing developed here are suitable for comparing modeled and measured aerosol forcing at specific locations and for use in climate models, provided aerosol composition and microphysical properties are known, calculated, or assumed. Results are presented showing the dependence of scattering efficiency, upscatter fraction, and normalized forcing (W m−2/g(SO24−) m−2 or W g(SO24−)−1) on dry particle size (expressed as mole(sulfate) per particle), composition ((NH4)2SO4, NH4HSO4, H2SO4), solar zenith angle, latitude, and season. Forcing is strongly dependent on dry particle size and RH but is relatively insensitive to composition. The normalized forcing can be integrated over a known or assumed size distribution to evaluate the sulfate aerosol forcing. Global and annual average values of the normalized forcing are evaluated as a function of particle size and RH. Depending on values of these variables, normalized forcing may be less than, intermediate to, or greater than the range of previous estimates of sulfate aerosol forcing.

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

No abstract