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Diffuse-Sky Downward Irradiance ( DFDI ) at Surface in Cloud-Free Atmospheres-A Closure Experiment

Radiative transfer models consistently overestimate surface diffuse downward irradiance (DFDI) in cloud-free atmospheres by 9% to 40% at two low altitude sites while correctly calculating direct-normal solar irradiance (DNSI). Of the 32 independent cases analyzed, the amount of overestimation is found to be larger than the combined uncertainties in model inputs, model calculations, and measured DFDI. But models correctly calculate the DNSI. For realistic aerosol optical properties, the only way to reconcile these findings is to reduce sunphotometer-inferred aerosol optical thickness (AOT) by an average 0.022 ± 0.01 at 550 nm, while at the same time increasing continuum-like atmospheric absorptance over the solar spectrum by an average 5% ± 3%. At two high-altitude sites, models and measurements agree to within their mutual uncertainties, suggesting that this phenomenon is present only in the boundary layer. The proposed excess absorption and corresponding reduction in AOT would have important consequences for climate and remote sensing.