A Comparison of Modeled and Measured Surface Shortwave Irradiance for a Molecular Atmosphere

There is a growing body of evidence that models overestimate clear-sky downward shortwave surface irradiances (Charlock and Alberta 1996, Kato et al. 1997, Kinne et at. 1997, Wild et al. 1981). Kato et al. (1997) showed that their model, with the best available gaseous absorption cross section data, accurately computes direct irradiance provided that the model has as input accurate water vapor and ozone profiles and extinction optical thickness of the atmosphere at a set of discrete wavelengths. However, the overestimate of the downward diffuse irradiance by their model is significant; the difference is largest near 400 nm and decreases with increasing wavelength. The two-stream approximation, which was used in their study, has a reported accuracy of better than 10% when computing the diffuse irradiance of a molecular atmosphere, based on comparisons with more sophisticated models (Toon et al. 1989). Our goal in this paper is to determine the agreement between the modeled and measured downward diffuse irradiance and to enhance our confidence level in our model calculations and measurements for a molecular atmosphere.