Comparison of Spectral Direct and Diffuse Solar Irradiance Measurements and Calculations for Cloud-Free Conditions

The comparison of spectral measurements and calculations provides an excellent foundation for analyzing the ability of models to compute the solar irradiance absorbed in the atmosphere. Since molecular absorption bands have distinctive spectral signatures, the absence of an atmospheric absorber in a model would result in spectral measurement-model residuals qualitatively different from those caused by other likely errors. This study presents comparisons between ground-based direct and diffuse irradiance measurements from the Rotating Shadowband Spectroradiometer (RSS) (Harrison et al. 1999) and corresponding multiple-scattering calculations performed by the Code for High Resolution Accelerated Radiative Transfer (CHARTS) (Moncet and Clough 1997). The direct beam residuals alone provide persuasive evidence against the existence of significant unmodeled molecular absorption, with the diffuse irradiance comparisons furnishing effective corroborating information. Additionally, the diffuse irradiance results provide vital information concerning aerosol scattering properties.