Modeling of Solar Spectral Irradiance Data from Cloudless to Overcast Skies

Solar spectral models applicable to partly cloudy and overcast skies as well as cloudless skies are developed by the analyses of standing spectral observation data in Japan. To characterize solar spectral irradiance in detail by decomposing a global spectrum into a direct and diffuse spectrum, not only global spectra but also diffuse spectra have been observed. The average photon energies of the diffuse spectra were changed largely by the cloud cover in contrast to the little dependence in the direct spectra, which confirms the importance of separate observations. Spectral models describing the effects of clouds on the direct and diffuse spectra were derived; these models were demonstrated by regression analyses of the observations. Additionally, we have revealed a relationship associating Rayleigh scattering intensity in diffuse irradiance with the diffuse broadband irradiance and the solar altitude, which permits us to estimate the model parameter from the available parameters. (C) 2012 The Japan Society of Applied Physics