An alternative cloud index for estimating downwelling surface solar irradiance from various satellite imagers in the framework of a Heliosat-V method

Abstract. We develop a new way of retrieving the cloud index from large variety satellite instruments sensitive to reflected solar radiation, embedded on geostationary and non-geostationary platforms. The is widely used proxy for effective transmissivity, also called “clear-sky index”. This study in framework development Heliosat-V method estimating downwelling irradiance at surface Earth (DSSI) imagery. To reach its versatility, uses simulations fast radiative transfer model estimate overcast (cloudy) clear-sky (cloud-free) scenes Earth’s reflectances. Simulations consider anisotropy reflectances caused by both atmosphere are adapted spectral sensitivity sensor. ground described bidirectional reflectance distribution function external satellite-derived data. An implementation applied visible imagery Meteosat Second Generation satellite, 11 locations where high-quality situ measurements DSSI available Baseline Surface Radiation Network. For 15 min means DSSI, results our preliminary ground-based show bias 20 W m−2, root-mean-square difference 93 correlation coefficient 0.948. statistics, except bias, similar operational corrected satellite-based data products HelioClim3 version 5 CAMS Service.