A modified algorithm for calculating the cloud index

The cloud index is an important component of the Heliosat algorithm, which estimates solar radiation components from Meteosat High Resolution Visible images. The cloud index quantifies the reflective properties of the atmosphere, and varies from 0 at clear conditions to 1 at overcast. The algorithm is semi-empirical in the way that it includes several constants that need to be tuned. Some of these were removed in the Heliosat-II algorithm where the Meteosat calibration constant was introduced to replace the “pseudo-reflectivity” with a “real reflectivity”. This approach is followed here, and two additional changes are made: (1) An analytical expression is introduced to correct for backscattered radiation from air molecules. (2) A correction is made for non-lambertian reflectivity, removing the time consuming need for determining the ground reflectivity for each month and each slot. The new cloud index is used to calculate global irradiances which are validated against hourly measurements from five European ground stations. The average root mean square deviation is 15.5% for a six-month spring/summer period, of comparable accuracy as using the more time consuming traditional algorithm used in the EU project Satel-Light.