The CMSAF hourly solar irradiance database (product CM54): Accuracy and bias corrections with illustrations for Romania (south-eastern Europe)

Product CM54 of CMSAF (Climate Monitoring Satellite Application Facility) consists of Surface Incoming Shortwave Radiation (SIS). This product is obtained by using the MAGICSOL algorithm, which needs only the broadband visible channel as Meteosat satellites input. Hourly, daily and monthly averaged data sets are available, covering on a regular 0.03×0.03° grid the Meteosat scene up to a scanning angle of 70°. The CM54 product has been tested by using global hourly averaged solar irradiance data measured in 2010 in five Romanian meteorological stations. The available satellite database is structured into three sub-databases. Two databases (Z85 and Z75) consist of recordings associated with solar zenith angle Z<85° and Z<75°, respectively. A third database (Z85SIS+) was obtained by removing from the database Z85 the null irradiance values. The databases Z85 and Z75 underestimate the measured values and their RMSE is relatively similar, around 35%. The database Z85SIS+ has MBE and RMSE values around 0.1% and 25%, respectively. Independent of the database, MBE increases while RMSE decreases by increasing the fractional cloudiness class. The database Z85SIS+ has an MBE between −1% and 1%, independent of the cloudiness class. The RMSE of Z85SIS+ database is about 9%, 20% and 37% for clear skies, vaguely and partly cloudy skies and overcast skies, respectively. The CM54 product overestimates the ground-based measurements at small zenith angles and underestimates at very large zenith angles. Regression relationships have been prepared to remove the bias errors from the database Z85SIS+. These relationships are function of location, total cloud cover amount and class of zenith angle.