Toward a solar climatological database: the HelioClim project

The HelioClim project aims at producing temporal series of solar radiation maps at ground level over Europe, Africa and Atlantic Ocean from 1985 up to now. Meteosat satellites images in visible channel and in the reduced resolution B2 format are used to produce this climatological database. The Heliosat II method was selected to convert these images into maps of the global irradiation at ground level. It is an improved version of the original Heliosat method which estimates the global irradiation by comparison of what is observed by the sensor to what should be over that pixel if the sky were clear. The improvements contained in the Heliosat II method consist mostly in a use of a more accurate clear sky model and explicit formulations of the radiance and the transmittance instead of empirical values. A comparison was performed between irradiation values retrieved by this method and measured by ground stations in Europe. A measuring station is not necessarily contained within a B2 pixel and an interpolation method is needed. The accuracy is less than that observed for the high-resolution images because of the smaller number of images and the interpolation process. Nevertheless, the observed relative root mean square error is around 20 %, a very satisfactory result. We conclude that B2 images processed by Heliosat II method gives good results especially for the estimation of daily irradiation or for the irradiation of larger period. They are also able to provide useful data to create a more than fifteen years solar climatological database.