Derivation of daylight and solar irradiance data from satellite observations

Abstract : The estimation of the downward surface shortwave irradiance from satellite observations has been subject to numerous investigations in the past. Different methods from simple transmittance-reflectance correlations to the extensive use of radiative transfer calculations have been applied leading to generally satisfying results with an accuracy almost independent of the chosen method. In the framework of the European Community research project SATELLIGHT an attempt is made to use satellite methods to derive daylight and solar irradiance data with a continuous spatial coverage forWestern and Central Europe from Meteosat images. For potential end users these data will be placed in a data base in the Internet (Fontoynont et al., 1998). In daylighting applications, knowledge of the luminance distribution of the sky is of primary concern. Thus, beyond the retrieval of surface global irradiance, the separation of diffuse and direct components as well as a better representation of these quantities for low sun elevations had to be derived in the scope of this project. We present improvements and additions to an existing method for the derivation of the global irradiance. This includes new correction schemes for the influence of atmospheric extinction processes. These have been partly developed by simulating the satellite signal for a cloudless atmosphere using the radiative transfer code MODTRAN. In addition, the derivation of the diffuse irradiance is briefly outlined.