The diffuse-to-global spectral irradiance ratio as a cloud-screening technique for radiometric data

Continuous measurements of solar spectral radiation using the multifilter rotating shadow band radiometer (MFRSR-7) are performed at the Actinometric Station of the National Observatory of Athens (ASNOA). The present study utilizes 4 days of continuous observations, from local sunrise to sunset, in order to investigate the daily variation of the radiation components (diffuse and global) as well as their ratio (diffuse-to-global irradiance ratio, DGR) under different atmospheric conditions. DGR has received a great scientific interest, as well as the respective diffuse-to-direct-beam ratio especially for investigating solar irradiance modifications under different atmospheric conditions, aerosol load and optical properties. Apart from this, the present study shows that the DGR can also constitute a powerful tool for cloud screening, i.e. for removing perturbed data due to cloud contamination from automated sun scanning radiometers. The relationship between DGR at a specific wavelength with the respective ratio for the whole MFRSR band (300–1100 nm) is found to exhibit a curvature; this curvature is strongly modified when perturbed irradiance data (possibly caused by clouds) occur. Even though the perturbed data can also be easily identified from the diurnal irradiance variation, the present study is the first to show the effect of perturbed solar spectral data on the DGR.