Clear-Sky Criteria and Cloud Cover over Desert Areas from Meteosat Data

In order to use the diurnal variation of Meteosat visible channel data to help determine the bidirectional reflectance function for desert areas, criteria must be established for identifying cloud-free scenes. The authors consider Meteosat observations, obtained in different seasons over four years, of desert and arid land areas of Africa and the Arabian peninsula. Normalizing apparent albedo to near-midday values, the authors show that clear and cloudy scenes can be distinguished using relative spatial heterogeneity parameters of this normalized albedo on the one hand, and of the midday infrared window radiance on the other. Further, it is shown that cloud cover estimates based on these parameters are in good agreement with the C1 results of the International Satellite Cloud Climatology Project. Difficulties related to Saharan dust clouds or to homogeneous overcast cases can be resolved by comparing midday IR window radiance with the maximum value over a 10-day period.