Abstract
Abstract For any given area, diurnal variation of the emitted longwave radiation reveals the response of the surface and atmosphere to the astronomical diurnal forcing. However, the diurnal rhythm can be masked locally by changes in cloud cover linked to passage of weather systems. To eliminate this weather “noise” we construct histograms of the pixel radiance counts from the METEOSAT IR window channel, for regions of dimensions 250 to 1000 km, and we study their evolution with time. We find that considerable information on the diurnal variation, both of surface temperature and of cloud cover at all levels, can be obtained from short sequences (3 days) of hourly METEOSAT data. In West Africa, diurnal variation of clear-sky emergent radiances, corresponding to amplitudes of 30–50 K in surface skin temperature, is found not only in the Saharan desert regions but also during the dry season in the Sahel–Sudan–Guinea zone. In the August wet season, in this last area, the amplitude is lower, while diurnally var...
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