Satellite-derived climatology of the Saharan aerosol

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Abstract
Mineral aerosol can be observed from satellite, with shortwave (VIS) imagery over oceanic surfaces and longwave (IR) imagery over land surfaces (especially for arid regions). Techniques appropriate to the VIS and IR channels of satellites of the current Meteosat series are described. Over ocean, the VIS technique involves a retrieval of aerosol optical depth obtained from the satellite-derived data. Over continent, the IR technique requires creating optimized reference images, constituted by maximum radiances associated with clear and clean pixels. Clouds and dust plumes are then separated from the permanent surface structures in difference images obtained by subtracting original images from the reference one. Both techniques are applied to data at 12 UTC in the format ISCCP B2. The respective patterns of retrieved aerosol optical depth over ocean and derived difference images over continent are observed to fit satisfactorily along the coast. Results of optical distribution and temporal variation of the Saharan aerosol occurrence over Africa and its bounding seas and oceans, north of the equator are presented. The seasonal character of the phenomenon is described. Observed maximum for these frequencies allows the sources of dust emission to be located and their seasonal rhythm of activity to be estimated.

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