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B. Pinty and G. Szejwach

Abstract

A technique for inferring the spatial and seasonal albodo changes over a whole climatic region from satellite data is developed. This technique uses the diurnal variation of radiances which is measured by geostationary satellites and requires the knowledge of a surface albedo value over at least one reference site. The proposed method is tested over western Africa, using METEOSAT data; and surface albedo maps representative of the wet and dry seasons are derived. With regard to the considered scales and to the achievable accuracies, the technique is shown to be relevant for climatological studies.

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B. Pinty and D. Ramond

Abstract

Surface albedo can be inferred from geostationary satellite measurements as long as the effects due to the atmosphere, the spectral response of the sensor, and the angular anisotropy of the reflected field are corrected. In this paper, we developed a method which includes ad hoc correction procedures for the three categories of effects. An application of the method is conducted over the Sahara and the African Sahel using METEOSAT radiances together with auxiliary data derived from other satellites (Tiros-N and Nimbus-7) and standard meteorological observations. The surface albedo maps are estimated over this region, at a spatial resolution compatible with one used in climate models, for 2 days representative of the dry and the wet seasons, respectively. The observed seasonal surface albedo change and the relationships between the surface and the planetary albedos are discussed in order to examine the validity of the method and the correction procedures.

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B. Pinty and D. Tanre

Abstract

The inference of surface reflectance from satellite observations requires the knowledge of the double-way transmittance through the atmosphere. Since the existing pyranometer networks routinely provide measurements of the incident transmittance over sensitive climatic regions, it would be useful for subsequent applications to relate this ground-based measurement to the corresponding double-way transmittance. A variety of satellite radiance simulations corresponding to clear sky conditions has been made in order to derive a suitable parameterized expression between the two quantities. The accuracy of this expression when making use of additional meteorological observations is shown and discussed. Finally, the derived expression is used to improve a method recently proposed by Pinty et al. for retrieving surface albedo over the African Sahel from METEOSAT radiances.

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B. Pinty, G. Szejwach, and J. Stum

Abstract

No abstract available.

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J. Stum, B. Pinty, and D. Ramond

Abstract

The conversion of radiances measured by the METEOSAT visible channel into broadband radiances can be performed as long as the appropriate conversion factors are known. A simple model allowing a spectral description of the optical properties of cloud free atmospheres and land surfaces is used to estimate these conversion factors. A sensitivity study of these factors indicates that a knowledge of the optical properties of the surfaces (described through spectrally averaged albedos and spectral band ratios) is decisive for retrieving broadband conversion factors. A parameterization is proposed which permits estimation of METEOSAT conversion factors for radiation budget calculations.

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