Conclusions

An advanced algorithm for characterizing the radiative state of the surface and the atmosphere over the Meteosat visible band has been designed and tested against actual data. The proposed algorithm capitalizes on the capability of the Meteosat instrument to acquire radiance data every 1/2 hour, suggesting that, for a given geophysical system, the successive relative locations of the Sun during the same day (or even half-day) provide a good angular sampling of the radiation field emerging at the top of the atmosphere. The main products delivered by this algorithm are

1. the quantitative characterization of surface radiative properties which can be used to document the state and monitor the evolution of the land surface,

2. an indication on the probable aerosol load provided as an effective optical thickness,

3. the description of the "clear sky" reflectance field at the top of the atmosphere, and

4. the detection of clouds and their associated shadows during the day.

Figure 1. Maps of the DHR (30°) values obtained over the Sahelian regions of North Africa, for the first ten days during the months of November, January and May on the basis of the accumulation procedure. Each value corresponds to the selection of the most representative "Likely" solution retrieved each day. The black color identifies the pixels for which no reliable solution was delivered by the algorithm. These DHR values are weighted by the spectral response of the Meteosat VIS channel which extends approximately from 0.4|itn to 1.1 ^m with a maximum response at 0.65)im

Figure 1. Maps of the DHR (30°) values obtained over the Sahelian regions of North Africa, for the first ten days during the months of November, January and May on the basis of the accumulation procedure. Each value corresponds to the selection of the most representative "Likely" solution retrieved each day. The black color identifies the pixels for which no reliable solution was delivered by the algorithm. These DHR values are weighted by the spectral response of the Meteosat VIS channel which extends approximately from 0.4|itn to 1.1 ^m with a maximum response at 0.65)im

Fire activities (1993)

December

B Knty and coll., 1SSS

Figure 2. Monthly maps of the active fire occurrence detected on the basis of AVHRR data for year 1993. The detection follows from a series of tests on the values measured in the various AVHRR channels (see Arino and Melinotte, 1998, for more details)

The analysis of the results obtained for the year 1996 shows that accurate surface albedo maps may help assessing large land cover changes at the continental scale. As a matter of fact, the interpretation of the monthly surface albedo changes strongly suggests that biomass burning activities may be the dominant environmental factor over large African regions, even masking the natural changes that would be induced by the North-South migration of the monsoon.

The results presented here are based on a small portion of the entire set of products generated by EUMETSAT for year 1996. An assessment of the quality of these products will require further work and is under way. However, these results already illustrate, on the one hand, the great potential of a radiation transfer algorithm based on multi-angular measurements and, on the other hand, the type and value of the information that may be retrieved from the geostationary satellites in general and Meteosat in particular.

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