Surface Albedo Changes

The algorithm described above was implemented in the operational processing chain of EUMETSAT and then applied to a full year (1996) of Meteosat-5 data. The operational version of the algorithm permitted the retrieval, on a daily basis and for every pixel, of the surface parameters characterizing the BRF shape and amplitude. On this basis, it is then possible to estimate the associated DHR values for any particular location of the Sun.

To provide spatially comparable values, the surface albedo has been computed everywhere for a standard zenith angle of 30°.

For a number of surface applications, it is desirable to ensure a good geographical coverage, which implies the temporal compositing of these products in time. Such procedures are justified to the extent that surface changes occur over time scales longer than the period of composition. Traditional analyses based on vegetation index products, such as NDVI, recommend the application of a simple algorithm, for instance selecting the maximum value during the compositing period, but it has been shown that this procedure biases the composite data sets by selecting results towards measurements collected under specific angular conditions (see for instance, Holben, 1986; Meyer et al., 1995). Here, we propose a different scheme, which allows the selection of the most representative conditions during a compositing period on the basis of a simple statistical analysis. This analysis is based on the inspection of the daily p 0 retrieved values for every period of ten consecutive days. The daily ¿o likely values have been analyzed for every period of ten consecutive days in order to select the most representative value. This latter step was implemented by estimating the temporal average and corresponding deviation ofthe pA0 values over the 10-day periods:

where T is the number of available values during the 10-days period of temporal accumulation, Pis the temporal averaged value estimated for parameter P and AV is the average deviation of the distribution.

The 10-days representative value for the p~0 parameter is the actual p0 value minimizing the quantity Since this solution corresponds to one of the daily "Likely" solutions selected in the complete 10-day time series, the associated discrete values for the and parameters are easily assessed. This procedure defines the most representative 10-day values of the three surface parameters characterizing the surface radiative properties, namely, and as well as the corresponding DHR (30°)

values. It also ensures that these selected values are sufficient to generate a radiation field consistent with at least one of the radiation fields actually measured during one of the 10-days period by the Meteosat instrument.

In order to deliver the most complete possible maps of geophysical products we implemented an accumulation procedure for every period of ten consecutive days during the year 1996. The accumulation procedure simply consists in the sequential filling of the remaining gaps in the results available for any {PA} parameter, starting from day 1 and ending on day 10 of the time series. In other words, we produced maps of geophysical products for every ten-day period, which are made up of the most representative retrievals selected over these ten days. According to this procedure, the actual conditions of observations, the performances and the results of the inversion procedure are always fully documented for these successful retrievals composing the maps. Figure 1 displays a sample of the maps of the DHR (30°) values obtained over the Sahelian region of North Africa, for the first ten days during the months of November, January and May, on the basis of this accumulation procedure. A very large North-South gradient (absolute DHR values of about 0.55 and 0.08 are observed over the Sahara and the Equatorial forest, respectively) with values decreasing with latitude is shown on these maps. The most striking feature is the relative decrease of the DHR values over the entire continent in quite a broad band of latitudes from November to January and, conversely, a relative increase from January to May.

The seasonal migration of the Inter-Tropical Convergence Zone (ITCZ) is the most important meteorological process over the western part of these African regions. The increase of rainfall associated with the northward displacement of the ITCZ over the continent, between April-May and AugustSeptember, translates into a corresponding growth of vegetation in these bands of latitude. Conversely, the southward migration of the ITCZ, which generally occurs from September to March-April, is associated with onset of the dry season and vegetation, mainly savanna, suffers from curing (see Cheney and Sullivan, 1997), i.e., plants are basically drying out and dying. The DHR (30°) values, as retrieved from the Meteosat-5 instrument, were simulated for a variety of leaves and underlying soil properties (Pinty et al., 2000b). These simulations have revealed that the Meteosat-5 DHR (30°) values should increase with a decrease in the chlorophyll content of the leaves. However, Figure 1 indicates that, instead, a significant decrease of roughly 0.1 is occurring during the onset of the dry season, while, on the contrary, a relative increase of about the same amplitude is observed from January to May.

These results cannot be interpreted solely on the basis of natural phenomena controlled by the tropical meteorology. As a mater of fact, these bands of latitude are also subject to major anthropogenic activities related to biomass burning. Interestingly, Figure 2 displays the location of the major fires which have been identified from AVHRR data in these African regions, accumulated during the months of December and April 1993 (Arino and Melinotte, 1998). Though similar results for 1996 are not yet available, the seasonality of fire activities is very well established (see for instance, Cooke et al., 1996 and Koffi et al., 1996). On this basis, it is reasonable to consider the results obtained in 1993 data as representative of usual conditions for the sake of the present discussion. This figure illustrates the intense biomass burning activities occurring during the onset of the dry season, typically in December, with some definite slowing down of these activities as the dry season goes on (during April, for instance). Comparing Figures 1 and 2 strongly suggests that fire activities constitute a major environmental land cover change able to significantly impact the surface albedo values at a continental scale. The co-location of the detected fires in December and the regions affected by a decrease in surface albedo between November and January is indeed quite obvious. The relative increase in surface albedo values from January to May may result from various phenomena including a slight re-growth of vegetation and also a change in soil cover due to the removal of the dark burnt material by winds. The simulated impacts of these processes on the variations of the surface albedo values (see Figure 17 in Pinty et al., 2000b) are in agreement with results from radiative transfer simulations. This provides some evidence that these processes are a priori good candidates to interpret these fast changes in surface albedo values at the continental scale.

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