Abstract. Land surface albedo constitutes a critical climatic variable, since it largely controls the actual amount of solar energy available to the Earth system. The purpose of this paper is to establish a theory for the exploitation of space observations to solve the atmosphere/surface radiation transfer problem on an operational basis and to generate surface albedo, aerosol load, and possibly land cover change products. Surface albedo is rather variable in space and time and depends both on the structure and on the radiative characteristics of the surface, as well as on the angular and spectral distribution of radiation at the bottom of the atmosphere. Weather and climate models often use preset distributions or simple parameterizations of this environment variable, even though such approaches do not accurately account for the actual effect of the underlying surface. From a mathematical point of view, the determination of the surface albedo corresponds to the estimation of a boundary condition for the radiation transfer problem in the coupled surface-atmosphere system. A relatively large database of 10 years or more of Meteosat data has been accumulated by EUMETSAT. These data, collected at half-hour intervals over the entire Earth disk visible from longitude 0 ø, constitute a unique resource to describe the anisotropy of the coupled surface-atmosphere system and provide the opportunity to document changes in surface albedo which may have occurred in these regions over that period. In addition, since the coupled surface-atmosphere radiation transfer problem must be solved, the proposed procedure also yields an estimate of the spatial and temporal distribution of aerosols. The proposed inversion procedure yields a characterization of surface radiative properties that may also be used to document and monitor land surface dynamics over the portion of the globe observed by Meteosat. Results from preliminary applications and an error budget analysis are discussed in a companion paper [Pinty et al., this issue]. IntroductionThe bulk of the solar radiation available to the Earth system is absorbed at or near the oceanic and continental surface and then ultimately released to the atmosphere through the fluxes of infrared radiation, as well as sensible and latent heat. The fraction of solar energy absorbed at the surface of the planet is controlled by its surface albedo, which is highly variable in space and time over terrestrial surfaces. Satellite-borne instruments constitute a priori a unique tool for monitoring surface albedo values at the global scale and at spatial and temporal resolutions adequate for meteorological and climate studies. However, the above assertion implies that the problems hindering the accurate estimation of surface albedo values from space measurements are correctly addressed.The contributions to the measured radiances due to the atmospheric layers and the variations due to the anisotropic reflectance of all terrestrial surfaces are of primary concern in this 18,099

Abstract. An advanced algorithm to retrieve the radiative properties of terrestrial surfaces sampled by the Meteosat visible instrument was derived in a companion paper IntroductionIn a companion paper, Pinty et al. [this issue] (hereinafter referred to as part 1) proposed a new approach to estimate land surface albedo from data acquired by the Meteosat instrument at a daily and pixel resolution. The cornerstone of this approach is the exploitation of the temporal sampling of Meteosat (data acquired every 30 min from sunrise to sunset) as if it were an instantaneous angular sampling. With the assumption that the geophysical system under investigation does not change significantly during the day, an analytical expression has been derived (part 1, equation (43)) to express the bidirectional reflectance field measured by Meteosat for "clear-sky" pixels. This expression accounts for the major radiation transfer processes occurring among the Sun, the Earth system, and the satellite and follows from mathematical developments aimed at deriving a formulation of the bidirectional reflectance field that can be operationally inverted against a set of daily Meteosat data. Application of this procedure permits estimating (1) the surface bidirectional reflectance factors (BRF) required to derive albedo-related quantities and (2) an "effective" aerosol load which, together with the surface BRF, provides a coherent interpretation of the daily sequence of This paper deals with a number of additional issues to be addressed when applying this approach to actual Meteosat-5 data. As such, it discusses and proposes methods to handle (1) the selection, for each pixel (location), of those time observations during the day which are not contaminated by cloud radiative effects, (2) the identification of the optimal solution for each pixel and each day through the set of potential solutions, and finally (3) the assessment of the limits inherent to the Meteosat-5 sensor itself. The application, performed on the basis of two sequences of Meteosat data sets available in June and November/December 1996, allows the production of an ensemble of geophysical maps and preliminary documentation of the seasonal changes in land surface over the regions studied. The impact of biomass burning on the surface albedo values of savannas and woodlands is examined and compared to the natural vegetation cycle related to the African monsoon events. 18,113

Large and unexpected variations in surface albedo at a continental scale over Africa could be explained by intense biomass burning activities during the dry seasons. These phenomena have been recorded in Meteosat data used to estimate surface albedo changes and Along Track Scanning Radiometer (ATSR‐2) data used to monitor fire. The seasonal albedo changes observed cannot be explained by seasonally driven phenological cycles of tropical vegetation. A more complex set of mechanisms incorporating the effects of anthropogenic activities must be at work.

The Meteosat Third Generation (MTG) Programme is the next generation of European geostationary meteorological systems. The first MTG satellite, MTG-I1, which is scheduled for launch at the end of 2018, will host two imaging instruments: the Flexible Combined Imager (FCI) and the Lightning Imager. The FCI will provide continuation of the SEVIRI imager operations on the current Meteosat Second Generation satellites (MSG), but with an improved spatial, temporal and spectral resolution, not dissimilar to GOES-R (of NASA/NOAA).Unlike SEVIRI on the spinning MSG spacecraft, the FCI will be mounted on a 3-axis stabilised platform and a 2-axis tapered scan will provide a full coverage of the Earth in 10 minute repeat cycles. Alternatively, a rapid scanning mode can cover smaller areas, but with a better temporal resolution of up to 2.5 minutes. In order to assess some of the data acquisition and processing aspects which will apply to the FCI, a simplified end-to-end imaging chain prototype was set up. The simulation prototype consists of four different functional blocks:-A function for the generation of FCI-like references images -An image acquisition simulation function for the FCI Line-of-Sight calculation and swath generation -A processing function that reverses the swath generation process by rectifying the swath data -An evaluation function for assessing the quality of the processed data with respect to the reference images This paper presents an overview of the FCI instrument chain prototype, covering instrument characteristics, reference image generation, image acquisition simulation, and processing aspects. In particular, it provides in detail the description of the generation of references images, highlighting innovative features, but also limitations. This is followed by a description of the image acquisition simulation process, and the rectification and evaluation function. The latter two are described in more detail in a separate paper.Finally, results from the prototype imaging chain are shown, including generated datasets, evaluation of results and conclusions derived from the first tests. An outline of planned extensions to the prototype and its role in the MTG Ground Segment development conclude the presentation.

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