Surface albedo retrieval from Meteosat - 1. Theory

Land surface albedo constitutes a critical climatic variable, since it larg ely controls the actual amount of solar energy available to the Earth syste m. The purpose of this paper is to establish a theory for the exploitation of space observations to solve the atmosphere/surface radiation transfer pr oblem 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 c haracteristics of the surface, as well as on the angular and spectral distr ibution of radiation at the bottom of the atmosphere. Weather and climate m odels often use preset distributions or simple parameterizations of this en vironment variable, even though such approaches do not accurately account f or the actual effect of the underlying surface. From a mathematical point o f view, the determination of the surface albedo corresponds to the estimati on of a boundary condition fur the radiation transfer problem in the couple d surface-atmosphere system. A relatively large database of 10 years or mor e of Meteosat data has been accumulated by EUMETSAT. These data, collected at half-hour intervals over the entire Earth disk visible from longitude 0 degrees, constitute a unique resource to describe the anisotropy of the cou pled surface-atmosphere system and provide the opportunity to document chan ges in surface albedo which may have occurred in these regions over that pe riod, In addition, since the coupled surface-atmosphere radiation transfer problem must be solved, the proposed procedure also yields an estimate of t he spatial and temporal distribution of aerosols. The proposed inversion pr ocedure 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 a nd an error budget analysis are discussed in a companion paper [Pinty et al ., this issue].