Operational derivation of land surface albedo and down-welling short-wave radiation in the framework of the Land-SAF project

摘要

In the framework of the EUMETSAT Satellite Application Facility for Land Surface Analysis (Land-SAF) we develop surface albedo and short-wave radiation products generated from observations provided by the SEVIRI instrument on board of the geo-stationary Meteosat Second Generation satellite. For the albedo product cloud-free top-of-atmosphere reflectances in the instruments 0.6 /spl mu/m, 0.8 /spl mu/m, and 1.6 /spl mu/m channels are atmospherically corrected with a simplified and fast radiative transfer code. The algorithm then exploits the diurnal variation of the illumination angle, which provides information on the angular variation of reflectance, in order to invert a linear kernel-based bi-directional reflectance model. Several albedo variants are derived by adequately integrating the constrained model functions. The down-welling short-wave radiation at the surface essentially depends on cloud cover and hence to derive a reliable estimate of this quantity, the cloud mask represents an important piece of information. Depending on the presence of clouds and potentially on cloud type, different physically based parameterization schemes are applied to approximate the radiative transfer problem. In the case of cloudy sky the top-of-atmosphere albedo is the most important input quantity. It is derived from the satellite measurements by applying a narrow - to broadband conversion and a suitable angular reflectance model.