5.3
Land surface albedos from Land SAF and CYCLOPES projects: an accuracy assessment based on the comparison against in situ measurements and other sensors products
Jean-Louis C.H. Roujean, Météo-France, Toulouse, France
The concept of validation, or "ground truth", for satellite-derived products becomes ever more important when the eyes of satellites are turned to making global measurements in support of weather forecast and possible climate change observations. The land surface albedo is a key parameter influencing the climate near the ground. Verifying its reliability at ground level must take into account the up-scaling issue. This latter is at the core of major problems in satellite remote sensing, in particular in regard to the patchiness and degree of clumping of agricultural and forested areas. A land surface albedo product is developed for the SAF (Satellite Application Facilities) project on Land Surface Analysis supported by EUMETSAT, the European meteorological space agency. This project is devoted to an operational production of surface variables that are primarily useful for climate and weather forecast. The present strategy is a synergistic use of geostationary and polar orbiting systems. Meteosat Second Generation is operational since early 2004 while Metop, boarding AVHRR-3, will be launched in 2005. For the SAF, three broadband albedo products are disseminated at full resolution of about 5 km: visible broadband, near-infrared broadband, and shortwave broadband. For each of them, black-sky albedos (directional-hemispherical reflectance) and white-sky albedos (bi-hemispherical reflectance) are available. Such albedo specifications comply with users requirements. A quality flag comes with the product. The validation exercise is twofold: a direct validation based on a comparison with in situ measurements and other sensor products; then, an indirect validation from use in numerical weather prediction models. Product analysis relies on a comparison with long-term series of broadband albedos collected at Baseline Surface Radiation Network (BSRN) stations. The landscape variability around sampled station is assessed up to the size of sensor pixel with high resolution imagery such as Landsat-7 and SPOT-5. To a certain extend, the validation strategy is similar to MODLAND. An inter-comparison of SAF albedos with MODIS and MISR albedos will be shown for selected BSRN stations and MODLAND sites within MSG disk. The CYCLOPES project, supported by the European Union, concerns a dissemination of albedo products based on sensors fusion (VEGETATION, AVHRR, MERIS, POLDER, MSG). The goal is to perform a product that keeps the peculiarity of each sensor in terms of resolution: spectral, angular, spatial, temporal. Results of albedo validation for SAF and CYCLOPES will be shown with uncertainties due to pre-processing physical steps (calibration, cloud mask, atmospheric correction, use of a kernel-driven model). They will be commented in regard to several applications in meteorology. .
Session 5, Land Surface Albedo and Its Impacts on Surface Climate (parallel with Joint Session 8)
Thursday, 13 January 2005, 8:30 AM-12:00 PM
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