Land surface albedo at high spatial resolution from a merging of Sentinel-2 and Landsat-8 data...

Land surface albedo at high spatial resolution from a merging of Sentinel-2 and Landsat-8 data; analysis of times series at landscape scale and validation (Full title)

Surface albedo is an Essential Climate Variable (ECV) that requires a routine and coherent update at global scale in order to ensure reliable estimates of the water and carbon fluxes. An operational method was implemented to map surface albedo at high resolution, which is based on an approach previously developed for PROBA-V satellite in the framework of the Copernicus Global Land Service. The method considers Level 2A Sentinel-2 (S2) and Landsat-8 (L8) images issued from MAJA (MACCS-ATCOR Joint Algorithm) processing chain aimed at removing atmospheric components (cloud, water, aerosol) on the signal. L8 images are resampled to obtain pixels of 10 meters resolution and also converted in Sentinel-2 like spectral channels. Then, a semi-empirical BRDF (Bidirectional Reflectance Distribution Function) kernel-driven model of reflectance is applied to Level 2A S2 and L8 images to retrieve BRDF parameters. For such, a composite period of two months – with sliding periods of ten days - is retained as it permits to get a sufficient number of clear scenes to constrain the model and also because it offers variations in solar geometry to counterbalance the low variations in viewing geometry due to narrow field-of-view (FOV) for S2 or nadir view for L8. Then, a weighed angular integration of the kernels allows deriving both directional-hemispherical (DHR) and bi-hemispherical reflectance (BHR) reflectance, namely surface albedo products. Finally, a narrow to broadband albedo conversion is achieved based on coefficients issued from a huge number of PROSAIL model simulations. Surface albedo products are disseminated with a quality control and an uncertainty assessment. Albedo products at 20 meters and 30 meters are also generated to scale heterogeneity.

The algorithm is fully operational and is able to process any S2 tile. In order to upgrade the BRDF sampling and refine the temporal resolution, Sentinel-3 (S3) images are also incorporated in the processing line for appraisal. Results of considering one or several satellites are analyzed. The validation is carried on for years 2018 and 2019 over ICOS (Integrated Carbon Observing System) anchor stations of Auradé and Lamasquère located near Toulouse (France) and operated by CESBIO. They are covered by crops (maize, wheat, and sunflower, merely) and measure in routine on hourly basis downwelling and upwelling shortwave radiation of both global and diffuse components. A gap-filling method serves to enhance the quality of the time series for the sake of a more consistent comparison. Assessment of the high resolution surface albedo at landscape scale is performed through a comparison with MODIS equivalent products.

The domains of applications concern primarily the agriculture and urban areas for which added-value of the high resolution is conspicuous. In particular, surface albedo contributes to quantitative estimates of the radiation forcing (RF). This latter exerts a cooling effect during wintertime periods when crops patches are covered by vegetation instead of bare soil. This yields a mean to enhance the role of the surface as an abatement of the carbon emissions. Magnitude of time evolving RF in and around the ICOS anchor stations will be discussed.