Preliminary assessment of 20-m surface albedo retrievals from sentinel-2A surface reflectance and MODIS/VIIRS surface anisotropy measures

Abstract

Satellite-based retrievals of land surface albedo at 20-m resolution are generated by coupling the surface reflectances from the recently launched Sentinel-2A satellite with surface anisotropy information (as described by Bidirectional Reflectance Distribution Function, BRDF) from either the MODerate-resolution Imaging Spectroradiometer (MODIS) or the Visible Infrared Imaging Radiometer Suite (VIIRS). The intrinsic black-sky albedo (BSA) and white-sky albedo (WSA) values of the surface are derived at the six shortwave spectral bands of Sentinel-2A's Multi Spectral Instrument (MSI). A specific set of narrow-to-broadband conversion coefficients is derived from radiative transfer simulations and presented for the generation of broadband albedos. Initial evaluation uses well-calibrated ground-based albedo measurements by pyranometers mounted on the towers at seven sites of the Surface Radiation Network (SURFRAD). Over those sites where pyranometer measurement footprints are not spatially representative of the landscape covered by the satellite pixels (i.e., spatially nonrepresentative) at the grid scales (500 m to 1 km) of the MODIS and VIIRS products, the finer-resolution Sentinel-2A albedos manifested a pronounced decrease in root mean squared error (RMSE) and mean bias in the evaluation against the ground-based data as compared to the coarser resolution albedo products of MODIS and VIIRS. This decrease occurs because the 20-m Sentinel-2A albedo values are better able to resolve the spatial details of surface albedo within the ground-based instrument footprints than the coarser resolution sensors. This preliminary evaluation also demonstrates the consistency of the Sentinel-2A albedo results whether using the MODIS BRDF or the VIIRS BRDF products for the surface anisotropy information. The RMSEs and mean biases of the Sentinel-2A albedos over all the seven validation sites are both within the accuracy requirement of ±0.05 absolute albedo units for satellite derived albedo products. This study, to generate Sentinel-2A MSI albedo with either MODIS or VIIRS BRDFs, extends previous efforts of Landsat TM, ETM+ and OLI albedo and enhances the continuity of finer-resolution albedo data. Such long-term and higher resolution records of surface albedo improve the investigations into the changes and drivers of local/regional surface energy balance over heterogeneous regions and increasingly fragmented landscapes across the globe due to natural and human-induced land cover changes.