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.

中文翻译：

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从哨兵 2A 表面反射率和 MODIS/VIIRS 表面各向异性测量中对 20 米表面反照率反演的初步评估

摘要 通过将最近发射的 Sentinel-2A 卫星的表面反射与来自中等分辨率的表面各向异性信息（如双向反射分布函数，BRDF 所述）相耦合，可以生成 20 米分辨率的基于卫星的地表反照率反演。成像光谱仪 (MODIS) 或可见光红外成像辐射计套件 (VIIRS)。表面的固有黑天反照率 (BSA) 和白天反照率 (WSA) 值来自 Sentinel-2A 多光谱仪器 (MSI) 的六个短波光谱带。一组特定的窄带转换系数源自辐射传输模拟，并用于生成宽带反照率。初步评估使用安装在地表辐射网络 (SURFRAD) 七个站点的塔上的总辐射表进行的校准良好的地面反照率测量。在 MODIS 和 VIIRS 产品的网格尺度（500 m 至 1 km）上，总辐射表测量足迹不能在空间上代表卫星像素覆盖的景观（即，在空间上不具有代表性）的站点上，更高分辨率的 Sentinel-2A与 MODIS 和 VIIRS 的较粗分辨率的反照率产品相比，反照率表现出均方根误差 (RMSE) 和对地基数据评估的平均偏差的显着降低。之所以会出现这种下降，是因为 20 米 Sentinel-2A 反照率值比较粗分辨率的传感器能​​够更好地解析地基仪器足迹内的地表反照率的空间细节。该初步评估还证明了 Sentinel-2A 反照率结果的一致性，无论是使用 MODIS BRDF 还是 VIIRS BRDF 产品获取表面各向异性信息。所有七个验证站点的 Sentinel-2A 反照率的 RMSE 和平均偏差均在 ±0.05 绝对反照率单位的精度要求内，用于卫星衍生的反照率产品。这项研究使用 MODIS 或 VIIRS BRDF 生成 Sentinel-2A MSI 反照率，扩展了 Landsat TM、ETM+ 和 OLI 反照率的先前努力，并增强了更高分辨率反照率数据的连续性。