Validation of Moderate Resolution Imaging Spectroradiometer (MODIS) albedo retrieval algorithm: Dependence of albedo on solar zenith angle

The Moderate Resolution Imaging Spectroradiometer (MODIS) bidirectional reflectance distribution function (BRDF)/albedo algorithm uses multiday, multiband MODIS surface reflectance products from Terra and Aqua to produce a global albedo product at a 500 m resolution (Collection 5). This paper evaluates the ability of the MODIS albedo product to represent albedos at all diurnal solar zenith angles through a comparison with field measurements from the Surface Radiation Budget Network (SURFRAD) and the Atmospheric Radiation Measurement Southern Great Plains (ARM/SGP) stations. The results show that, for most of the sites, the overall accuracy of the MODIS albedo is within 0.05 and shows an increasing negative bias and increased RMSE as zenith angle increases beyond 70 degrees-75 degrees as compared to the ground observations. The full inversion of the MODIS BRDF/albedo algorithm has a higher inversion quality than the backup algorithm. Site heterogeneity and spatial-scale mismatch between the MODIS and ground observations are the major factors contributing to the discrepancy between the MODIS albedo and the field measurements.