Upscaling of Single-Site-Based Measurements for Validation of Long-Term Coarse-Pixel Albedo Products

The in situ measurements from globally distributed sparse networks provide a valuable data source for the validation of satellite products. However, the representativeness errors resulting from the spatial scale mismatch between in situ-satellite measurements and surface heterogeneity have generally limited past validation to only very few spatially representative sites, which cannot meet the requirement of a comprehensive validation. In response to this challenge, this article offers a strategy for upscaling sparse in situ measurements and removing the impact of representativeness errors on the evaluation of coarse-pixel albedo products. The main idea of the upscaling method is to establish the correspondence relationship between each subpixel albedo time series within a coarse pixel and in situ albedo time series by using high-resolution albedo maps as the prior knowledge. Furthermore, the performance of the upscaling method is carefully evaluated over the sites featured by different degrees of spatial representativeness. The results indicate that the upscaling method improves the representativeness of single-site measurements with respect to a coarse pixel, and the improvement is most significant over the sites with relatively low representativeness. Therefore, the upscaling method is particularly useful for the validation at heterogeneous sites in strengthening the reliability of validation results. It is expected to open the door to maximizing the use of existing sparse networks and generating a time series of globally distributed reference data sets with sufficient length, consistency, and continuity.