Re-understanding of land surface albedo and related terms in satellite-based retrievals

Abstract

Land surface albedo is a critical variable in determining surface energy balance, and regulating climate and ecosystem processes through feedback mechanisms. Therefore, climatic modelers and radiative monitoring require accurate estimates of land surface albedo. With the instrument development, algorithm upgrade, spectral-band-adjustment in wavelength center or band width, and the increasing distinct requirement from diversified communities, various albedo terms have been generated in related satellite-based products. The lack of understanding on the divergence of these terminologies can introduce potential considerable errors in the subsequent applications, or an elevated probability to invert the deduced conclusion. We surveyed the basic concepts of reflectance quantities, retrieval strategies, and models developed since the 1970s, and discuss both strength and opportunity for improvements on land surface albedo extraction, and product generation. In addition, we exemplified the difference of albedo terms using the daily MODIS product (MCD43A) to emphasize the potential risk of the ambiguous usage, over typical IGBP land covers in Northern Kazakhstan. Our investigation shows that relative differences among various albedo terms can reach up to 181% and 50%, while 0.266 and 0.118 of absolute variance respectively in the narrow and broad-band surface albedo, which illuminated cautions against the ambiguous understanding of albedo terminologies or erroneous usage of albedo products.

Cross-Comparison of Global Surface Albedo Operational Products-MODIS, GLASS, and CGLS

“…The magnitude of surface albedo products discussed above shows different mean values and various distribution feature of the differences among albedo quantities, and spectral bands or products over the global land surface. The global mean WSA albedo shows a persistent higher value than that of related BSA in MODIS, GLASS, and CGLS, which is determined by the definition of WSA and BSA [27]. BSA is calculated by the integral of BRDF over the view hemisphere, while WSA has a further integral over the illumination hemisphere.…”

“…We investigated the consistency or difference of satellite-based land surface albedo products that have been frequently used in numerous applications. As the important standard products routinely providing continuous surface albedo over the global region, MODIS, GLASS, and CGLS, generating the same albedo quantities, were adopted to prevent effects introduced by various definitions of albedo in this investigation [27]. We organized BSA and WSA at VIS, NIR, and SW bands of three discussed data suites on all of the available identical days of 2000-2017 to avoid the potential uncertainty caused by date switches [27].…”

“…As the important standard products routinely providing continuous surface albedo over the global region, MODIS, GLASS, and CGLS, generating the same albedo quantities, were adopted to prevent effects introduced by various definitions of albedo in this investigation [27]. We organized BSA and WSA at VIS, NIR, and SW bands of three discussed data suites on all of the available identical days of 2000-2017 to avoid the potential uncertainty caused by date switches [27]. Our above efforts indicate that an apparent difference exists in the retrieval validity, consistency of global annual mean, spatial distribution and temporal dynamics, in terms of the cross-comparison at VIS, NIR, and SW bands in the global region and the direct comparison with field measurements at SW band over nine ground stations.…”

“…These products are generated at temporal intervals, spanning from a daily to an annual mode, in sporadic individual years or a continuous series of years, with the spatial resolution varying from 30 m to 55 km in order to fit the requirement of energy balance in numerous research communities. The albedo algorithm in the early stage usually adopts the singleangle model with an isotropic assumption of land surface, while the modern approach has adequately considered anisotropy features through multi-angle observations, first to inverse a Bidirectional Reflection Distribution Function (BRDF) model of the target, then to integrate an illumination or view hemisphere to estimate certain albedo quantities [27]. Compared with other BRDF-based approaches, the semi-empirical kernel-driven linear model is widely adopted by most current operational surface albedo products due to its simplicity and operability [28], especially the kernel-driven linear model Ross-Thick/Li-Sparse-Reciprocal (RTLSR) [29].…”

“…Quantities of albedo with different definitions are frequently generated by diverse approaches from various satellite-based observations which usually have diverse spatial and temporal resolutions. Shuai et al pointed out that a non-ignorable relative difference exists among albedo terms defined over certain observed geometries or at different spectral regimes, which can potentially introduce considerable errors in subsequent applications [27]. He et al [36] discussed nine shortwave (SW) albedo datasets spanning 1981-2010 including MODIS, MERIS (Medium-Resolution Imaging Spectrometer), GLASS, GlobAlbedo, CLARA-SAL (Clouds, Albedo, and Radiation-Surface Albedo), ERBE (Earth Radiation Budget Experiment), and three surface SW radiation datasets: ISCCP (International Satellite Cloud Climatology Project), GEWEX (Global Energy and Water Exchanges Project), and CERES (Clouds and the Earth's Radiant Energy System), and subsequently claimed about 0.02-0.03 difference in global mean albedo, and better agreement in summer than winter, particularly at high latitudes where the maximal difference range varies from 0.1 to 0.4.…”