White-sky Albedo Research Articles

First operational BRDF, albedo nadir reflectance products from MODIS

With the launch of NASA's Terra satellite and the MODerate Resolution Imaging Spectroradiometer (MODIS), operational Bidirectional Reflectance Distribution Function (BRDF) and albedo products are now being made available to the scientific community. The MODIS BRDF/Albedo algorithm makes use of a semiempirical kernel-driven bidirectional reflectance model and multidate, multispectral data to provide global 1-km gridded and tiled products of the land surface every 16 days. These products include directional hemispherical albedo (black-sky albedo), bihemispherical albedo (white-sky albedo), Nadir BRDF-Adjusted surface Reflectances (NBAR), model parameters describing the BRDF, and extensive quality assurance information. The algorithm has been consistently producing albedo and NBAR for the public since July 2000. Initial evaluations indicate a stable BRDF/Albedo Product, where, for example, the spatial and temporal progression of phenological characteristics is easily detected in the NBAR and albedo results. These early beta and provisional products auger well for the routine production of stable MODIS-derived BRDF parameters, nadir reflectances, and albedos for use by the global observation and modeling communities.

Using a multikernel least-variance approach to retrieve and evaluate albedo from limited bidirectional measurements

When a multikernel modelling approach is being applied to remotely sensed data, a new criteria — least variance of white-sky albedo — selects semiempirical bidirectional reflectance distribution function (BRDF) kernel combinations better than the more conventional least-squares fitting criteria. The BRDF describes the scattering of light from surface as a function of illumination and view geometry. Semiempirical kernels are nonlinear geometric functions derived from a simplification of physical models of scattering. These are combined linearly to fit observed bidirectional reflectance measurements. White-sky albedo (bihemispherical reflectance) is the integral of directional reflectance for all viewing and illumination directions and is a true surface property. The variance of retrieved white-sky albedo is a function of the noises of measurement, the specific viewing and illumination geometry of the surface scattering measurements, and the number of observations used in the inversion. By selecting the kernel combination that provides the least variance of white-sky albedo, our studies show that a more stable estimate of white-sky and black-sky (directional–hemispherical) albedo is produced.