The reflectance of natural surfaces depends on the view geometry, i.e., the position of the observer or measuring instrument relative to the Sun. Land surfaces usually appear darker in the forward scattering direction (i.e, with the Sun in front of the observer) and significantly brighter in the backscattering direction (i.e., with the Sun behind the observer). This dependence arises to a large extent from the 3-D structure or roughness of the surface as the end result of the interplay between shadowed and sunlit facets. For example, shadows are absent in the backscattering direction, which creates a reflectance “hot spot,” whereas they are maximal in the forward scattering direction, effectively darkening the surface. Solar photons may also be trapped bouncing between surface elements such as tree leaves before escaping. The number of such "internal" scattering events is modulated by the general brightness of the surface, creating additional angular dependence. Snow, the brightest type of surface at visible wavelengths, has maxima at both forward and backscattering angles, whereas water reflects light primarily in the forward scattering, or “glint,” direction. Wind roughens water surface, creating waves; stronger wind speeds lead to steeper waves, widening the glint "cone" and reducing its peak brightness.
Surface anisotropy is described by models of the bidirectional reflectance distribution function (BRDF) which is studied not just out of scientific curiosity, but for its many practical applications. BRDF defines the sun-angle dependence of surface albedo, one of the key parameters included in climate and weather forecasting models that constrain the surface energy budget. It is also required in atmospheric and land remote sensing applications to obtain accurate assessments of atmospheric aerosol and thin cloud properties, surface type classification, snow characterization (e.g., grain size retrievals), detection of seasonal and rapid surface change, and many other applications. The development of accurate satellite data analysis methods for deriving spectral surface BRDF and albedo is one of the important research directions of the Climate and Radiation Laboratory (CRL).
Surface BRDF model parameters are currently provided by the Earth Observing System (EOS) MODIS and MISR instruments. MISR obtains this information by measuring reflected radiance at nine different view angles ranging from 70˚ in the forward direction through nadir to 70˚ in the backward direction along the satellite track during overpass. MODIS on the other hand accumulates multi-angle measurements up to 55-60˚ using several days of observations of the same region from different orbits. To obtain surface BRDF, these data have to be corrected for the effects of atmospheric scattering and absorption of sunlight. The MODIS and MISR spectral surface reflectance, BRDF, and albedo are used by the land-user community for a wide range of basic science problems as well as variety of applications, such as mapping burn scars, land cover classification, characterization of vegetation properties and modeling gross primary production, a major component of the Earth system global carbon cycle.
The EOS instruments acquire global views of Earth, but they sample BRDF at a limited number and range of view angles. For full angular sampling of the surface BRDF with the unprecedented resolution of ~1˚, members of the CRL use the Cloud Absorption Radiometer (CAR) instrument. CAR is an aircraft sensor that obtains the most detailed BRDF data available when flown over different surfaces during dedicated NASA field campaigns and sub-orbital experiments. During ~18 years of operation, CAR has produced an extensive "library" of surface reflectance measurements over a wide variety of surfaces. These data have been used so far to develop and test models of snow reflectance and water sunglint, as well as to validate satellite BRDF data.
Contact: Alexei Lyapustin