REFLECTANCE AND ALBEDO, SURFACE

‘Albedo’ is Latin, meaning whiteness. The albedo of a surface is the fraction of the incident sunlight that the surface reflects. Radiation that is not reflected is absorbed by the surface. The absorbed energy raises the surface temperature, evaporates water, melts and sublimates snow and ice, and energizes the turbulent heat exchange between the surface and the lowest layer of the atmosphere. The surface albedo is a key ingredient in the remote sensing of surface and atmospheric properties from space. The spectral and angular dependence of reflected sunlight is used to infer surface properties such as the extent and nature of vegetation cover. It must also be allowed for when determining atmospheric composition such as the amount, size, and optical properties of haze particles. Over continents, the largest component of the reflected sunlight under cloud-free conditions is due to reflection by the surface. Consequently, the determination of atmospheric composition from reflected sunlight requires accurate knowledge of the contribution made by the reflecting surface. Different surfaces have different albedos. Oceans, lakes, and forests reflect relatively small fractions of the incident sunlight and have low albedos. Snow, sea ice, and deserts reflect relatively large fractions of the incident sunlight and have large albedos. While estimates of albedos for various surfaces are presented later (Table 3), it should be recognized that an albedo is not an intrinsic property of a surface. Instead, for any surface, the albedo depends on the spectral and angular distributions of the incident light, which in turn are governed by atmospheric composition and the direction of the beam of light from the sun. Ideally, surfaces may be broken into components, each component reflecting light following its own intrinsic optical rules for the wavelength and angular dependence of reflection. In practice, however, when viewed at the few-meters to several-kilometers resolution typical of spaceborne sensors, surfaces exhibit complex mixtures of components and geometric structures. At such scales, surfaces can contain any mixture of soil, vegetation, twigs, branches, rocks, snow, ice, water, and so on. The list is seemingly endless. Even if the scene is spatially uniform, reflection by the individual surface elements, such as grains of sand in a desert, or snowflakes in a drift, can be so difficult to treat according to the principles of optics as to render predictions of reflectance futile. In addition, topography and the consequent shading add further to the complexity. Thus, for practical treatments, surfaces are reduced to having few components, even as few as one or two, and both the spectral and angular distributions of the reflectances for the individual components are highly simplified. Simplifications of the spectral dependence of surface reflection are often dealt with through use of wavelength intervals over which the reflectivity of the surface remains reasonably constant. Simplifications of the angular dependence of surface reflection are often dealt with through the separation of the incident light into two components: one for the direct beam of incident light, which is the sunlight that has not been absorbed or scattered in the atmosphere; and one for incident sunlight that has been scattered into directions that differmarkedly from that of the direct beam. These simplifications are used below in combination with a global inventory of surface types to produce global maps of surface albedos. These maps, however, should be taken as providing only rudimentary estimates. Because of the complexities noted earlier, systematic attempts to construct global maps of surface albedos from satellite observations are just now being undertaken.