Anisotropic Reflection by Melting Glacier Ice: Measurements and Parametrizations in Landsat TM Bands 2 and 4

This article deals with the anisotropic reflection of radiation by melting glacier ice. Ground-based measurements of the directional distribution of the reflected radiation over the hemisphere (so-called BRDFs=bidirectional reflectance distribution functions) were made on the Morteratschgletscher (Switzerland) in Landsat TM bands 2 (520–600 nm) and 4 (760–900 nm). These BRDFs cover a wide range of solar zenith angles (26–75°) and surface characteristics (quantified by a variation in the spectrally integrated albedo between 0.14 and 0.50). All BRDFs exhibit a similar pattern with a minimum in the nadir direction and a maximum in the forward limb, but the amount of anisotropy increases with increasing wavelength, with increasing solar zenith angle and with decreasing albedo. The data were used to derive parametrizations (one for each TM band) which relate the bidirectional reflectance (the reflectance in a specific direction) to the albedo for a given solar-view geometry. Specific parametrizations (one for each TM band) for “near-nadir reflection” are also presented. All these parametrizations can be used to convert satellite-derived bidirectional reflectances into surface albedos and thus to correct for anisotropic reflectance. The residual uncertainty in the albedo due to inaccuracy of the correction is estimated to be 0.02 in both TM bands.