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 t he reflected radiation over the hemisphere (so-called BRDFs=bidirectional r eflectance distribution functions) were made on the Morteratschgletscher (S witzerland) in Landsat TM bands 2 (520-600 nm) and 4 (760-900 nm). These BR DFs cover a wide range of solar zenith angles (26-75 degrees) and surface c haracteristics (quantified by a variation in the spectrally integrated albe do between 0.14 and 0.50). All BRDFs exhibit a similar pattern with a minim um in the nadir direction and a maximum in the forward limb, but the amount of a anisotropy increases with increasing wavelength, with increasing sola r zenith angle and with decreasing albedo. The data were used to derive par ametrizations rations (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 parametrisations (one for each TM band) for near-nadir reflection" are also presented. All these parameterizations can be used to convert satellite-derived bidirectional reflectances into s urface albedos and thus do correct for anisotropic reflectance. The residua l uncertainty in the albedo due to inaccuracy of the correction is estimate d to be 0.02 in both TM bands. (C) Elsevier Science Inc., 1999.