Anisotropy of the Reflected Radiation Field Over Melting Glacier Ice: Measurements in Landsat TM Bands 2 and 4

In this article we analyze the anisotropy of the reflected radiation field over melting glacier ice using 118 ground-based radiance measurements in Landsat TM Bands 2 and 4. The measurements were carried out on the tongue of the Morteratschgletscher, Switzerland. On the basis of directional measurements several bidirectional reflectance distribution functions (BRDFs) were derived for dirty and clean glacier ice and for solar zenith angles between 46° and 60°. The anisotropic reflectance factor ranges between 0.70 and 1.60 for TM 2 and between 0.56 and 1.72 for TM 4. The BRDFs exhibit nadir darkening and maximum brightening near the forward limb. Dirty and wet ice reflect more anisotropically than clean ice. We give an example to illustrate the effect of anisotropic reflectance on the TM-derived albedo along a cross-section of the glacier. For solar zenith angles around 48° and in the nadir viewing direction we found differences of 0.05 (in TM 2) and 0.08 (in TM 4) between the albedo (α) and bidirectional reflectance (r). The differences prove to be more or less uniform for both clean ice and dirty ice. It is therefore suggested that one can convert r to α without having to classify the glacier surface.