Effect of surface roughness on bidirectional reflectance of Antarctic snow

The angular pattern of sunlight reflected by snow is altered by surface roughness, which in the interior of Antarctica is usually in the form of meter-scale longitudinal erosional features (sastrugi), whose axes align with the direction of strong winds. The bidirectional reflectance distribution function (BRDF) changes over the course of a day as the solar azimuth changes relative to the sastrugi axis. The normalized BRDF, or "anisotropic reflectance factor" R, was measured at South Pole Station from a 22-m tower at 600, 660, and 900 nm wavelengths. The R pattern was similar at the three wavelengths; it probably varies little from 300 to 900 nm. Measurements were made at solar zenith angles theta(o) from 67 degrees to 90 degrees, over the full range of viewing zenith angle (theta(r)), azimuth angle between Sun and view (phi), and azimuth angle between Sun and sastrugi (phi(sus)). Variation of R with phi(sas) was notable; sastrugi oriented perpendicular to the solar beam cause a reduction of the forward peak, and sastrugi at an oblique angle cause R to lose its symmetry about the solar azimuth. However, the effects of sastrugi are mostly restricted to large viewing zenith angles, so remote sensing of albedo and atmospheric properties can be carried out accurately without knowledge of sastrugi height and orientation if only near-nadir views are used. This recommendation is opposite that for observations of broken clouds over dark surfaces, for which large theta(r) is preferred. A parameterization of R is developed, valid for viewing angles theta(r) less than or equal to 50 degrees. Sastrugi can cause a reduction of the snow albedo by altering the angle of incidence and by trapping of photons. For the small sastrugi of the Antarctic Plateau, the albedo is unaffected at visible wavelengths but can be reduced by a few percent at near-infrared wavelengths when the Sun is low.