Session 5.3 Spectral bidirectional reflectance of Antarctic snow: Measurements and parameterization

Tuesday, 11 July 2006: 9:00 AM
Hall of Ideas G-J (Monona Terrace Community and Convention Center)
Stephen R. Hudson, Univ. of Washington, Seattle, WA; and S. G. Warren, R. Brandt, T. C. Grenfell, and D. Six

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The bidirectional reflectance distribution function (BRDF) of snow was measured from a 32-meter tower at Dome C, at latitude 75°S on the East Antarctic Plateau. These measurements were made at 96 solar zenith angles between 51° and 87°, and cover wavelengths 350—2400 nm, with 3- to 30-nanometer resolution, over the full range of viewing geometry.

From these measurements, parameterizations were developed that allow users to quickly and accurately calculate the anisotropic reflectance function of the East-Antarctic snow surface. The parameterizations cover nearly all viewing angles and are applicable to the high parts of the Antarctic Plateau that have small surface roughness, and, at viewing zenith angles less than 55°, elsewhere on the plateau, where larger surface roughness affects the BRDF at larger viewing angles. The root-mean-squared error of the parameterized reflectances is between 2% and 4% at wavelengths less than 1400 nm and between 5% and 8% at longer wavelengths.

The effect of the snow-surface roughness on the BRDF is examined by comparing the parameterization results with the BRDF predicted by a plane-parallel radiative transfer model for a perfectly flat snow surface. The presence of the roughness reduces the anisotropy of the reflected radiance field. Finally, the effect of having a cloud above the surface is investigated using observations from Dome C of the reflected radiance field from a thin fog layer above the snow surface. It appears that the presence of a cloud or fog layer above the snow surface acts to mask the roughness of the snow, thereby enhancing the anisotropy of the reflected radiance.

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