Abstract
Measurements of the hemispherical-directional reflectance of snow at fine spectral and angular resolution
Institute for Computational Earth System Science, Department of Geography, University of California, Santa Barbara, California, USA
Donald Bren School of Environmental Science and Management, University of California, Santa Barbara, California, USA
We present 2 days' measurements of the hemispherical-directional reflectance factor (HDRF) of snow made at fine spectral and
angular resolution with the Automated Spectro-Goniometer (ASG) for the range of solar zenith angles (
0 = 40°–50°) and snow textures (surface grain size = 80–280 μm). Measurements of the stratigraphy of snow texture and density
accompanied each day's suite of measurements. These measurements represent the most detailed available in terms of angular
and spectral resolution. The HDRF for fine grain, faceted snow exhibited a local backscattering peak at the view zenith near
the solar zenith angle, whereas those for medium grain, clustered snow did not have a local backscattering peak. The HDRF
decreased at all wavelengths for an increase in grain radius from 80 μm to 280 μm. However, the decrease in HDRF in the visible
wavelengths was largest at
r
= 80° in the forward direction and largest for λ > 1.8 μm near
r
= 30° in the backward direction. As solar zenith angle decreased from 47° to 41°, the HDRF increased near nadir for λ ≤ 1.03
μm but decreased with coherent angular structure for λ > 1.03 μm. We compared forward radiative transfer modeling results
with the HDRF measurements. The forward model used single-scattering parameters for ice spheres with radii that matched the
surface-area-to-volume ratio derived from stereological analysis of snow samples and a stratigraphic distribution of optical
depths from measured density and modeled extinction efficiency. All HDRF models underestimated reflectance for λ > 1.30 μm
and had large absolute errors in the perpendicular plane. Mean absolute RMS errors in reflectance for the fine grain, faceted
snow case were 0.09 at λ = 1.3 μm and 0.14 at λ = 1.85 μm. Mean absolute RMS errors for the medium grain, clustered snow were
0.04–0.06 at λ = 1.3 μm and 0.04–0.06 at λ = 1.85 μm. The models for the more spherical medium grain snow had better overall
spectral and angular fits than those for the nonspherical fine grain snow. The spherical radii inferred from the surface-area-to-volume
ratio from stereological analysis of snow with nonspherical particles have a greater effective path length than the actual
snow particles, resulting in underestimates of hemispherical-directional reflectance.
Received 16 December 2003; accepted 8 June 2004; published 29 September 2004.
Citation: (2004), Measurements of the hemispherical-directional reflectance of snow at fine spectral and angular resolution, J. Geophys. Res., 109, D18115, doi:10.1029/2003JD004458.
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