Anisotropic Reflectance Factor of the snow surface measured at
South Pole Station from 1990-1992.


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File description:

column 1:  viewing zenith angle (0 deg=nadir, 90 deg=horizon)

column 2:  viewing azimuth angle (0 deg = backscattering direction,
           180 deg = forward scattering direction, positive to the
           right of 0 deg, negative to the left of 0 deg.)

column 3:  Anisotropic Reflectance Factor


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The ARF measurements correspond to figures in:

Warren, S.G., R.E. Brandt, and P. O'Rawe Hinton. Effect of surface roughness 
on bidirectional reflectance of Antarctic snow. 
J. Geophys. Res. (Planets), 103, 25,789-25,807. 1998


Abstract.  

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 from 67 deg to 90 deg, 
over the full range of viewing zenith angle, azimuth angle between Sun and 
view, and azimuth angle between Sun and sastrugi.  Variation of R with azimuth angle 
between Sun and sastrugi 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
viewing zenith angle is preferred. A parameterization of R is developed, valid for 
viewing angles <=50 deg. Sastrugi can cause a reduction of the snow albedo by altering
the angle of incidence unaffected at visible wavelengths but can be reduced by a few
percent at near-infrared wavelengths when the Sun is low.