Hemispherical-Directional Reflectance (HDRF) of windblown snow-covered Arctic tundra at large solar zenith angles

Ground-based measurements of the hemispherical-directional reflectance factor (HDRF) of windblown snowcovered Arctic tundra were measured at large solar zenith angles (79◦–85◦) for six sites near the international research base in Ny-Ålesund, Svalbard. Measurements were made with the Gonio RAdiometric Spectrometer System over the viewing angles 0◦–50◦ and the azimuth angles 0◦–360◦, for the wavelength range 400–1700 nm. The HDRF measurements showed good consistency between sites for near-nadir and backward viewing angles, with a relative standard deviation of less than 10% between sites where the snowpack was smooth and the snow depth was greater than 40 cm. The averaged HDRF showed good symmetry with respect to the solar principal plane and exhibited a forward scattering peak that was strongly wavelength dependent, with greater than a factor of 2 increase in the ratio of maximum to minimum HDRF values for all viewing angles over the wavelength range 400– 1300 nm. The angular effects on the HDRF had minimal influence for viewing angles less than 15◦ in the backward viewing direction for the averaged sites and agreed well with another study of snow HDRF for infrared wavelengths, but showed differences of up to 0.24 in the HDRF for visible wavelengths owing to light-absorbing impurities measured in the snowpack. The site that had the largest roughness elements showed the strongest anisotropy in the HDRF, a large reduction in forward scattering, and a strong asymmetry with respect to the solar principal plane