Hemispherical-Directional Reflectance (HDRF) of Windblown Snow-Covered Arctic Tundra at Large Solar Zenith Angles

Abstract

Ground-based measurements of the hemisphericaldirectional 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.

High Angular Resolution Measurements of the Anisotropy of Reflectance of Sea Ice and Snow

“…In the literature several methods are used to describe the extent of the anisotropy of the reflectance and the dependency of HDRF( θ v , θ s , ω s ,Δ ϕ , λ ) to illumination and wavelength (e.g., Ball et al, ; Bourgeois et al, ; Doctor et al, ; Sandmeier et al, ). Here we used the anisotropy index, ANIX( λ ), to evaluate the extent of the reflectance anisotropy over a single surface.…”

“…Here we used the anisotropy index, ANIX( λ ), to evaluate the extent of the reflectance anisotropy over a single surface. ANIX( λ ) is calculated as the ratio of the maximum to the minimum spectral reflectance factor for a single measurement cycle (Ball et al, ; Bourgeois et al, ; Sandmeier et al, ). For the CE600 measurements, ANIX( λ ) was estimated as the ratio of the median over the 1% highest and lowest values across the entire hemisphere.…”

“…Near‐surface spectroradiometer field measurements have also been made by Li and Zhou () over snow‐covered sea ice and by Bourgeois et al (), Painter and Dozier (), and Ball et al () on different snow surfaces for a large range of solar zenith angles. Bourgeois et al () observed significant variations in snow reflectance anisotropy for solar zenith angles above 65°.…”

“…Below this threshold the authors found that snow reflectance anisotropy was mainly driven by the physical characteristics of the snow layer. In addition, Bourgeois et al () and Ball et al () found that reflectance anisotropy is wavelength dependent and a function of the surface roughness. Both studies illustrate the need to provide reflectance anisotropy representative at larger spatial scales with high angular and spectral resolutions.…”

“…This nonexhaustive list of publications shows the wide range of measurement setups established to evaluate reflectance anisotropy over ice and snow surfaces. However, as mentioned by Ball et al (), there is a need for more systematic measurements of hyperangular and hyperspectral reflectance anisotropy as well as for a larger panel of snow and ice surfaces to meet the requirements of numerous applications. In addition, there is a need in directional reflectance measurements ranging from the scale of scene individual elements to composite scenes and ultimately to airborne and satellite pixels (Milton et al, ).…”