Bidirectional reflectance of snow and sea ice: field, laboratory and modeling studies

摘要

Field measurements of the Hemispherical-Conical Reflectance Factor (HCRF) of Arctic snow-covered tundra were carried out using the GonioRAdiometric Spectrometer System (GRASS); over the viewing angles 0° to 50°, for the wavelength range 400 nm to 1300 nm. The HCRF measurements agreed well between sites where the snowpack was smooth and snow depth was greater than 40 cm, with a relative standard deviation of less than 10 % for backward and near nadir viewing angles. The site with the largest roughness elements had no forward peak and had a strong asymmetry in the HCRF with respect to the solar principal plane. The Conical-Conical Reflectance Factor (CCRF) of laboratory-generated sea ice was measured for the viewing angles 0° to 60°, for the wavelength range 410 nm to 730 nm. The CCRF of sea ice and the averaged HCRF of snow had forward scattering peaks, and an anisotropy that was strongly wavelength dependent; with the relative strength of the forward peak typically increasing with wavelength. The radiative-transfer model, PlanarRad, was able to reproduce the CCRF of the sea ice with a root-meansquared-error (RMSE) of less than 9 %, with differences in the reflectance factors of typically less than 0.05. The change in the hemispherical reflectance of Spectralon over the 19 °C phase transition of PTFE was calculated by measuring the change in the output flux from a temperature-controlled Spectralon integrating sphere at 633 nm. The relative change in hemispherical reflectance was calculated as 0.09 ± 0.02 %, and the change in output flux was 1.82 ± 0.21 %. The change in the hemispherical reflectance of Spectralon is small, but the effect is amplified for integrating spheres; thus the influence of the phase transition on PTFE based integrating spheres should be considered for operating temperatures near to the 19 °C PTFE phase transition temperature.