Optical Properties Of Ice And Snow In The Polar Oceans. I: Observations

Optically sea ice is a complex material with an intricate and highly variable structure which includes brine pockets, air bubbles, brine channels and internal platelet boundaries. Large variations in the optical properties of the surface layer can occur on horizontal scales of only a few meters, complicating efforts to quantify larger scale interactions between shortwave radiation and the ice-ocean system. Radiative transfer in sea ice is dominated at visible wavelengths by scattering rather than absorption. Because scattering in the ice is essentially independent of wavelength, spectral variations in the optical properties are primarily the result of differences in absorption. Observations show that albedos are particularly sensitive to the presence of liquid water in the surface layers, the effect being most pronounced at wavelengths above 600 nm. Albedos and extinction coefficients in the ice vary inversely with brine volume, and thus temperature. Below the eutectic point, precipitation of solid salts causes a sharp increase in scattering and corresponding increases in albedo and absorption. Biological activity in natural sea ice often affects light transmission and absorption, particularly in coastal regions and in the Southern Ocean. Phase function measurements indicate that the scattering distribution in sea ice is only weakly dependent on wavelength and brine volume.