Observations of the polarization of light reflected from sea ice

As part of a large, interdisciplinary program investigating the electromagnetic properties of sea ice, we made spectral measurements of the albedo, reflectance, and Stokes vector of the reflected radiance field. The overall program encompassed observations of sea ice physical properties, optical properties and microwave properties, plus an extensive modeling effort. Measurements were made of an evolution sequence including young sea ice, pancake ice, snow-covered ice, first-year ice, and ponded ice. The effects of surface roughness were investigated by artificially roughening part of a 31-cm-thick, smooth, young ice sheet. Spectral and total albedos were sensitive to surface conditions, consistent with earlier studies. Stokes vector observations exhibited the greatest variability in the plane of incidence of the solar beam. Reflectances at 0° azimuth increased sharply with zenith angle. Smoother surfaces, such as melt ponds, pancakes and bare ice, exhibited a larger increase than the snow-covered cases with their “rougher” surfaces. Since the solar zenith angle was close to Brewster's angle, specularly reflected light was highly polarized. In the “smooth” ice cases, there was significant polarization associated with the increase in reflectance, implying a substantial contribution from specular reflection. This contribution was greater at longer wavelengths, where specular reflection was a larger component of the reflected radiance. Away from the plane of incidence, the Stokes vector showed little angular variation, and the reflected light at these angles was largely unpolarized.