Structural-optical relationships in first-year sea ice

Abstract

The optical properties of sea ice are governed by the distribution of brine and gas inclusions, and precipitated salt crystals within the ice. Laboratory experiments designed to understand structural-optical relationships and their dependence on temperature in first-year sea ice were carried out. Detailed observations of the microstructure of isothermal samples of natural sea ice were obtained for temperatures between -33 and -2°C. Changes in apparent optical properties of cylindrical samples cut from the same ice core were monitored simultaneously. A cylindrical Monte Carlo radiative transfer model was developed to infer inherent optical properties from the radiance data. Experimental results were used to develop and test a structural-optical model necessary for detailed radiative transfer modeling in sea ice.Microstructure observations were initially carried out at -15°C to obtain inclusion size distributions. Brine pocket dimensions were found to range from 0.01 mm to 10 mm, with number densities averaging about 30 mm-3. Observed vapor bubbles had radii less than 0.2 mm and number densities approximately 1 mm-3. Both these estimates are an order of magnitude larger than number densities previously reported.Results indicate that structural-optical relationships in sea ice can be described by three regimes. At temperatures below -23°C, optical properties change dramatically, and are most affected by the precipitation of hydrohalite. At temperatures between -23 and -8°C, they remain fairly constant where effects from changes in the mass of precipitated mirabilite crystals are offset by changes in the size of brine inclusions. At temperatures between -8 and -2°C, only small changes in the optical properties of the ice were observed, despite large observed increases in the cross-sectional area of the inclusions. This was discovered to be related to a significant increase in bulk asymmetry parameter resulting from a decrease in the refractive index of brine. We expect this general pattern will be found in most types of sea ice, regardless of the exact distribution of inclusions. These results suggest that it is possible to develop simple parameterizations of radiative transfer in sea ice appropriate for incorporation into large-scale climate models and GCMs.