|
Effects of Temperature on the Optical Properties of Sea Ice |
Department of Atmospheric Sciences University of Washington
bonnie@atmos.washington.edu
The optical properties of sea ice describe how shortwave radiation is backscattered, transmitted, and absorbed by the ice. They are important not only for calculating surface melting, internal heat storage and solar heat input to the ocean mixed layer, but also for characterizing a variety of ice-albedo feedback processes in small-scale and large-scale models. These optical properties depend primarily on the distribution of inclusions of brine, vapor, precipitated salts, and particulates within the ice. The size and number distributions of these inclusions undergo large changes with temperature, but establishing a relationship between these changes and observed changes in the ice optical properties has proven to be difficult. We describe results from laboratory experiments designed to understand structural-optical relationships and their dependence on temperature in first-year sea ice. These results have been used to develop and test a structural-optical model necessary for detailed radiative transfer modeling in sea ice. n an effort to link structural observations to inherent optical properties of the ice, we measured apparent optical properties of cylindrical, isothermal samples of natural sea ice with known structure at temperatures between -33 and -2°C. Inherent optical properties were interpreted from albedo and transmissivity data using a cylindrical Monte Carlo radiative transfer model. As expected, scattering associated with the precipitation of hydrohalite crystals increased when the ice was cooled below -23 C. Also as expected, optical properties remained relatively constant between -23 and -8°C where effects from changes in the mass of precipitated mirabilite crystals were offset by changes in the size of brine pockets. At temperatures between -8 and -2°C, the structural data indicated there was a large increase in the cross-sectional area of the inclusions, although there were only small changes in the optical properties of the ice. The primary reason for this was discovered to be related to the strong decrease in the refractive index of the brine with increasing temperature. This caused scattering from brine pockets to become more forward peaked, thus offsetting the effects of increased cross-sectional area.
Results from these observations and our structural-optical model indicate that the optical properties of sea ice: (1) change dramatically at temeratures below -23°C, where they are determined primarily by bulk ice salinity, brine pocket size and number distribution being relatively unimportant, (2) remain fairly constant between -23 and -8°C, although the magnitude of the optical properties still depend on bulk salinity, and (3) change only weakly above -8°C due to the temperature dependence of the brine refractive index. We expect this general pattern will be found in most types of natural sea ice, regardless of the exact distribution of inclusions.
