Observations and modeling of structural-optical properties in first-year sea ice (INVITED)

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 climate models. Observations show large temporal changes in the state of the ice and in its apparent optical properties, particularly during the summer melt season. In order to model and predict such changes, we need a basic understanding of how absorption and scattering within the ice are controlled by the ice microstructure, specifically inclusions of brine, gas, and precipitated salts.

Since little is known about how these inclusions are distributed, how they evolve with temperature, or how they affect the optical properties of the ice, a study was designed to simultaneously collect detailed information on the microstructure of sea ice and its optical properties over a wide range of temperatures. An imaging system, capable of resolving inclusion sizes of 0.01 mm, was used to examine the microstructure of natural, first-year ice samples in a temperature-controlled laboratory. Experiments were initially carried out at –15 C to characterize size distributions for brine pockets and gas bubbles in cold ice. Both populations were observed to have number densities an order of magnitude larger than previously reported. Large changes in the microstructure were observed as samples were cooled to –30 C, and subsequently warmed to –2 C. Interactions between brine pockets, the disappearance of gas bubbles as the ice cooled, and the presence of precipitated salts within brine pockets were documented.

Structural and optical results were used to develop and test a structural-optical model necessary for detailed radiative transfer modeling in sea ice. Observational and theoretical results indicate that the optical properties of sea ice: (1) change dramatically at temperatures 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 depends on bulk salinity, and (3) change only weakly above –8 C due to evolution of the microstructure and the dielectric properties of brine. We expect this general pattern will be found in most types of natural sea ice, regardless of the exact distribution of inclusions.