A temperature‐dependent, structural‐optical model of first‐year sea ice

Abstract

A model has been developed that relates the structural properties of first‐year sea ice to its inherent optical properties, quantities needed by detailed radiative transfer models. The structural‐optical model makes it possible to calculate absorption coefficients, scattering coefficients, and phase functions for the ice from information about its physical properties. The model takes into account scattering by brine inclusions in the ice, gas bubbles in both brine and ice, and precipitated salt crystals. The model was developed using concurrent laboratory measurements of the microstructure and apparent optical properties of first‐year, interior sea ice between temperatures of −33°C and −1°C. Results show that the structural‐optical properties of sea ice can be divided into three distinct thermal regimes: cold (T < −23°C), moderate (−23°C < T < −8°C), and warm (T > −8°C). Relationships between structural and optical properties in each regime involve different sets of physical processes, of which most are strongly tied to freezing equilibrium of the brine and ice. Volume scattering in cold ice is dominated by the size and number distribution of precipitated hydrohalite crystals. Scattering at intermediate temperatures is controlled by changes in the distribution of brine inclusions, gas bubbles, and mirabilite crystals. Total volume scattering in this regime is approximately independent of temperature because of a balance between increasing and decreasing scattering related to the thermal evolution of these inclusions and scattering by drained inclusions. In warm ice, scattering is controlled principally by temperature‐dependent changes in the real refractive index of brine and by the escape of gas bubbles from the ice. Model predictions indicate that scattering coefficients can exceed 3000 m−1 for cold ice, averaging ∼450 m−1 for moderate and warm ice and reaching a minimum of ∼340 m−1 at −8°C. Scattering in all three regimes is very strongly forward peaked, with values of the asymmetry parameter g generally falling between 0.975 (T = −8°C) and 0.995 (T = −33°C).