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Effects of Temperature on the Microstructure of Sea Ice |
Department of Atmospheric Sciences University of Washington
bonnie@atmos.washington.edu
The microstructure of sea ice directly determines its optical properties. Surface melting, ice-albedo feedback processes, solar heating of the upper ocean, and heat storage within the ice are all dependent on the backscattering, transmission, and absorption of shortwave radiation by the ice cover. SHEBA observations show large temporal changes in the state of the ice and in its albedo and transmissivity, particularly during the summer melt season. While the apparent optical properties of sea ice vary with ice type and temperature throughout the annual cycle, they depend more fundamentally on how inclusions of brine, gas, and precipitated salts are distributed within the ice.
Since little is known about these distributions and how they evolve with temperature, experiments were designed to collect detailed information on the microstructure of sea ice over a wide range of temperatures. An imaging system, capable of resolving inclusion sizes of 0.01 mm and smaller, was used to examine the microstructure of first-year ice in a temperature-controlled laboratory. Experiments were initially carried out at -15°C to obtain size distributions for brine pockets and gas bubbles in cold ice. Brine pocket dimensions were found to range from less than 0.01 mm to approximately 10 mm, with number densities averaging about 30 pockets per mm3. This is an order of magnitude larger than number densities previously reported. Vapor bubbles in our laboratory samples and in a SHEBA field sample had radii less than 0.2 mm and number densities of approximately 1 per mm3, which is also 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. The link between structural and optical properties of sea ice is closely tied to the total cross-sectional area of the inclusions. We show that this area increases dramatically when the ice cools below -23°C, and when it warms above -5°C.
