Diffusion of Light in Antarctic Sea Ice
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DIFFUSION OF LIGHT IN ANTARCTIC SEA ICE R G BUCKLEY* AND H J TRODAHL** *Physics and Engineering Laboratory, Department of Scientific and Industrial Research, P 0 Box 31313, Lower Hutt, New Zealand **Department of Physics, Victoria University of Wellington, P 0 Box 600, Wellington, New Zealand ABSTRACT It is important to understand the diffusion sun light in sea ice as this interaction exerts a strong influence over the weather in polar regions and over the growth of microbial communities in the sub-ice water column. Sea ice is a granular composite of ice, brine, air and at low temperatures precipitated salts, in which the proportion and disposition of these components vary with depth, with temperature and thermal history. It is demonstrated that by using a simple in situ technique, supported by Monto Carlo simulations, the diffusive transport of light through a weakly absorbing composite such as sea ice can be characterised. The radiation field both within and emergent from the ice is found to reflect the anisotropic structure of the ice, in particular, the scattering is found to be highly anisotropic so that the light is channelled predominantly vertically. The technique will be illustrated with experimental results collected in McMurdo Sound, Antarctica. INTRODUCTION Sea ice is a granular composite that grows at the interface between the ocean and the atmosphere in polar regions and has a significant impact on polar environments. In the Antarctic the winter growth of sea ice effectively doubles the area of the continent and thus exerts a strong influence over southern climates and over the microbial communities that live under the ice and form the basis of the Antarctic food chain. The interaction of sunlight with sea ice is thus an important parameter in determining the input of solar energy into the Antarctic environment. Understanding this interaction has become particularly relevant recently due to the increased UV surface radiance in the Antarctic associated with the ozone hole [1] as the increase in UV radiance is thought to be harmful to ice algae [2]. Below we discuss a technique [3,4,5] that characterises the diffusion of light through sea ice and the nature of the transmitted radiation field. This is accomplished by coupling a simple experimental technique with Monto Carlo modelling. SEA ICE Sea ice is a complex composite of ice, brine, trapped air and at low temperatures, precipitated salts. The proportions of these components vary with depth, with temperature and history of the ice. The material displays an extreme temperature gradient over its thickness with the top at ambient air temperature and the bottom at its melting point.
Mat. Res. Soc. Symp. Proc. Vol. 195. 01990 Materials Research Society
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When the temperature of sea water is lowered to approximately -1.8 0 C (depending on the salinity) ice crystals form in the water column and for ice types we discuss here eventually forms a solid layer approximately 0.2 m thick that is an isotropic mixture of ice, brine and air. Because ice possesse
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