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HEM bird measurements of ice thickness and freeboard over the McMurdo Sound and implications for ice density

HEM bird measurements of ice thickness and freeboard over the McMurdo Sound and implications for ice density
Wolfgang Rack


W. Rack1, C. Haas2 and P. Langhorne3

  1. Gateway Antarctica, University of Canterbury, Christchurch, NZ
  2. Department for Earth and Space Science, York University, Toronto, CAN
  3. Department of Physics, University of Otago, Dunedin, NZ


We present airborne measurements to investigate the thickness, freeboard, and density of sea ice and ice shelf in McMurdo Sound in the western Ross Sea. In November 2009 and 2011 we used a helicopter-borne laser and electromagnetic induction sounder (HEM bird) to measure thickness and freeboard profiles across the ice shelf and the sea ice. In both years, airborne data were validated with field measurements for ice thickness, snow depth and density, and thickness of the sub-ice platelet layer. The quality of the helicopter measurements was analysed at cross-over points, yielding an accuracy of about 10% for both thickness and freeboard over sea ice and ice shelf. It was found that the EM sounder systematically overestimates the ice thickness in the presence of a sub-ice platelet layer, which is observed in areas of outflow of super-cooled ice shelf water. This offers the opportunity to use the HEM bird for mapping the sub-ice platelet layer by airborne remote sensing. Using freeboard and thickness, the bulk density was calculated assuming hydrostatic equilibrium. Significant density steps were detected between first-year and multiyear sea ice, with higher values for the younger sea ice. On the ice shelf, bulk ice densities were sometimes higher than that of pure ice, which can be partly explained by the accretion of marine ice and glacial sediments. The thickness and density distribution reflects a picture of areas of basal freezing and supercooled Ice Shelf Water emerging from below the central ice shelf cavity into the McMurdo Sound.

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This page was last modified on 3 September 2013.