Mapping the Thickness of Pancake Ice Using Ocean Wave Dispersion in SAR Imagery

Abstract

During the early to midwinter period pancake ice is a major component of the Antarctic sea ice cover, occupying a belt extending 200-300 km from the outer ice edge with an area of about 6 million km2. Experience in the Arctic suggests that the sea ice thickness in this region can be mapped by monitoring the penetration of ocean waves into it. Spectral analysis of subscenes from ERS-2 synthetic aperture radar (SAR) images yields the wavelength and direction of the principal spectral component both outside and inside the ice cover. There is a change of wavelength at the ice edge associated with a change in the wave dispersion, which can be quantitatively related to the thickness of the ice. The analysis is complex because the true wave spectrum must be retrieved from the SAR spectrum, which involves an inversion technique requiring a “first-guess” spectrum. The analysis technique is described, and the wave theory which predicts the change in wavelength. Experience in the Chukchi Sea and especially in the Odden ice tongue in the Greenland Sea is reviewed. Preliminary data from the Ross Sea have been analysed, yielding principal wave components, but the ambient wavelength was too long for the effect of ice to be detectable. The next application will be to the outer ice edge, when midwinter data from this region are available at the end of the current season.