The Use of SAR to Measure Ocean Wave Dispersion in Frazil–Pancake Icefields

Abstract

Icefields composed of frazil and pancake ice play important roles in both polar regions in winter. During the early to midwinter period pancake ice is a major component of the Antarctic sea ice cover, while in the Arctic the Odden ice tongue in the Greenland Sea, associated with deep convection, is composed mainly of pancake ice. The retrieval of sea ice thickness by remote sensing is, in general, a very difficult task. In this paper the change in dispersion of ocean waves as they penetrate into pancake ice is considered so as to gain insight into its possible relationship with thickness. Spectral analysis of subscenes from ERS-2 (second European Remote Sensing Satellite) 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 decrease of wavelength at the ice edge, as well as refraction toward the normal and a loss of amplitude. The analysis is complex because the true wave spectrum must be retrieved from the SAR spectrum, involving an inversion technique that requires a ?first-guess? spectrum. This analysis technique is described, together with a newer cross-spectral technique and a discussion of the wave theories that predict the change in wavelength. Results are reported from two experiments in the Odden ice tongue, in 1993 and 1997, both of which involved surface truth measurements from ships and wave buoys. In the Antarctic, results are reported from imagery of the outer ice-edge zone of the Weddell Sea sector in midwinter (July) 1997.