SAR ice thickness mapping in the Beaufort Sea using wave dispersion in pancake ice - a case study with intensive ground truth

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 wide marginal ice zones are composed mainly of pancake/frazil ice. The concept of using the change in the wavelength of ocean waves on entering a frazil-pancake icefield, as measured by spectral analysis of SAR images, as a way of mapping the thickness of the ice, was originated several year ago by a seminal paper (Wadhams & Holt, 1991) and fully developed in the following years by two of the present authors (Wadhams et al., 2002, Wadhams et al., 2004). In our current work in the EU SPICES project (www.nersc.no/nb/project/spices) we plan to use the results of theory and observations so far in order to develop a processing scheme for routinely deriving ice thicknesses in frazil-pancake regions of the Arctic and Antarctic and hence assess their mass and heat balance. To do this we require more ground truth than had henceforth been available, the only direct comparison between satellite and shipborne work dating from more than ten years ago (Wadhams et al., 2004). In autumn 2015 (Sep 30 - Nov 10) the University of Alaska research ship "Sikuliaq" carried a team funded by the Office of Naval Research “Sea State Project” to study marginal ice zone processes in the Beaufort Sea, particularly waveice interaction processes. One such study involved a line of wave buoys laid out along a pre-declared line, which could thus be covered by simultaneous Cosmo-SkyMed images; in particular, the image dated from October 11 covered an area where frazil-pancake ice was the dominant ice type along the entire length of the survey line out to open water. The ground truth facilities deployed included: i) directional wave buoys placed in the water between pancakes; ii) "Swift" directional wave buoys placed outside of the ice edge; and iii) measurements of pancake ice thickness by recovery of cakes, and of frazil ice mass per unit area using a collector (a "frazilometer") giving a sample that is allowed to melt out. We report on the comparison between the wave fields measured by the buoys and the wave spectra derived from the SAR; and on the comparison between the ice thicknesses measured in situ and the thickness inferred from SAR wave number analysis with the application of a viscous theory.