Pancake Ice Thickness Mapping in the Beaufort Sea From Wave Dispersion Observed in SAR Imagery

The early autumn voyage of RV Sikuliaq to the southern Beaufort Sea in 2015 offered very favorable opportunities for observing the properties and thicknesses of frazil-pancake ice types. The operational region was overlaid by a dense network of retrieved satellite imagery, including synthetic aperture radar (SAR) imagery from Sentinel-1 and COSMO-SkyMed (CSK). This enabled us to fully test and apply the SAR-waves technique, first developed by Wadhams and Holt (1991), for deriving the thickness of frazil-pancake icefields from changed wave dispersion. A line of subimages from a main SAR image (usually CSK) is analyzed running into the ice along the main wave direction. Each subimage is spectrally analyzed to yield a wave number spectrum, and the change in the shape of the spectrum between open water and ice, or between two thicknesses of ice, is interpreted in terms of the viscous equations governing wave propagation in frazil-pancake ice. For each of the case studies considered here, there was good or acceptable agreement on thickness between the extensive in situ observations and the SAR-wave calculation. In addition, the SAR-wave analysis gave, parametrically, effective viscosities for the ice covering a consistent and narrow range of 0.03-0.05 m(2) s(-1). Plain Language Summary It is known from work at open ocean boundaries such as Antarctica and the Greenland Sea that ocean waves interact with icefields to break up the ice and strip it away from the ice-water boundary, enhancing ice retreat. Also, in a wave field, new ice is forced to grow in the form of pancakes, an array of small cakes. The Beaufort Sea in late summer now counts as an open ocean boundary, and on the 2015 cruise of Sikuliaq it was found that vast fields of pancake ice were being created at the advancing edge as summer ended. We were able to map the thickness of these pancakes, and hence their contribution to the volume of the icefield, using a satellite technique developed by two of the authors in 1991. This uses synthetic aperture radar images of the ice to determine the frequency spectrum of wave energy, and since the speed of waves in ice depends on the frequency and the ice thickness, we can extract thicknesses from this analysis. The results were tested against direct measurements from the ship. This technique can be used worldwide to determine the overall contribution of pancake ice to the world's sea ice.