Relating wave attenuation to pancake ice thickness, using field measurements and model results

Wave attenuation coefficients (a, m^-1) were calculated from in situ data transmitted by custom wave buoys deployed into the advancing pancake ice region of the Weddell Sea. Data cover a 12day period as the buoy array was first compressed and then dilated under the influence of a passing low-pressure system. Attenuation was found to vary over more than 2 orders of magnitude and to be far higher than that observed in broken-floe marginal ice zones. A clear linear relation between a and ice thickness was demonstrated, using ice thickness from a novel dynamic/thermodynamic model. A simple expression for a in terms of wave period and ice thickness was derived, for application in research and operational models. The variation of a was further investigated with a two-layer viscous model, and a linear relation was found between eddy viscosity in the sub-ice boundary layer and ice thickness.