A comparison of theory and laboratory measurements of wave propagation and attenuation in grease ice

Abstract

In an experimental study using a wave tank in a laboratory cold room we determine the dispersion relation and amplitude attenuation for surface waves propagating through different thicknesses of grease ice. We compare our results to two ice rheology models: the mass-loading model, which predicts a wavelength decrease relative to open water, and an infinite depth viscous fluid model, which predicts an increasing wavelength as the wave Reynolds number decreases. For a thick grease ice layer in which the waves are strongly damped we observe that the wavelength increases by up to 30% over its open water value in the frequency range of 1.0 Hz < f < 1.6 Hz. This trend agrees with the viscous model, and the agreement improves as the ice thickness increases and at higher wave frequencies where conditions approach those of the infinite depth approximation. The Reynolds number decreases approximately exponentially with frequency and is in the range 1 < R < 10 for our experimental conditions. From the model the inferred viscosity of grease ice is at least 4 orders of magnitude larger than the open water value and increases with frequency, suggesting that grease ice is non-Newtonian. For the observed parameter values our analysis shows that the mass-loading model of grease ice is inapplicable while a one-layer viscous model provides a better match to laboratory observations. Copyright 1997 by the American Geophysical Union.