Ice particle grouping under waves: Experimental investigation of the initial stage of pancake ice formation

Accurate modeling of the wave-ice interaction processes is essential in improving the predictability of the marginal ice zone environment, which is exposed to energetic wind waves. Here, the ice floe formation process under waves was investigated in laboratory experiments, using an 8 m-long, 1.5 m-wide wave flume filled with 0.6 m-deep fresh water enclosed in a freezer room. Under continuous agitation of waves, water froze as numerous small pieces like grease ice. They formed band-like structures (“groups”), which eventually consolidated to form ice floes with raised rims. Based on the observed process, a theoretical scaling of group widths $D/\lambda \propto a^{-1/2}$ (D, $\lambda$, and $a$ denote group widths, wavelength, and wave amplitude, respectively) is derived by modeling the ice as collection of small elements, where their bondings would break when the tensile stress induced by the wave orbital motion exceeds a certain value. The measured group widths generally followed the scaling, which suggests that the major dynamics of initial group formation is explained by the tensile fracture by wave orbital motion. However, the widths showed some systematic deviation from the theoretical scaling positively correlated with frequency, suggesting an influence of unaccounted processes. It is also pointed out that there is a large discrepancy in the reported coefficient of the scaling, which is relevant to ice bonding strength, between laboratory experiments and field measurements. Such analyses and other observations suggest potential influence other processes such as wave-to-ice momentum transfer in the pancake formation process.