Laboratory study of frazil ice accumulation under wave conditions

Abstract

Ice growth in turbulent seawater is often accompanied by the accumulation of frazil ice crystals at its surface, forming a grease ice layer. The thickness and volume fraction of this ice layer play an important role in shaping the gradual transition from a loose to a solid ice cover, however, observations are very sparse. Here we analyse an extensive set of observations of frazil ice, grown in two parallel tanks with controlled wave conditions and thermal forcing, focusing on the first one to two days of grease ice accumulation. The following unresolved issues are addressed: (i) at which volume fraction the frazil crystals' rising process starts and how densely they accumulate at the surface, (ii) how the grease ice solid fraction and salinity evolve with time until solid ice starts to form and (iii) how do these conditions affect, and are affected by, waves and heat loss from the ice. We obtained estimates of the minimum initial grease ice solid fraction (0.03–0.05) and the maximum solid fraction to which it accumulates before freezing into pancakes (0.23–0.31). The equivalent thickness of solid ice that needs to be accumulated until grease ice packs close to maximum (95% of the compaction accomplished), was estimated as 0.4 to 1.2 cm. Comparison of grease ice thickness and wave observations indicates that a grease ice layer first begins to affect the wave field significantly when its thickness exceeds the initial wave amplitude. These results are relevant for modelling frazil ice accumulation and freeze-up of leads, polynyas and along the seasonal ice zone.