Grease-ice thickness parameterization

Abstract

Grease ice is a mixture of sea water and frazil ice crystals forming in Arctic and Antarctic waters. The initial grease-ice cover, or the grease ice forming during winter in leads and polynyas, may therefore have mixed properties of water and ice. Most sea-ice models use a lower thickness limit on the solid sea ice, representing a transition from grease ice to solid ice. Before grease ice solidifies it is often packed into a layer by the local wind. Existing field measurements of grease ice are compared and used to evaluate a new thickness parameterization including the drag from the wind as well as the ocean current. The measurements support a scaling of the wind drag and the back pressure from the greaseice layer using a nonlinear relation. The relation is consistent with an increasing grease-ice thickness towards a solid boundary. Grease-ice data from Storfjorden, Svalbard, confirm that tidal currents are strong enough to add significant drag force on the grease ice. A typical wind speed of only 10 ms-1results in a 0.3 m thick layer of grease ice. Tidal currents of 0.5 m s-1 will pack the grease ice further towards a stagnant boundary to a mean thickness of 0.8 m.