A New Parameterisation of Frazil and Grease Ice Formation in a Climate Sea Ice Model

Abstract:

An idealised model describing frazil ice formation in the ocean mixed layer beneath a lead in the sea ice cover is developed and incorporated into the sea ice climate model CICE. The frazil ice model assumes a steady state formation of single size frazil ice crystals. The crystals are uniformly distributed under the lead over the mixed layer depth and the lead width. The basic processes affecting the frazil ice mass balance is the rate of frazil ice formation due to the heat loss from the open water to the atmosphere, advection of heat and frazil ice volume into the lead from the water under sea ice, and precipitation of frazil ice crystals to the ocean surface and formation of grease ice. The grease ice is pushed against one of the lead edges by wind and water drag keeping the lead open. The frazil ice model is incorporated into CICE and used to simulate the sea ice state in the Arctic Basin and Southern Ocean.

In contrast to the original frazil ice treatment in CICE which produces sea ice with only around 10% frazil ice fraction, the new model produces of order of 50% of frazil-derived sea ice, which corresponds better to observations. While the original model can be re-tuned in order to produce a similar average fraction of frazil ice by having a frazil collection thickness of 30 cm in the Antarctic and 5 cm in the Arctic, the new model's collection thickness is dynamically calculated, allowing for a larger collection thickness in large leads whereas the old model assumes it to be equal for wide and narrow leads. The new model keeps leads open for a longer period thus increasing the period of frazil ice formation. This is particularly important in the central Arctic where the new model's increased frazil ice production results in sea ice 0.5 m thicker than in the old model.