Frazil Ice Formation in the Polar Oceans

Areas of open ocean within the sea ice cover, known as leads and polynyas, expose ocean water directly to the cold atmosphere. In winter, these are regions of high sea ice production, and they play an important role in the mass balance of sea ice and the salt budget of the ocean. Sea ice formation is a complex process that starts with frazil ice crys- tal formation in supercooled waters, which grow and precipitate to the ocean surface to form grease ice, which eventually consolidates and turns into a layer of solid sea ice. This thesis looks at all three phases, concentrating on the rst. Frazil ice comprises millimetre- sized crystals of ice that form in supercooled, turbulent water. They initially form through a process of seeding, and then grow and multiply through secondary nucleation, which is where smaller crystals break o from larger ones to create new nucleii for further growth. The increase in volume of frazil ice will continue to occur until there is no longer super- cooling in the water. The crystals eventually precipitate to the surface and pile up to form grease ice. The presence of grease ice at the ocean surface dampens the e ects of waves and turbulence, which allows them to consolidate into a solid layer of ice. The ice then mostly grows through congelation ice forming beneath the layer of ice. A mathematical model describing the above processes is formulated and used to simulate ice growth. The model consists of conservation equations for mass and heat, with an imposed momentum budget. Simulations are realistic and numerical sensitivity experiments are used to investigate the dependence of ice growth on the ambient environment.