Cracks in the sea-ice cover, known as leads, can form due to mechanical forcing and expose ocean water directly to the cold atmosphere. The formation of sea ice in these leads plays an important role in the mass balance of the sea ice and the salt budget of the ocean. The first stage of sea-ice formation is the creation of frazil ice, which are small millimetre-sized crystals that are formed as a result of supercooling in turbulent waters. Current Global Climate Models (GCMs) do not represent this stage accurately and the aim of this PhD is to create a model which will do so and which can be incorporated into a GCM. The processes involved in the formation of frazil ice are of both a thermal and dynamical nature. The thermal processes are that of growth/melting of the individual crystals due to supercooling or superheating (secondary nucleation is also included) and the heat energy balance at the surface of the ocean. The dynamical processes are vertical and horizontal turbulent mixing and precipitation of the crystals out of the water column. We will present simulations of the evolution of frazil-ice concentration, ocean temperature, and precipitated ice crystals in the Arctic lead.