The early stages of sea ice formation often involve frazil ice, formed as a crystal suspension in supercooled turbulent water. Frazil ice formation is known to play a significant role in the development of Antarctic Bottom Water, and is thus a key component of the global ocean circulation. However, despite its significance, frazil ice formation is still a poorly understood sea-ice process. 
Understanding how frazil ice impacts the water column requires a model for the number, size and velocities of frazil ice crystals. All of these quantities change as the crystals collide and potentially fracture or flocculate. However, the conditions required for crystals to fracture or flocculate remain uncertain.
We develop a model for frazil ice collisions which includes the effect of hydrodynamic interactions between the crystals. This model allows us to determine the collision efficiency of crystal interactions, a quantity previously not included in frazil ice models. We find that the collision efficiency strongly depends on the level of turbulence and in more quiescent flow, hydrodynamic interactions significantly reduce the number of crystal collisions.
Our model also suggests that nucleation is more likely to occur by small dendritic structures breaking off from the crystal surface, rather than from crystals snapping. This has implications for the number and size distribution of the resulting crystals.
We discuss the utility of our model for parameterising frazil ice population models and discuss the implications for understanding the role of frazil ice in the development of Antarctic Bottom Water.
