On the Rate of Frazil Ice Formation in Polar Regions in the Presence of Turbulence

Abstract

The purpose of this paper is to present the results of a series of laboratory experiments aimed at understanding the frazil ice growth in polar regions during the summer whence a freshwater layer at temperature 0°C spreads between an old ice sheet and underlying salty seawater that is at its freezing point. The aim of the experiments was to study the influence of external turbulence on the rate of new frazil ice formation. The experiments were conducted in a large walk-in freezer at a temperature near 0°C. To produce controlled turbulence, two oscillating grids were installed in a tank filled with two layers of water: fresh water at temperature 0°C in the upper layer and salty (35 psu) cold water at temperature ?1.9°C in the bottom layer. During the experiments, the bottom layer was cooled from below, using Peltie elements, and its temperature was near the freezing point. The turbulence induced in both layers facilitates the transport of heat across the density interface between layers, and as the time progresses the lower boundary of the upper layer becomes overcooled, and small crystals of frazil ice intensively form in this overcooled zone. These buoyant crystals rise to the surface, and with time a sheet of frazil ice is formed at the surface of the fresh water. It is found that the rate of frazil ice formation in the presence of turbulence is a function of the interfacial Richardson number and is much higher (30?100 times) than the case where there is no turbulence. A theoretical explanation is given to explain thew observed high ice formation rates.