A laboratory study on supercooling and frazil ice production processes in winter coastal polynyas

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The rates of frazil ice production and salt flux in wind‐generated open water were estimated on the basis of results from laboratory experiments that model coastal polynyas in a severe winter. The measurements were made under various conditions of wind speed, air temperature, and water salinity. The rate of ice production increased with increasing wind speed and water salinity, and with decreasing air temperature. The high rates of ice production are attributed to the vigorous production of underwater frazil crystals. Salt fluxes due to frazil production were also much greater than those of sheet ice growing vertically under calm atmospheric conditions. The important factor that governs such a process of high ice production lies in supercooling and underwater ice production. In the case of common seawater of salinities larger than 25‰, supercooled water that formed on the surface sinks into the underlying seawater at its freezing point by a density instability as well as by a wind‐forced convection. The sinking of the supercooled water and the resulting production of numerous frazil crystals under the water lead to the maintenance of open water for a long period. Thus large heat fluxes occur through the open water that is continuously exposed to cold air, producing a large amount of the underwater ice at high rates. This mechanism for ice production in open waters is driven by a strong, cold wind and is enhanced by increasing water salinity.