A LABORATORY STUDY ON SUPERCOOLING AND FRAZIL ICE PRODUCTION PROCESSES IN WINTER COASTAL POLYNYAS

The rates of frazil ice production and salt flux in wind-generated ope n water were estimated on the basis of results from laboratory experim ents 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. Th e high rates of ice production are attributed to the vigorous producti on of underwater frazil crystals. Salt fluxes due to frazil production were also much greater than those of sheet ice growing vertically und er 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 th an 25 parts per thousand, 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 th e supercooled water and the resulting production of numerous frazil cr ystals under the water lead to the maintenance of open water for a lon g period. Thus large heat fluxes occur through the open water that is continuously exposed to cold air, producing a large amount of the unde rwater ice at high rates. This mechanism for ice production in open wa ters is driven by a strong, cold wind and is enhanced by increasing wa ter salinity.