PROCESSES OF HIGH-SEA ICE PRODUCTION (II) : CONVECTION WITH FRAZIL ICE PRODUCTION

元データ 国立極地研究所

概要

A semi-permanent area of open water has been frequently observed within the winter sea ice cover in the polar region. The continuous existence of the open water polynya even in the severe winter shows that two phenomena, namely vigorous frazil ice production and sweeping it away with a strong wind blowing, simultaneously occur there. The open water polynya, therefore, serves as an efficient ice factory much more than the sheet ice growth. The purpose of this study is to clarify the atmosphere-ocean interaction through the processes of high-sea ice production. Laboratory experiments were performed in which the processes of frazil ice production and the resulting convection were examined as a function of air temperature (-10∿-30℃) and wind speed (2-10m/s). A plexiglass test tank (0.4×2×0.6m in dimension) was filled with salt water (32 permils in salinity) and set in a large cold room. The wind was blowing on the water surface from one side. The convection phenomena in the test tank were observed with a schlieren optical system. The edge position of a frazil ice layer which was accumulated on the lee was advancing against the wind. The processes of frazil ice production markedly changed with wind speed. The higher the wind speed, the thicker frazil ice layer composed of fine crystals formed, whereas at the lower wind speed the thinner frazil ice sheet was advancing windward. The advancing rate of frazil ice layer increased with lowering air temperature but did not depend on wind speed noticeably. The rate of frazil ice production increased with increasing wind speed and lowering air temperature, the wind effect was much larger. At the higher wind speed, a significant amount of supercooled water tank through the wind-driven circulation as well as the instability in density, which results in the production of a large amount of frazil ice in the interior of the water tank. From these experimental observations, it was found that the following four typical regions exist during the processes of frazil ice production; (1) Fast ice sheet (most close to the wind and common ice growth); (2) Open water (region of frazil ice production); (3) Ice edge (brine exclusion mainly occurs in this region); (4) Frazil ice layer (this forms on the lee and is gradually advancing windward).

著者

Ushio Shuki Institute Of Low Temperature Science Hokkaido University
WAKATSUCHI Masaaki Institute of Low Temperature Science, Hokkaido University
Wakatsuchi Masaaki Institute Of Low Temparature Science Hokkaido University

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