Author(s): Takenobu Toyota; Haruka Koda
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Abstract: The crystal alignments of sea ice are categorized mainly into two types: columnar ice and granular ice. Whereas columnar ice is produced through the bottom freezing under calm conditions and its growth rate can be estimated with good accuracy, the formation processes of granular ice produced under turbulent conditions have not been fully understood yet because of its complexity and the lack of observation. Given that granular ice occupies significant fraction of total ice thickness especially in the seasonal ice zone, the understanding of granular ice formation is quite important. One of the typical processes is as follows: frazil ice produced in leads under cold windy conditions is herded to the down-wind edge of the lead and about 10 cm thick grease ice forms there. The waves gradually attenuate in grease ice and beyond the dead zone named by Martin and Kauffman (1981), grease ice loses fluidity and starts to be solidified. In this study, we focus on the production rates of frazil ice in leads and the solidification process at the dead zone. For this purpose, we conducted a laboratory experiment using two thermally insulated tanks (0.3 x 0.3 x 0.65 m) filled with natural seawater: one was used for producing frazil ice under turbulent conditions generated by a stirrer and the other for growing sea ice under calm conditions for comparison. After ice grew up to about 5 cm at various room temperatures, we stopped the stirrer and observed the solidification process. As a result, it was found that 1) the production rate of frazil ice was about 1.3 times faster than that of columnar ice, 2) the surface 4-5 cm thick grease ice was solidified rapidly within two hours after the stirrer was stopped, 3) An addition of a small amount of snow particles to water can enhance the production rates efficiently. Though still a preliminary step, the results implicate some important properties of granular ice formation.
Year: 2016