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Initial sea-ice growth in open water: properties of grease ice and nilas

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Author(s): A. K. Naumann | D. Notz | L. Håvik | A. Sirevaag

Journal: The Cryosphere Discussions
ISSN 1994-0432

Volume: 6;
Issue: 1;
Start page: 125;
Date: 2012;
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ABSTRACT
To investigate initial sea-ice growth in open water, we carried out an ice-tank study with three different setups: grease ice grew in experiments with waves and in experiments with a current and wind, while nilas formed in a quiescent experimental setup. In this paper we focus on the differences in bulk salinity, solid fraction and thickness between these two ice types. We find that the bulk salinity of the grease-ice layer remains almost constant until the ice starts to consolidate. In contrast, the bulk salinity of nilas is in the first hours of ice formation well described by a linear decrease of 2.1 g kg−1 h−1 independent of air temperature. Such rapid decrease in bulk salinity can be understood qualitatively in the light of a Rayleigh number, the maximum of which is reached while the nilas is still less than 1 cm thick. Comparing three different methods to measure solid fraction in grease ice based on (a) salt conservation, (b) mass conservation and (c) energy conservation, we find that the method based on salt conservation does not give reliable results if the salinity of the interstitial water is approximated as being equal to the salinity of the upper water layer. Instead the increase in salinity of the interstitial water during grease-ice formation must be taken into account. We find that the solid fraction of grease ice is relatively constant with values of 0.25, whereas it increases to values as high as 0.5 as soon as the grease ice consolidates at its surface. In contrast, the solid fraction of nilas increases continuously in the first hours of ice formation. The ice thickness is found to be twice as large in the first 24 h of ice formation in the setup with a current and wind compared to the other two setups, since the wind keeps parts of the water surface ice free. The development of the ice thickness can be reproduced well with simple, one dimensional models given only the air temperature or the ice surface temperature.
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