Modelling Antarctic Fast-Ice Growth

Abstract

ABSTRACT. An eXlstmg thermodynamic ice-growth model (Semtner, 1976) has been tested for its ability to predict the growth of fast ice in McMurdo Sound, Antarctica. Significant discrepancies between observed and predicted ice thicknesses were found to occur, primarily due to the presence of sub-ice platelets and the formation of a snow-ice layer. Although these ice-growth processes are not well enough understood to permit rigorous physical modelling, it is shown that fairly simple modifications to the model greatly improve the accuracy of the thickness predictions, ano serve to highlight the importance of these processes in the Antarctic fast-ice environment.Surface flooding and snow-ice formation are assumed to occur immediately upon the establishment of a positive hydrostatic water level, and a surface temperature in excess of a critical value, above which interconnecting channels in the ice matrix permit the flow of water to the surface. The presence of the sub-ice platelet layer is assumed to increase columnar ice growth at a rate proportional to the volume fraction of ice in the platelet layer, a simple technique but one that permits estimates of platelet-enhanced growth without detailed knowledge of oceanographic conditions. The resulting model predictions are in close agreement with measurements of fast-ice growth and decay in McMurdo Sound; however, data suitable for testing the model over a complete range of conditions and over multi-year cycles are not available at the present time.

Structural characteristics of congelation and platelet ice and their role in the development of antarctic land-fast sea ice

“…and Wadhams, 1989). In the austral winters of 1984, Barry (1988 and Crocker and Wadhams ( 1989) respectively reported that, prior to basal melting, the locations of the thickest platelet-ice accumulations corresponded to the areas of maximum surface supercooling reported by Lewis and Perkin (1985). The pattern of occurrence and quantity of platelet ice in our…”

“…There, annual ice growth is dominated by the accretion of congelation ice formed as a result of the conduction of heat from the ocean to the atmosphere through the developing ice layer. The cellular sub-structure of ice plates and brine layers, crystal morphology and size variations, and the influence of water currents on the development of preferred crystal caxis orientations in congelation ice have been described by Weeks and Hamilton (1962), Cherepanov ( 1973), Cow and Weeks ( 1977 ), Cow (1978, 1980) Platelet ice has only been observed in Antarctica, where it appears to be commonest in fast ice and coastal pack ice (Paige, 1966;Buynitskiy, 1967;Cherepanov and Kozlovskiy, 1972, 1973a, b, 1977Kozlovskiy and Cherepanov, 1977;Gow and others, 1982;Lange, 1988;Lange and others, 1989;Crocker and Wadhams, 1989;Eicken and Lange, 1989;Kipfstuhl, 1991;Jeffries and Weeks, 1991a, b, in press). This ice type commonly occurs as aggregates of platelets, which either underlie or have become incorporated into congelation ice.…”

“…In McMurdo Sound it is believed that the platelets grow from adiabatically supercooled low-density water that flows northward into the Sound from below McMurdo Ice Shelf (Lewis and Perkin, 1985;Crocker <J Fig. 4 .…”

“…Using the McMurdo Sound fast ice as an example, Crocker and Wadhams (1989) successfully applied a thermodynamic model to the sequence of Antarctic fastice development by congelation ice growth and plateletice solidification. Analytical studies of the McMurdo Sound fast ice have included investigations of congelation ice-crystal dimensions and plate-width spacings (Paige, 1966), and congelation ice crystal c-axis alignments (Cow and others, 1982).…”

Winter snow cover variability on East Antarctic sea ice

“…Higher ice temperatures resulting from depth hoar formation would increase the ice permeability if they were above •-8øC [Crocker and Wadhams, 1989], creating interconnecting channels that may enhance the influx of seawater onto the ice surface if the freeboard is negative. Figure 14a clearly illustrates that T 1 appears to have exceeded this threshold from at least August 4 until about August 14 in the 1995 East Antarctic experiment.…”