Sea ice growth processes
Sea ice is a complex composite composed primarily of pure ice in various states of crystallization, but including air bubbles and pockets of brine. Understanding its growth processes is important for climate modellers and remote sensing specialists, since the composition and microstructural properties of the ice affect how it reflects or absorbs sunlight.
Sea ice growth models for predicting the ice distribution and extent are also valuable for shipping. An ice growth model can be combined with remote sensing measurements in an assimilation model as a means of generating more accurate ice charts.
Overview
Several formation mechanisms of sea ice have been identified. At its earliest stages, sea ice consists of elongated, randomly oriented
One of the more interesting processes to occur within consolidated ice packs is changes in the saline content. As the ice freezes, most of the salt content gets rejected and forms highly saline
The main physical processes of sea-ice desalination are gravity drainage and flushing of surface
Vertical growth
The downward growth of consolidated ice under assumption of zero heat flux from the ocean is determined by the rate of conductive heat flux, Q*, at the ice-water interface. The ocean heat fluxes substantially vary both spatially and temporally, and strongly contribute to the summer sea ice melt and the absence of sea ice in some parts of the Arctic Ocean. If we also assume a linear temperature profile within ice and no effect from ice thermal intertia, we can determine latent heat flux Q* by solving the following equation:
where Tsi is the snow-ice interface temperature, Ts is the air-snow interface temperature, hi and hs are the ice and snow thicknesses. The water temperature, Tw, is assumed to be at or near
which are latent, sensible, longwave and shortwave radiation fluxes, respectively. For a description of the approximate
While Cox and Weeks assume thermal equilibrium,
The rate of ice growth can be calculated from heat flux by the following equation:
where L is the latent heat of fusion for water and is the density of ice (for pure ice). For sea ice, L is the effective latent heat of sea ice and is the density of sea ice. These two parameters depend on sea-ice salinity, temperature, and volumetric gas fraction. The growth rate of sea ice in turn determines the saline content of the newly frozen ice. Empirical equations for determining the initial brine entrapment in sea ice have been derived by Cox and Weeks[12] and Nakawo and Sinha[14] and take the form:
where S is ice salinity, S0 is the salinity of the parent water and f is an empirical function of ice growth rate, e.g.:
where g is in cm/s.[14]
Salt content
Brine entrapped in sea ice will always be at or near freezing, since any departure will either cause some of the water in the brine to freeze, or melt some of the surrounding ice. Thus, brine salinity is variable and can be determined based strictly on temperature—see
The relative brine volume, Vb, is defined as the fraction of brine relative to the total volume. It too is highly variable, however its value is more difficult to determine since changes in temperature may cause some of the brine to be ejected or move within the layers, particularly in new ice. Writing equations relating the salt content of the brine, the total salt content, the brine volume, the density of the brine and the density of the ice and solving for brine volume produces the following relation:
where S is sea ice salinity, Sb is brine salinity, is the density of the ice and is brine density. Compare with this empirical formula from Frankenstein and Garner:[15]
where T is ice temperature in degrees
In new ice, the amount of brine ejected as the ice cools can be determined by assuming that the total volume stays constant and subtracting the volume increase from the brine volume. Note that this is only applicable to newly formed ice: any warming will tend to generate air pockets as the brine volume will increase more slowly than the ice volume decreases, again due to the density difference. Cox and Weeks provide the following formula determining the ratio of total ice salinity between temperatures, T1 and T2 where T2 < T1:[12]
where c=0.8 kg m−3 is a constant. As the ice goes through constant warming and cooling cycles it becomes progressively more
The figure above shows a scatter plot of salinity versus ice thickness for ice cores taken from the Weddell Sea, Antarctica, with an exponential fit of the form, , overlaid, where h is ice thickness and a and b are constants.
Horizontal motion
The horizontal motion of sea ice is quite difficult to model because ice is a non-Newtonian fluid. Sea ice will deform primarily at
See also
References
- ^ G. Maykut; T. Grenfell & W. Weeks (1992). "On estimating spatial and temporal variations in the properties of ice in the polar oceans". Journal of Marine Systems. 3 (1–2): 41–72. .
- ^ a b c W. B. Tucker; D. K. Prerovich; A. J. Gow; W. F. Weeks; M. R. Drinkwater (eds.). Microwave Remote Sensing of Sea Ice. American Geophysical Union.
- ISSN 0148-0227.
- ^ T. Maksym & T. Markus (2008). "Antarctic sea ice thickness and snow-to-ice conversion from atmospheric reanalysis and passive microwave snow depth". Journal of Geophysical Research. 113 (C02S12). .
- ^ S. Tang; D. Qin; J. Ren; J. Kang & Z. Li (2007). "Structure, salinity and isotopic composition of multi-year landfast sea ice in Nella Fjord, Antarctica". Cold Regions Science and Technology. 49 (2): 170–177. .
- ^ a b Hajo Eicken (1992). "Salinity Profiles of Antarctic Sea ice: Field Data and Model Results". Journal of Geophysical Research. 97 (C10): 15545–15557. .
- .
- S2CID 225267189.
- hdl:10037/30456.
- ^ a b c G. Cox & W. Weeks (1988). "Numerical simulations of the profile properties of undeformed first-year sea ice during the growth season". Journal of Geophysical Research. 93 (C10): 12449–12460. .
- ^ a b
G. Heygster, S. Hendricks, L. Kaleschke, N. Maass, P. Mills, D. Stammer, R. T. Tonboe and C. Haas (2009). L-Band Radiometry for Sea-Ice Applications (Technical report). Institute of Environmental Physics, University of Bremen. ESA/ESTEC Contract N. 21130/08/NL/EL.
{{cite tech report}}
: CS1 maint: multiple names: authors list (link) - ^ a b M. Nakawo & N. K. Sinha (1981). "Growth rate and salinity profile of first-year sea ice in the high Arctic". Journal of Glaciology. 27 (96): 315–330. .
- ^ S2CID 129064888.