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 crystals. This is called frazil, and mixed with water in the unconsolidated state is known as grease ice. If wave and wind conditions are calm these crystals will consolidate at the surface, and by selective pressure begin to grow preferentially in the downward direction, forming nilas. In more turbulent conditions, the frazil will consolidate by mechanical action to form pancake ice, which has a more random structure. Another common formation mechanism, especially in the Antarctic where precipitation over sea ice is high, is from snow deposition: on thin ice the snow will weigh down the ice enough to cause flooding. Subsequent freezing will form ice with a much more granular structure. 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 brine inclusions between the crystals. With decreasing temperatures in the ice sheet, the size of the brine pockets decreases while the salt content goes up. Since ice is less dense than water, increasing pressure causes some of the brine to be ejected from both the top and bottom, producing the characteristic C-shaped salinity profile of first-year ice. Brine will also drain through vertical channels, particularly in the melt season. Thus multi-year ice will tend to have both lower salinity and lower density than first-year ice. Sea-ice density is relatively stable during winter with values close to 910 kg/m3, but may decrease up to 720 kg/m3 during warming mainly due to increase in air volume. Air volume of sea ice in can be as high as 15% in summer and 4% in late autumn. The main physical processes of sea-ice desalination are gravity drainage and flushing of surface meltwater and melt ponds. During winter, desalination is governed mostly by gravity drainage, while flushing becomes important during summer. Gravity drainage can be triggered both by atmospheric heat and bottom melt from oceanic heat. A typical salinity of first-year ice by the end of winter season is 4–6, while typical salinities of multiyear ice is 2–3. Snowmelt, surface flooding, and the presence of under-ice meltwater may affect sea-ice salinity. During the melt season, the only process of ice growth is related to the formation of false bottoms.

Vertical growth The downward growth of consolidated ice under the 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 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 inertia, we can determine latent heat flux Q* by solving the following equation:

Q ∗ = k i T s i − T w h i = k s T s − T s i h s {\displaystyle Q^{*}=k_{i}{\frac {T_{si}-T_{w}}{h_{i}}}=k_{s}{\frac {T_{s}-T_{si}}{h_{s}}}}

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 freezing (Stefan problem). We can approximate the ice and snow thermal conductivities ki and ks, as an average over the layers. The surface heat budget defines the snow surface temperature Ts and includes four atmospheric heat fluxes:

Q ∗ = Q E [ e ( T s ) ] + Q H ( T s ) + Q L W ( T s 4 ) + Q S W {\displaystyle Q^{*}=Q_{E}\left[e(T_{s})\right]+Q_{H}(T_{s})+Q_{LW}(T_{s}^{4})+Q_{SW}}

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