to Sea-Ice Nomenclature Overview page
Microstructure vs. Formation Mechanisms
The following classification scheme classifies ice according to its formation or origin, and links its formation to the resulting crystal texture.
Frazil generation is the formation of ice crystals in supercooled waters. As described in Section A. Sea Ice Microstructure there are 3 mechanisms of granular frazil generation. These include wave and wind-induced turbulence, contact between water masses of different salinity and thermohaline convection initiated by surface cooling and freezing. Blade or platelet crystals may also be considered frazil ice. They are formed close to the front of ice shelves as water masses rise from the bottom of ice shelves and undergo adiabatic supercooling.
Congelation ice grows as heat is conducted away from the ice-water interface, through the ice cover. Crystals with horizontal c-axes are favoured due to geometrical selection.
Snow ice forms as a result of seawater infiltration into the snow / ice interface and grows as heat is conducted away from the interface through the snow cover.
Wind- and wave-induced turbulence
In open water, winds and waves induce
turbulence and mixing. Heat removal provides the amount of
supercooling required for crystal nucleation. Mixing may
introduce more nuclei and may lower the amount of necessary
supercooling. Once there are nuclei, minute spheres of pure ice
begin to form. Brine is rejected to in between grains. As
crystals rise from the upper meters of the water column, they
accummulate at the sea surface in a thin skim and eventualy congeal
into a solid ice cover.
Contact
between two water masses of significantly different
salinity
In the summer, drainage holes in multi-year sea ice allow fresh meltwater to drain from the upper ice surface out to the ice-water interface. As a result, a stable layer of warmer, fresh meltwater (0 deg C) can be found between the lower surface of the ice and the underlying colder, dense seawater (-1.8 deg C). Double diffusion occurs, whereby heat diffuses out of the fresh meltwater at a faster rate than salt diffusing into the fresh meltwater. As the fresh water cools, ice crystals form at the interface between the fresh and the seawater.
Thermohaline
convection initiated by surface cooling and freezing
Cooling of ice surface causes cold,
dense brine plumes to descend into the water column below the
ice-water interface. Double diffusion occurs, whereby heat
diffuses from the surrounding water into the brine plume at a higher
rate than salt diffusing out of the brine plume. As heat is
diffused away, the surrounding water is cooled to its freezing
point. Frazil ice crystals nucleate and rise to the ice-water
interface.
Adiabatic supercooling of rising water masses
At depth, water has a depressed freezing point due to the pressure. Once this water is brought to the surface, the pressure is released, the freezing point rises and ice forms directly from the water. This type of process is frequently found near the front of ice shelves. Water is forced to great depths underneath the thick ice sheets. The pressure is released as the flow follows the bottom of the ice sheet and surfaces.
Link to Shad's page
To: Section A. Sea Ice
Microstructure
To: Section C. Parameterizing Uniaxial
Compressive Strength
To: Section D. References