Sea-Ice Nomenclature Overview page
This term is used to describe the ocean surface when it is free of an ice cover, as is the case in the photo on the left (for the most part).
A term used to describe newly formed, thin ice ( < 10 cm thick). New ice types include frazil, nilas, slush, pancake, and grease ice.
The word frazil means fine spicules or blobs, which gracefully describes the morphology of the crystal. Sea ice growth always begins with frazil ice production, the only true dendritic ice growth phase. Active convection aids sensible heat transport at the ocean surface, thereby super cooling the water and allowing dendritic growth of ice crystals. Vertical density instability and convection are a result of the fact that standard sea water has a freezing point greater than its temperature of maximum density. Frazil ice crystals nucleate on impurities in the water or on snow grains that have fallen onto the ocean surface. Nucleated spheres morph to discs, (which can be seen at the center of frazil crystals grown in extremely calm laboratory conditions), then true stellar crystals. If interlocking stellar crystals grow on the ocean and cover the surface with a sheet of interlocking crystals, it is called sheet ice. However, natural turbulence normally mixes the frazil crystals, prohibiting the development of stellar crystals (Weeks and Ackley, 1982). The depth of wind mixing is governed by surface waves, with larger perturbations inducing deeper convection in the upper ocean. In fact, under stormy conditions frazil ice may be mixed to depth of a meter or more. Wind speed and fetch ultimately control what kind of frazil ice development will follow its initiation in open water.
To see a picture of frazil ice, click
If the sea is calm, Nilas will form. Nilas is composed of both frazil and congelation ice crystals, the frazil ice crystals being above the congelation ice in vertical cross section. Upon forming a continuous frazil cover, congelation ice growth begins and composes the rest of the thickness of the ice. Rather than being a microstructural term, the term nilas classifies ice by thickness. Nilas ice is a thin ( <10 cm) elastic rind of ice that will easily bend and flex in a wave field. Nilas is subdivided into two categories based on both thickness and optical properties. Dark Nilas is thin (0-5 cm), therefore having similar albedo (~0.07)as open water. Upon thickening to >5 cm nilas is termed light nilas and has a lighter color/ higher albedo (~0.14).
Thicker, soupy accumulations of frazil ice which are often herded by wind action are termed grease ice. Open leads are often home to grease ice. Once again, optical properties are used to distinguish grease ice; because of its extremely low albedo it has a matte appearance on the sea surface. Grease ice thickness is proportional to wind speed and fetch, while being inversely proportional to air temperature, such that maximum grease ice production and thickness occurs with minimum surface air temp, high winds and long fetch distances (Bauer and Martin, 1983) .
Thick accumulations of frazil ice, usually indicative of rough surface conditions which induce turbulent mixing in the upper water column.
Strong wave action that herds slush or grease ice often forms pancake ice. Herded crystals are bonded by regelation forces in the process of colliding with other crystals. Eventually round pancake formations with raised rims (from bonking each other) result. Ice strength is greater than in the case of frazil or slush, but the cakes are still fairly unconsolidated. What little structural integrity the cakes possess is a result of topography above sea level, which allows brine drainage from the ice, thus stronger bonds between grains. Pancake shape is mainly governed by location. If the cakes are formed in open water, they will be more or less round in shape. If however, they form along a shoreline, they will be elongate in form with the long axis paralleling the shore.
A cyclic process termed the "pancake cycle" has been described by Lange (1990). After pancakes are formed, continued wind may raft pancakes on top of each other. Rafting increases the percentage of open water available for additional frazil ice production. With more frazil ice present, further pancake production occurs, followed by rafting and continued frazil ice production. The cycle will continue until pancake rafts have reached thickness' ~0.5 meters, depending on wind velocity.
Pancake ice may alternatively be formed by
wave/ice interaction. Incoming ocean swell may subject an ice
cover to forces large enough to fracture an ice sheet and form
pancake ice. Both amplitude and wavelength contribute to the
stresses felt by the the ice floe. Wavelength increases with
wave amplitude, but stress on the ice does not necessarily increase
with increasing amplitude, since long wavelength waves will not bend
the ice as radically as short wavelength waves. Stress is
therefore a function of the ratio of amplitude to wavelength,
although the exact manner in which stress is transmitted to the ice
in unclear. A medium amplitude, medium wavelength wave is most
likely to break a sheet of grey ice into pancake ice.
Additionally, if the wave is composed of multiple frequency waves
superimposed on each other, the chances for breakup is of an ice floe
is greatly enhanced.
Young ice has been defined as ice that is thicker than nilas, yet thinner than mature first year ice. Young ice is independent of type, solely dependent on thickness. To be considered young ice, the sea ice thickness must be between 10 and 30 cm. Young ice may be divided into two categories based on optical properties.
Grey ice is the thinner of the two sub-classes of young ice and has a lower albedo than the thicker grey-white ice. To be considered grey ice the ice must be between 10-15 cm thick. Grey ice is much less elastic than thinner nilas ice and more susceptible to breakup into pancakes by incoming swell.
The thicker of the two young ice types must fall between 15-30 cm in thickness. The albedo of grey-white ice is higher than of grey ice.
Usually snow hinders ice growth due to its lower thermal conductivity. However, large accumulations of snow may depress an ice floe causing the upper surface to be flooded. Flooding creates snow ice by freezing sea water and snow together. This process is thought to contribute substantially to the thickness of the Antarctic sea ice cover (Ackley et. al, 1990).
Ice growth occurs in three time dependent phases that can be identified by ice microstructural properties, although these characteristics are not accounted for in the currently used nomenclature system. In order of development, the phases are frazil ice, transition ice, and congelation ice.
As ice growth changes from frazil to congelation growth, it is termed transition ice (Gow et al, 1987). Upon formation of continuous ice surface, direct freezing of water on the underside of the ice skim begins and ice growth is no longer governed by super cooling of the water column. Since the surface is completely frozen, heat must be transferred through the overlying ice, thus reducing the ice growth velocity. A degree of freedom is lost for growth direction, since the option to grow crystals horizontally has been eliminated. As vertical growth continues, crystals with their c-axis oriented horizontally are favored, eliminating crystals with other orientations. C-axis alignment serves to identify the lower boundary of transition ice. The total thickness of the transition ice layer is ~5-10 cm.
Beneath the transition ice layer one finds congelation or columnar ice. Congelation ice accretes to the underside of the ice floe by direct freezing. Brine is trapped between crystals giving the ice a morphology consisting of ice plates and brine lamellae. Ice crystals grow elongate in form, with the long axis of the crystal parallel to heat flow (Eicken and Lange, 1991). The growth rate of congelation ice is much slower than of frazil ice since heat must be transferred trough all overlying ice, but the volume of congelation ice usually dominates a given ice floe. In the Arctic the congelation ice layer may grow to be >2 meters thick in areas of undeformed ice.
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