3 Role of SAR in Surface Energy Flux Measurements Over Sea Ice
39
ent conditions return to winter norms these changes can be retained within the snow
crystal structure retaining a thermal history (Barber et al. 1992).
During melt onset free water is continuously present within the snow pack and the
ice surface becomes damp at the snow-ice interface. Polycrystalline aggregates form
locally at the snow base and superimposed ice layers sometimes form on first-year ice
(Holt and Digby 1985). During this stage snow can be characterized as being either in
the pendular or the funicular regime. The pendular regime occurs when air occupies
continuous paths throughout the pore space and the funicular regime when liquid
occupies these paths. This transition occurs at approximately 7% water by volume. In
dry snow, grain growth occurs slowly, but with the addition of small amounts of water
(2-5% water by volume) grain growth increases markedly (Colbeck 1982).
The melt -freeze cycles create both surface and sub-surface layers (Colbeck 1982). During the early melt season, ice salinities at the surface decrease rapidly. The spatial distribution of these surface salinities is heterogeneous, probably due to the non-uniform
distribution of brine drainage channels. The average surface temperature is near freezing. The season ends when the majority of the snow cover is saturated.
During advanced melt the snow cover is first saturated throughout its volume and then
melts rapidly. During this period liquid is drained from the saturated snow. Tightly
packed grain clusters occur after the liquid has drained. Colbeck (1982) reports that grain
clusters under these conditions are typically two to four crystal arrangements. These
multi-crystal forms can also form aggregates of several hundred crystals, often called
polycrystalline aggregates. With drainage of the snow pack a gradient of water volume
is set up within the snow with minimum values at the surface and saturation at the base.
The ice surface undergoes melting, often in a cyclical diurnal fashion. The development of drainage networks begins primarily through seal holes, cracks and leads. The
salinity of first-year ice decreases. The ice cover decomposes completely in some areas.
Transient freezing periods can occur, but the average trend is for melting conditions.
Melt waters change the halocline of the local ocean and promote erosion at the interface between the ocean and ice.
At freeze-up new ice formation begins with the appearance of frazil, slush or columnar ice crystals developing within the water column. Frazil ice typically occurs under
conditions when the ocean surface is roughened bywind. The frazil platelets form within the top few centimeters of the surface and float to the top. These particles aggregate
to form shuga or grease ice. Further cooling of this slushy layer results in consolidation
of the surface into a dark thin ice type called nilas. This primary ice layer (frozen frazil)
is characterized by platelet-shaped crystals in the millimetre size range. Columnar
crystals are typical of calm growth conditions and slush ice is a mixture of snow, blown
in from surrounding surfaces, and sea water.
As the number of freezing days increases the ice starts to consolidate into the water
column. This is termed secondary ice growth and results in ice crystals which have
columnar-shaped grains.As the surface continues to cool, brine pockets within the frazil
layer are extruded to the surface, resulting in frost flowers (Martin 1979; Hollinger et
al.1984; Drinkwater and Crocker 1988). The salinities of this layer can be considerably
higher (45-100 ppt; Drinkwater and Crocker 1988; Martin 1979) than the bulk salinities of the ice. These frost flowers consist of delicate crystalline shapes which typically
grow to heights of 2-4 cm above the ice surface (Martin 1979). Brine is also expelled
either into the ocean beneath or into brine pockets within the young ice mass as it grows.
39
ent conditions return to winter norms these changes can be retained within the snow
crystal structure retaining a thermal history (Barber et al. 1992).
During melt onset free water is continuously present within the snow pack and the
ice surface becomes damp at the snow-ice interface. Polycrystalline aggregates form
locally at the snow base and superimposed ice layers sometimes form on first-year ice
(Holt and Digby 1985). During this stage snow can be characterized as being either in
the pendular or the funicular regime. The pendular regime occurs when air occupies
continuous paths throughout the pore space and the funicular regime when liquid
occupies these paths. This transition occurs at approximately 7% water by volume. In
dry snow, grain growth occurs slowly, but with the addition of small amounts of water
(2-5% water by volume) grain growth increases markedly (Colbeck 1982).
The melt -freeze cycles create both surface and sub-surface layers (Colbeck 1982). During the early melt season, ice salinities at the surface decrease rapidly. The spatial distribution of these surface salinities is heterogeneous, probably due to the non-uniform
distribution of brine drainage channels. The average surface temperature is near freezing. The season ends when the majority of the snow cover is saturated.
During advanced melt the snow cover is first saturated throughout its volume and then
melts rapidly. During this period liquid is drained from the saturated snow. Tightly
packed grain clusters occur after the liquid has drained. Colbeck (1982) reports that grain
clusters under these conditions are typically two to four crystal arrangements. These
multi-crystal forms can also form aggregates of several hundred crystals, often called
polycrystalline aggregates. With drainage of the snow pack a gradient of water volume
is set up within the snow with minimum values at the surface and saturation at the base.
The ice surface undergoes melting, often in a cyclical diurnal fashion. The development of drainage networks begins primarily through seal holes, cracks and leads. The
salinity of first-year ice decreases. The ice cover decomposes completely in some areas.
Transient freezing periods can occur, but the average trend is for melting conditions.
Melt waters change the halocline of the local ocean and promote erosion at the interface between the ocean and ice.
At freeze-up new ice formation begins with the appearance of frazil, slush or columnar ice crystals developing within the water column. Frazil ice typically occurs under
conditions when the ocean surface is roughened bywind. The frazil platelets form within the top few centimeters of the surface and float to the top. These particles aggregate
to form shuga or grease ice. Further cooling of this slushy layer results in consolidation
of the surface into a dark thin ice type called nilas. This primary ice layer (frozen frazil)
is characterized by platelet-shaped crystals in the millimetre size range. Columnar
crystals are typical of calm growth conditions and slush ice is a mixture of snow, blown
in from surrounding surfaces, and sea water.
As the number of freezing days increases the ice starts to consolidate into the water
column. This is termed secondary ice growth and results in ice crystals which have
columnar-shaped grains.As the surface continues to cool, brine pockets within the frazil
layer are extruded to the surface, resulting in frost flowers (Martin 1979; Hollinger et
al.1984; Drinkwater and Crocker 1988). The salinities of this layer can be considerably
higher (45-100 ppt; Drinkwater and Crocker 1988; Martin 1979) than the bulk salinities of the ice. These frost flowers consist of delicate crystalline shapes which typically
grow to heights of 2-4 cm above the ice surface (Martin 1979). Brine is also expelled
either into the ocean beneath or into brine pockets within the young ice mass as it grows.