3 Role of SAR in Surface Energy Flux Measurements Over Sea Ice
37
Radiative and energy balances of sea ice are intimately linked with the spatial distribution of the snow/sea ice/ocean matrix. The alternating presence and absence of
sea ice has significant impacts over various temporal and spatial scales. This is particularly important during ice formation and within openings of the Arctic sea ice cover
(leads, cracks and polynyas) during winter. It has been found that the small spatial area
of cracks and leads has a sufficiently strong enthalpy flux to dominate the total oceanatmosphere heat flux of the northern hemisphere (Maykut 1978). A snow cover on sea
ice acts to decrease the thermal diffusivity of the sea ice, thereby decreasing the heat
flux over all ice types. The spatial pattern of snow-covered sea ice and open water also
has a strong influence on the radiation balance. The albedo for the ocean surface may
range from 3 to 10%, while that for snow-covered sea ice ranges from 80 to 90%.
Ecologically, the radiative, energy and hydrological cycles of the ocean-sea iceatmosphere interface affect all components of the system. Sub-ice (epontic) primary
production within ice-covered regions is known to be light-limited, and higher levels
of the trophic system have evolved specific niches within average conditioris of ice and
snow thickness, growth, ablation, motion and advection. The ecological character and
diversity of open water areas, particularly those that occur within the perennial ice cover, may act as sensitive indicators of climate variability and ecological change in the
Arctic (Stirling 1980 and Dunbar 1981).
3.2.1
Physical and Electrical Properties of Sea Ice
Although the development of the physical properties of sea ice is a continuous process,
it is advantageous to compartmentalize unique stages of growth. Following Livingstone
et al. (1987), seasonality is defined within the context of microwave remote sensing of
a snow-covered sea ice surface into winter, early melt, melt onset, advanced melt, and
freeze-up.
Winter is the dominant season, lasting from November to May at least. During this
season first-year ice types cover a range of thickness conditions from >30 to <200 cm.
Younger ice forms such as nilas, shuga and frazil are rare (Maykut 1978). Typically, firstyear ice consists of a snow layer covering a primary layer of frazil ice overlaying columnar crystals. Total snow cover increases throughout the winter season, reaching accumulations from 5 cm to over 1.0 m. Maximum rates of snow precipitation occur during
the fall and spring seasons when relatively warm air masses exist. When snow is
deposited onto the sea ice surface various metamorphic processes begin to operate, creating the "typical" winter structure of the snow cover.
Vertically, snow on sea ice consists of small grains and high density in the surface
layers of the snow cover. The grain size and density increase towards the basal layer of
the snow volume. Observational evidence (Barber et al.1993, 1994) shows the existence
of a high-brine-volume, low-density, large-grain-size basal layer when snow occurs
over first-year sea ice. Immediately beneath the basal snow layer there exists a sea ice
frazillayer. This layer is formed in the early stages of ice growth when the ice forms
under moderately turbulent conditions. The columnar-oriented crystals occur at deeper levels and are a result of the ice growing under the quiescent conditions of a frazil
or shuga layer of new ice. The vertical profile of ice salinity is usually characterized as
a "e" shape [5-16 parts per thousand (ppt) at the surface, 4-5 ppt in the centre, and near
37
Radiative and energy balances of sea ice are intimately linked with the spatial distribution of the snow/sea ice/ocean matrix. The alternating presence and absence of
sea ice has significant impacts over various temporal and spatial scales. This is particularly important during ice formation and within openings of the Arctic sea ice cover
(leads, cracks and polynyas) during winter. It has been found that the small spatial area
of cracks and leads has a sufficiently strong enthalpy flux to dominate the total oceanatmosphere heat flux of the northern hemisphere (Maykut 1978). A snow cover on sea
ice acts to decrease the thermal diffusivity of the sea ice, thereby decreasing the heat
flux over all ice types. The spatial pattern of snow-covered sea ice and open water also
has a strong influence on the radiation balance. The albedo for the ocean surface may
range from 3 to 10%, while that for snow-covered sea ice ranges from 80 to 90%.
Ecologically, the radiative, energy and hydrological cycles of the ocean-sea iceatmosphere interface affect all components of the system. Sub-ice (epontic) primary
production within ice-covered regions is known to be light-limited, and higher levels
of the trophic system have evolved specific niches within average conditioris of ice and
snow thickness, growth, ablation, motion and advection. The ecological character and
diversity of open water areas, particularly those that occur within the perennial ice cover, may act as sensitive indicators of climate variability and ecological change in the
Arctic (Stirling 1980 and Dunbar 1981).
3.2.1
Physical and Electrical Properties of Sea Ice
Although the development of the physical properties of sea ice is a continuous process,
it is advantageous to compartmentalize unique stages of growth. Following Livingstone
et al. (1987), seasonality is defined within the context of microwave remote sensing of
a snow-covered sea ice surface into winter, early melt, melt onset, advanced melt, and
freeze-up.
Winter is the dominant season, lasting from November to May at least. During this
season first-year ice types cover a range of thickness conditions from >30 to <200 cm.
Younger ice forms such as nilas, shuga and frazil are rare (Maykut 1978). Typically, firstyear ice consists of a snow layer covering a primary layer of frazil ice overlaying columnar crystals. Total snow cover increases throughout the winter season, reaching accumulations from 5 cm to over 1.0 m. Maximum rates of snow precipitation occur during
the fall and spring seasons when relatively warm air masses exist. When snow is
deposited onto the sea ice surface various metamorphic processes begin to operate, creating the "typical" winter structure of the snow cover.
Vertically, snow on sea ice consists of small grains and high density in the surface
layers of the snow cover. The grain size and density increase towards the basal layer of
the snow volume. Observational evidence (Barber et al.1993, 1994) shows the existence
of a high-brine-volume, low-density, large-grain-size basal layer when snow occurs
over first-year sea ice. Immediately beneath the basal snow layer there exists a sea ice
frazillayer. This layer is formed in the early stages of ice growth when the ice forms
under moderately turbulent conditions. The columnar-oriented crystals occur at deeper levels and are a result of the ice growing under the quiescent conditions of a frazil
or shuga layer of new ice. The vertical profile of ice salinity is usually characterized as
a "e" shape [5-16 parts per thousand (ppt) at the surface, 4-5 ppt in the centre, and near