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c. BERTOIA, J. FALKINGHAM, F. FETTERER
of this soupy mixture of ice crystals on the ocean surface suppresses Bragg waves and
reduces backscatter to a level sufficient to allow detection of this very thin ice (Wohl
1995).
As grease ice thickens into nilas, characteristic finger rafting and tearing patterns are
visible; the relatively low resolution of the visible and infrared imagery at the ice centers had in the past masked these features. Nilas exhibits a tone between light and dark
gray but has no clear ridge lines. The analyst must use the texture in the image in conjunction with tone to identify this ice type. With continued cold temperatures, new ice
freezes into young ice (by definition 10-30 cm thick). Gray ice, at 10-15 cm, will continue to exhibit finger rafting, but it will retain small random convection holes, areas
where the relatively warm water has prevented ice from forming. These small ice free
areas are sometimes evident in SAR imagery as splotchy areas of dark return within
the lighter-toned gray ice area. Gray-white (15-30 cm) ice is the first form of ice to exhibit ridging. Ridges will be evident as narrow radiometrically bright features, although
gray-white ice floe forms will remain mostly undefined.
First-year ice is in the 30-200 cm thickness range; generally, un deformed ice thicker than 200 cm is not found. First-year ice can exhibit a significant range in brightness
but is normally brighter in tone than new ice and darker than multiyear ice. The most
characteristic feature of first-year ice is its distinctive floe shapes. First-year floes are
angular and well defined. Ridge lines are bright and long and run completely through
the floes. Floe perimeters can appear bright due to raised edges produced by inter-floe
dynamic interaction. Extensive very bright rubble fields may be evident in areas which
have undergone or are undergoing dynamic activity.
Multiyear ice, or old ice, is ice that has survived at least one melt season and normally
exceeds a thickness of 3 m. Lower salinity and greater thickness make old ice the most
dangerous ice type to shipping. Prior to the advent of SAR imagery, the only available
source for distinguishing first-year and multiyear fractions within the ice pack were
brightness temperatures derived from passive microwave. With active systems, the low
salinity of multiyear ice allows microwave penetration into the ice; thus, return from
this ice type is dominated by volume scattering. The brine pockets and bubbles characteristic of multi-year ice have dimensions in the 1-6 cm range, matching the scale of
the C- and X-band radars now in use. The combination ofthese features makes a bright
return characteristic of multiyear ice, particularly in winter conditions. Multiyear ice
floes are distinctly rounded and have a mottled texture in SAR imagery due to the prevalent drainage pattern which develops during the summer melt season (Fig. 8). Multiyear floes are often conglomerate, composed of many smaller floes frozen together. Second-year ice (ice that has survived only one summer's melt) can sometimes be distinguished from older ice through identification of a curvilinear pattern of fractures.
The preceding discussion summarized typical ice features found under ideal conditions. In the real world of operational ice analysis, environmental conditions may significantly alter or mask expected SAR signatures from sea ice. SAR signatures are significantly affected by surface moisture, produced by either melting ice and snow or wet
precipitation. Wind speeds over 10 cmls can produce very bright returns from open
water, sometimes masking lighter concentrations of ice. Experienced analysts will not
usually confuse the bright return from windy open water with that of multiyear ice due
to the textural differences between these two ice features.
c. BERTOIA, J. FALKINGHAM, F. FETTERER
of this soupy mixture of ice crystals on the ocean surface suppresses Bragg waves and
reduces backscatter to a level sufficient to allow detection of this very thin ice (Wohl
1995).
As grease ice thickens into nilas, characteristic finger rafting and tearing patterns are
visible; the relatively low resolution of the visible and infrared imagery at the ice centers had in the past masked these features. Nilas exhibits a tone between light and dark
gray but has no clear ridge lines. The analyst must use the texture in the image in conjunction with tone to identify this ice type. With continued cold temperatures, new ice
freezes into young ice (by definition 10-30 cm thick). Gray ice, at 10-15 cm, will continue to exhibit finger rafting, but it will retain small random convection holes, areas
where the relatively warm water has prevented ice from forming. These small ice free
areas are sometimes evident in SAR imagery as splotchy areas of dark return within
the lighter-toned gray ice area. Gray-white (15-30 cm) ice is the first form of ice to exhibit ridging. Ridges will be evident as narrow radiometrically bright features, although
gray-white ice floe forms will remain mostly undefined.
First-year ice is in the 30-200 cm thickness range; generally, un deformed ice thicker than 200 cm is not found. First-year ice can exhibit a significant range in brightness
but is normally brighter in tone than new ice and darker than multiyear ice. The most
characteristic feature of first-year ice is its distinctive floe shapes. First-year floes are
angular and well defined. Ridge lines are bright and long and run completely through
the floes. Floe perimeters can appear bright due to raised edges produced by inter-floe
dynamic interaction. Extensive very bright rubble fields may be evident in areas which
have undergone or are undergoing dynamic activity.
Multiyear ice, or old ice, is ice that has survived at least one melt season and normally
exceeds a thickness of 3 m. Lower salinity and greater thickness make old ice the most
dangerous ice type to shipping. Prior to the advent of SAR imagery, the only available
source for distinguishing first-year and multiyear fractions within the ice pack were
brightness temperatures derived from passive microwave. With active systems, the low
salinity of multiyear ice allows microwave penetration into the ice; thus, return from
this ice type is dominated by volume scattering. The brine pockets and bubbles characteristic of multi-year ice have dimensions in the 1-6 cm range, matching the scale of
the C- and X-band radars now in use. The combination ofthese features makes a bright
return characteristic of multiyear ice, particularly in winter conditions. Multiyear ice
floes are distinctly rounded and have a mottled texture in SAR imagery due to the prevalent drainage pattern which develops during the summer melt season (Fig. 8). Multiyear floes are often conglomerate, composed of many smaller floes frozen together. Second-year ice (ice that has survived only one summer's melt) can sometimes be distinguished from older ice through identification of a curvilinear pattern of fractures.
The preceding discussion summarized typical ice features found under ideal conditions. In the real world of operational ice analysis, environmental conditions may significantly alter or mask expected SAR signatures from sea ice. SAR signatures are significantly affected by surface moisture, produced by either melting ice and snow or wet
precipitation. Wind speeds over 10 cmls can produce very bright returns from open
water, sometimes masking lighter concentrations of ice. Experienced analysts will not
usually confuse the bright return from windy open water with that of multiyear ice due
to the textural differences between these two ice features.