Currently only first year ice, multiyear ice, and open water can be unambiguously distinguished using multispectral techniques. For these ice types and open water, the brightness temperature measured by the satellite can have contributions from first year and multiyear sea ice or open water. In equation form, this would be expressed as

TB = ewTw(1-C) + efyTfyC(1-F) + emyTmyCF


TB = the brightness temperature measured at the satellite
ew = the emissivity of water
Tw = the sea surface temperature
C = the total sea ice concentration, the fraction of ocean surface covered by sea ice within the satellite's field of view
efy = the emissivity of first year sea ice
Tfy = the surface temperature of first year sea ice
F = the multiyear ice fraction, the fraction of the total ice concentration, C, which is multiyear sea ice
emy = the emissivity of multiyear sea ice
Tmy = the surface temperature of multiyear sea ice

In terms of brightness temperatures, the equation becomes

TB = TBw(1-C) + TBfyC(1-F) + TBmyCF

Therefore, the goal here is to determine the total sea ice concentration, C, and the fraction of the ice concentration that is multiyear sea ice, F, from the brightness temperature measured by the satellite, TB. Because the emissivities of these three surfaces are so very different, their brightness temperatures have fundamentally different properties, which then allows us to distinguish one surface from another. Two important characteristics of brightness temperatures of both sea ice types and open ocean are

  1. The difference between the vertical and horizontal brightness temperatures is consistently greater (i.e., more polarized) for open water than for either ice type at all wavelengths.
  2. The discrimination between ice types increases with decreasing wavelength, and is greatest at the 0.81 cm wavelength.

These two physical characteristics form the basis for the sea ice algorithm (Figure 2.05).

The general difference between vertical and horizontal polarization brightness temperatures for open water and either ice type at all wavelengths is used to estimate the total sea ice concentration, C. Look at the slope of the brightness temperature lines in Figure 2.05 for sea ice and open ocean from 0.81 to 1.7 cm. It is

Thus we can use the special variation of the slope in brightness temperature with wavelength for differentiating between the two ice types and determine the multiyear fraction, F. The sea ice algorithm is then based on

  1. the ratio of the horizontal to vertical polarization at 0.8 or 1.7 cm to discriminate sea ice from open ocean
  2. the ratio of the difference in the vertically polarized brightness temperatures at 0.8 and 1.7 cm to their sum to discriminate first year sea ice from multiyear sea ice.

The use of these radiance ratios serves to reduce the effects of physical temperature variations in the radiating medium.