528
M. Hallikainen
a single ice type dominates. It cannot be used successfully to determine ice type. For an
area of sea ice and water with brightness temperatures of TBi and TBw , respectively, and with
atmospheric effects ignored, the observed brightness temperature is
(19.1)
where C denotes the sea-ice concentration, and the fractional area covered by open water,
therefore, is (I-C). If brightness temperatures TBi and TBw are known or can be estimated
with sufficient accuracy, sea ice concentration can be determined from Equation (19.2):
(19.2)
Equation (19.2) can be used in areas where a single ice type dominates. In the Baltic Sea, only
first-year ice exists. SMMR data from the Nimbus-7 satellite were used to map the ice extent
in the Gulf of Bothnia (northern Baltic Sea) using 18 GHz and 37 GHz horizontally polarized
data. However, in spring conditions involving dry snow on top of the ice layer in north and wet
snow in south, reasonably good accuracy was obtained only by assuming that TBi is equal to
the brightness temperature of snow-covered terrain in coastal areas, Figure 19.1b. Attempts to
estimate TBi from local sea ice data did not produce a good estimate for the ice concentration,
Figure 19.1a. For Antarctic areas, the sea-ice concentration can be estimated with an accuracy
of about 15% using Equation (19.2) together with estimated ice physical temperatures (Comiso
and Zwally, 1982; Zwallyet al., 1983). The physical temperature of sea ice is usually somewhat
warmer than the air temperature, except near the melting point (Steffen et al., 1992).
In areas, where open water, first-year ice and multiyear ice are present, the observed brightness
temperature is
(19.3)
where Cw, CFy and CMY are the fractions of water, first-year ice (FY) and multiyear ice (MY);
they add to unity. The values of both first-year ice and multiyear ice fractions cannot be
obtained from the brightness temperatures measured by a single-channel sensor.
19.2.1.1.2 Multichannel radiometry Several algorithms have been developed for estimating the sea-ice concentration and multiyear ice fraction from multifrequency, multi polarization
microwave radiometer data (Steffen et al., 1992). As an example of these, the NASA Team
Algorithm is presented here. It is originally based on the algorithm presented by Cavalieri et al.
(1984).The algorithm employs two concepts defined below for the 18 GHz vertically (T18V ) and
horizontally (T18H) and 37 GHz vertically (T37V) polarized brightness temperatures, namely
the polarization ratio (P R) and the spectral gradient ratio (G R):
PR(18) = T 18V - T 18H
T18V + T18H
GR(37V/18V) = T 37 V - T 18 V
T37V + T18V
(19.4)
(19.5)
The main advantage of using ratios of radiances is that they reduce the dependence of icetemperature variations. Equations (19.6) to (19.8) are used together to solve the first-year and
multiyear ice concentrations:
C FY = Fo + F1PR + F2GR + F3(PR)(GR)
D
(19.6)
M. Hallikainen
a single ice type dominates. It cannot be used successfully to determine ice type. For an
area of sea ice and water with brightness temperatures of TBi and TBw , respectively, and with
atmospheric effects ignored, the observed brightness temperature is
(19.1)
where C denotes the sea-ice concentration, and the fractional area covered by open water,
therefore, is (I-C). If brightness temperatures TBi and TBw are known or can be estimated
with sufficient accuracy, sea ice concentration can be determined from Equation (19.2):
(19.2)
Equation (19.2) can be used in areas where a single ice type dominates. In the Baltic Sea, only
first-year ice exists. SMMR data from the Nimbus-7 satellite were used to map the ice extent
in the Gulf of Bothnia (northern Baltic Sea) using 18 GHz and 37 GHz horizontally polarized
data. However, in spring conditions involving dry snow on top of the ice layer in north and wet
snow in south, reasonably good accuracy was obtained only by assuming that TBi is equal to
the brightness temperature of snow-covered terrain in coastal areas, Figure 19.1b. Attempts to
estimate TBi from local sea ice data did not produce a good estimate for the ice concentration,
Figure 19.1a. For Antarctic areas, the sea-ice concentration can be estimated with an accuracy
of about 15% using Equation (19.2) together with estimated ice physical temperatures (Comiso
and Zwally, 1982; Zwallyet al., 1983). The physical temperature of sea ice is usually somewhat
warmer than the air temperature, except near the melting point (Steffen et al., 1992).
In areas, where open water, first-year ice and multiyear ice are present, the observed brightness
temperature is
(19.3)
where Cw, CFy and CMY are the fractions of water, first-year ice (FY) and multiyear ice (MY);
they add to unity. The values of both first-year ice and multiyear ice fractions cannot be
obtained from the brightness temperatures measured by a single-channel sensor.
19.2.1.1.2 Multichannel radiometry Several algorithms have been developed for estimating the sea-ice concentration and multiyear ice fraction from multifrequency, multi polarization
microwave radiometer data (Steffen et al., 1992). As an example of these, the NASA Team
Algorithm is presented here. It is originally based on the algorithm presented by Cavalieri et al.
(1984).The algorithm employs two concepts defined below for the 18 GHz vertically (T18V ) and
horizontally (T18H) and 37 GHz vertically (T37V) polarized brightness temperatures, namely
the polarization ratio (P R) and the spectral gradient ratio (G R):
PR(18) = T 18V - T 18H
T18V + T18H
GR(37V/18V) = T 37 V - T 18 V
T37V + T18V
(19.4)
(19.5)
The main advantage of using ratios of radiances is that they reduce the dependence of icetemperature variations. Equations (19.6) to (19.8) are used together to solve the first-year and
multiyear ice concentrations:
C FY = Fo + F1PR + F2GR + F3(PR)(GR)
D
(19.6)