2 Passive Microwave Remote Sensing of the Ocean
25
There are two common retrieval algorithms for sea ice, the NASA team algorithm
and the bootstrap algorithm. Both algorithms use the polarization and gradient ratios
to determine ice concentration and type. At 19 GHz the difference between the vertical and horizontal polarizations is small for sea ice (both first-year and multiyear)
and large for ocean. The two polarizations are different for first-year versus multiyear ice at 37 GHz (Cavalieri et al., 1984). The primary error in the NASA team
algorithm is due to the effects of surface glazing and layering on these channel ratios
(Comiso et al., 1997). Newer team algorithms use the 89 GHz gradient ratio to minimize these errors (Markus and Cavalieri, 2000). The bootstrap algorithm uses the
polarization and gradient ratios, combining different channels, such as the 19 and
37 vertical polarization ratio (Comiso et al., 1997). Both algorithms use different
methodologies to filter weather effects.
Validation of the sea ice retrievals has been completed through inter-comparison
between different algorithms and comparison to visible and infrared satellite measurements. The NASA team algorithm and bootstrap algorithm generally agree
with each other but differ by 10–35% in areas within the ice pack (Comiso and
Steffen, 2001).
2.5.6 Sea Surface Temperature
Sea surface temperature is a key climate and weather measurement routinely made
each day by satellite infrared (IR) and PMW radiometers, in-situ moored and drifting buoys, and ships of opportunity. These measurements are used to create daily
spatially-complete global maps of SST that are then used for weather prediction,
ocean forecasts, and in coastal applications such as fisheries forecasts, pollution
monitoring, and tourism. They are also widely used by oceanography, meteorology,
and climate scientists for research. Prior to 1998, SSTs were only available globally
from IR satellite retrievals, but with the launch of TMI, PMW retrievals became
possible. While IR SSTs have a higher resolution than PMW SSTs (1–4 km as compared to 25 km), their retrieval is prevented by clouds giving PMW SSTs improved
coverage since they are retrieved through clouds.
Between 4 and 11 GHz the vertically polarized TB of the sea-surface has an
appreciable sensitivity to SST. In addition to SST, TB depends on the sea-surface
roughness and on the atmospheric temperature and moisture profile. Fortunately, the
spectral and polarimetric signatures of the surface-roughness and the atmosphere
are quite distinct from the SST signature, and the influence of these effects can be
removed given simultaneous measurements at multiple frequencies and polarizations. Both TMI and AMSR-E measure multiple frequencies that are more than
sufficient to remove the surface-roughness and atmospheric effects. Sea-surface
roughness, which is tightly correlated with the local wind, is usually parameterized
in terms of the near-surface wind speed and direction. The additional 7 GHz channel present on AMSR-E and not TMI, provides improved estimates of sea-surface
roughness and improved accuracy for SSTs less than 12 ◦ C (Gentemann et al., in
press). All channels are used to simultaneously retrieve SST, wind speed, columnar
25
There are two common retrieval algorithms for sea ice, the NASA team algorithm
and the bootstrap algorithm. Both algorithms use the polarization and gradient ratios
to determine ice concentration and type. At 19 GHz the difference between the vertical and horizontal polarizations is small for sea ice (both first-year and multiyear)
and large for ocean. The two polarizations are different for first-year versus multiyear ice at 37 GHz (Cavalieri et al., 1984). The primary error in the NASA team
algorithm is due to the effects of surface glazing and layering on these channel ratios
(Comiso et al., 1997). Newer team algorithms use the 89 GHz gradient ratio to minimize these errors (Markus and Cavalieri, 2000). The bootstrap algorithm uses the
polarization and gradient ratios, combining different channels, such as the 19 and
37 vertical polarization ratio (Comiso et al., 1997). Both algorithms use different
methodologies to filter weather effects.
Validation of the sea ice retrievals has been completed through inter-comparison
between different algorithms and comparison to visible and infrared satellite measurements. The NASA team algorithm and bootstrap algorithm generally agree
with each other but differ by 10–35% in areas within the ice pack (Comiso and
Steffen, 2001).
2.5.6 Sea Surface Temperature
Sea surface temperature is a key climate and weather measurement routinely made
each day by satellite infrared (IR) and PMW radiometers, in-situ moored and drifting buoys, and ships of opportunity. These measurements are used to create daily
spatially-complete global maps of SST that are then used for weather prediction,
ocean forecasts, and in coastal applications such as fisheries forecasts, pollution
monitoring, and tourism. They are also widely used by oceanography, meteorology,
and climate scientists for research. Prior to 1998, SSTs were only available globally
from IR satellite retrievals, but with the launch of TMI, PMW retrievals became
possible. While IR SSTs have a higher resolution than PMW SSTs (1–4 km as compared to 25 km), their retrieval is prevented by clouds giving PMW SSTs improved
coverage since they are retrieved through clouds.
Between 4 and 11 GHz the vertically polarized TB of the sea-surface has an
appreciable sensitivity to SST. In addition to SST, TB depends on the sea-surface
roughness and on the atmospheric temperature and moisture profile. Fortunately, the
spectral and polarimetric signatures of the surface-roughness and the atmosphere
are quite distinct from the SST signature, and the influence of these effects can be
removed given simultaneous measurements at multiple frequencies and polarizations. Both TMI and AMSR-E measure multiple frequencies that are more than
sufficient to remove the surface-roughness and atmospheric effects. Sea-surface
roughness, which is tightly correlated with the local wind, is usually parameterized
in terms of the near-surface wind speed and direction. The additional 7 GHz channel present on AMSR-E and not TMI, provides improved estimates of sea-surface
roughness and improved accuracy for SSTs less than 12 ◦ C (Gentemann et al., in
press). All channels are used to simultaneously retrieve SST, wind speed, columnar