THE NEAR-SURFACE LAYER OF THE OCEAN
have allowed studies of processes on unprecedented space and time scales
compared to traditional in situ observations. Important data acquired with
satellites include sea surface temperature, surface wind speed and direction,
surface wave height and direction, ocean color, ocean current velocity, and
sea ice thickness and extent in the high latitudes. Remote sensing methods
have found numerous applications in studies of the coastal and shelf waters
adjacent to continental margins where inflow and mixing of river waters
substantially contribute to the observed patterns. Obvious advantages of
satellite remote sensing methods are in the frequent global coverage and
generally consistent accuracy of the data. However, the interpretation of the
remote sensing data often requires detailed knowledge of the processes at the
air-sea interface.
7.1.1 Remote sensing of surface winds
The surface wind stress (as well as heat and moisture fluxes) over the
global ocean represents the boundary condition for global circulation
models. Satellites provide exciting opportunities to measure wind stresses at
the sea surface remotely, with global coverage.
Passive (radiometric) and active (radar) ocean sensors have orbited
Earth, at various times since SeaSat in 1975. Presently flying is NASA’s
QuikSCAT, one of the most sophisticated satellites, equipped with the
SeaWinds radar scatterometer. It is capable of measuring wind speeds from 3
to 20 m s
-1 with an accuracy of 2 m s
-1 and an accuracy of 20
o for direction
with 25 km resolution. In order to utilize these measurements in the most
effective way, understanding of both the detailed structure and dynamics of
the ocean surface and the physics of electromagnetic wave scattering from
the sea surface is required.
The remote sensing of the wind velocity vector is possible due to the
amplitude and directional correlation between the wind and sea state
parameters. The sea surface roughness almost entirely determines the
scattering of radar signals. Capillary-gravity waves with wavelengths of
~0.01 m directly influence microwave scattering and radiance emission; a
precise knowledge of the short-scale-wave roughness is therefore required.
Due to nonlinear interactions, short waves are intimately coupled with
intermediate and long waves. Full wavenumber models of the surface
roughness such as that of Donelan and Pierson (1987), Apel (1994),
Elfouhaily et al. (1997) have been widely used in microwave radar scattering
studies. The problem, however, has not yet been completely solved because
of the complex nature of the processes governing sea surface roughness and
the physics of electromagnetic wave interaction with the sea surface
(Lemaire et al., 1999; Voronovich and Zavorotny, 2001). Research in this
440
have allowed studies of processes on unprecedented space and time scales
compared to traditional in situ observations. Important data acquired with
satellites include sea surface temperature, surface wind speed and direction,
surface wave height and direction, ocean color, ocean current velocity, and
sea ice thickness and extent in the high latitudes. Remote sensing methods
have found numerous applications in studies of the coastal and shelf waters
adjacent to continental margins where inflow and mixing of river waters
substantially contribute to the observed patterns. Obvious advantages of
satellite remote sensing methods are in the frequent global coverage and
generally consistent accuracy of the data. However, the interpretation of the
remote sensing data often requires detailed knowledge of the processes at the
air-sea interface.
7.1.1 Remote sensing of surface winds
The surface wind stress (as well as heat and moisture fluxes) over the
global ocean represents the boundary condition for global circulation
models. Satellites provide exciting opportunities to measure wind stresses at
the sea surface remotely, with global coverage.
Passive (radiometric) and active (radar) ocean sensors have orbited
Earth, at various times since SeaSat in 1975. Presently flying is NASA’s
QuikSCAT, one of the most sophisticated satellites, equipped with the
SeaWinds radar scatterometer. It is capable of measuring wind speeds from 3
to 20 m s
-1 with an accuracy of 2 m s
-1 and an accuracy of 20
o for direction
with 25 km resolution. In order to utilize these measurements in the most
effective way, understanding of both the detailed structure and dynamics of
the ocean surface and the physics of electromagnetic wave scattering from
the sea surface is required.
The remote sensing of the wind velocity vector is possible due to the
amplitude and directional correlation between the wind and sea state
parameters. The sea surface roughness almost entirely determines the
scattering of radar signals. Capillary-gravity waves with wavelengths of
~0.01 m directly influence microwave scattering and radiance emission; a
precise knowledge of the short-scale-wave roughness is therefore required.
Due to nonlinear interactions, short waves are intimately coupled with
intermediate and long waves. Full wavenumber models of the surface
roughness such as that of Donelan and Pierson (1987), Apel (1994),
Elfouhaily et al. (1997) have been widely used in microwave radar scattering
studies. The problem, however, has not yet been completely solved because
of the complex nature of the processes governing sea surface roughness and
the physics of electromagnetic wave interaction with the sea surface
(Lemaire et al., 1999; Voronovich and Zavorotny, 2001). Research in this
440