THE NEAR-SURFACE LAYER OF THE OCEAN
between the U.S. (NASA) and Japan (National Space Development Agency:
NASDA). The results of the intercomparison between this satellite and the
TOGA COARE rain radar shown in Figure 7-4 are very promising. Note
that the satellite derived rain rates can be used to force the mixed layer
model aimed at enhancing the remote sensing of SST (see the previous
section).
7.1.4 Surface ocean currents
Ocean surface currents can be measured remotely with high-frequency
(HF) radio waves due to the Bragg scattering effect. This effect results from
coherent reflection of the transmitted energy by ocean surface waves whose
wavelength is exactly one-half of the transmitted radar waves (Paduan and
Graber, 1997). These coherent reflections produce a strong peak in the
backscatter spectrum. Note that scatterometers exploit Bragg scattering from
capillary waves (~1 cm) to obtain information about winds. HF radars
exploit Bragg scattering from surface gravity waves (O~10 m) to obtain
information about currents (as well as waves and winds). Combining
simultaneous measurements from two separated sites, maps of surface
current vectors can be created.
At this point, HF radars have found important applications in coastal
ocean studies (Shay et al, 2002). There have also been attempts to employ
the HF radar for offshore surface current measurements (Skop and Peters,
1997). Ship-based observations, however, suffer from motion of the ship,
which complicates the measurement due to the degradation of spectral
returns. Pierre Flament and Cedric Chavanne are working on the solution to
this problem. (Ship motion used to be a problem for atmospheric radars on
ship as well; but it is solved at this point.)
HF radar provides information about currents from the near-surface
layer of the ocean with effective thickness equal to one half of the radio
wavelength, O/2 ~5 m. The slippery near-surface layer of the ocean that
develops due to solar heating and/or freshwater influx (Chapter 4) can affect
the interpretation of the HF radar data relative to surface currents. This
problem, however, has not yet received proper attention.
Determination of surface currents is also possible from SAR imagery
(Chapron et al., 2004), which opens a new exciting opportunity to study
global ocean circulation from space. This method exploits electromagnetic
waves of a several-cm wavelength and is based on the Doppler shift in the
radio signal reflected from the surface waves of comparable wavelength. The
short surface waves are advected by the near-surface shear and modulated by
the orbital velocities of longer waves. In addition to the surface intensified
446
jets (Section 4.2), the aqueous viscous sublayer (Section 2.1.1) appears to be
between the U.S. (NASA) and Japan (National Space Development Agency:
NASDA). The results of the intercomparison between this satellite and the
TOGA COARE rain radar shown in Figure 7-4 are very promising. Note
that the satellite derived rain rates can be used to force the mixed layer
model aimed at enhancing the remote sensing of SST (see the previous
section).
7.1.4 Surface ocean currents
Ocean surface currents can be measured remotely with high-frequency
(HF) radio waves due to the Bragg scattering effect. This effect results from
coherent reflection of the transmitted energy by ocean surface waves whose
wavelength is exactly one-half of the transmitted radar waves (Paduan and
Graber, 1997). These coherent reflections produce a strong peak in the
backscatter spectrum. Note that scatterometers exploit Bragg scattering from
capillary waves (~1 cm) to obtain information about winds. HF radars
exploit Bragg scattering from surface gravity waves (O~10 m) to obtain
information about currents (as well as waves and winds). Combining
simultaneous measurements from two separated sites, maps of surface
current vectors can be created.
At this point, HF radars have found important applications in coastal
ocean studies (Shay et al, 2002). There have also been attempts to employ
the HF radar for offshore surface current measurements (Skop and Peters,
1997). Ship-based observations, however, suffer from motion of the ship,
which complicates the measurement due to the degradation of spectral
returns. Pierre Flament and Cedric Chavanne are working on the solution to
this problem. (Ship motion used to be a problem for atmospheric radars on
ship as well; but it is solved at this point.)
HF radar provides information about currents from the near-surface
layer of the ocean with effective thickness equal to one half of the radio
wavelength, O/2 ~5 m. The slippery near-surface layer of the ocean that
develops due to solar heating and/or freshwater influx (Chapter 4) can affect
the interpretation of the HF radar data relative to surface currents. This
problem, however, has not yet received proper attention.
Determination of surface currents is also possible from SAR imagery
(Chapron et al., 2004), which opens a new exciting opportunity to study
global ocean circulation from space. This method exploits electromagnetic
waves of a several-cm wavelength and is based on the Doppler shift in the
radio signal reflected from the surface waves of comparable wavelength. The
short surface waves are advected by the near-surface shear and modulated by
the orbital velocities of longer waves. In addition to the surface intensified
446
jets (Section 4.2), the aqueous viscous sublayer (Section 2.1.1) appears to be