GUIDE WAVE ANALYSIS AND FORECASTING - WMO

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small compared with that of the long wavelength of
interest, so that the radial velocity measured by the radar
Doppler shift is dominantly due to the orbital motion of
the long waves. By analysis of the fluctuation of the
observed Doppler shift, the spectrum of the waves travelling
along the radar beam is deduced. Directional wave
spectrum measurements are achieved by making
measurements in 6 different directions (using a steerable
beam) for a claimed wave-height range 0.1–40 m and
period 3–90 s.
8.5.4 The satellite scatterometer
The satellite-borne scatterometer (Jones et al., 1982) is
another oblique-looking radar sensor. The total echo
power from its radar beam footprint is used to estimate
wind speed, and the relative return power from different
look directions gives an estimate of the wind direction,
because the small-scale roughness of the sea surface,
seen by the radar, is modulated by the longer wind
waves. Calibration is achieved by comparison with nearsurface
wind measurements. The scatterometer does not
give information about the waves, except for the direction
of the wind waves, but its estimates of wind velocity
and hence estimates of wind stress on the sea surface are
proving to be very useful inputs to wave models, particularly
in the Southern Ocean where few conventional
measurements are made.
8.5.5 Synthetic aperture radar (SAR)
Practical aircraft and satellite-borne antennas have beam
widths too large to permit wave imaging. In the synthetic
aperture technique, successive radar observations are
made as the aircraft or satellite travels horizontally.
WAVE DATA: OBSERVED, MEASURED AND HINDCAST 95
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Figure 8.5 — Mean significant wave height (in metres) for the period January–March 1996 from the Topex altimeter (courtesy
D. Cotton, Southampton Oceanography Centre)
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Subsequent optical or digital processing produces
narrow focused beams and high-grade imaging of the
longer waves, as evidenced by the variation of the radarecho
intensity (“radar brightness”) produced by the
mechanisms shown in Figure 8.4. Figure 8.6(a) shows
an example from SEASAT of wave imagery and Figure
8.6(b) shows a wave directional spectrum (with 180°
ambiguity) achieved by analysis of an image. Figure 8.7
shows a high resolution scene in north-western Spain
taken from the ERS-1 SAR.
SAR has the advantage of being a broad-swath
instrument, with swath width and resolution of about
100 km and 25 m, respectively. However, the physical
processes underlying its imaging of waves are complex
and still not universally agreed. The main difficulty in
interpreting the images of ocean waves is that the sea
surface is not at rest, as assumed by the synthetic processor,
and the orbital velocities of the longer waves,
which transport the ripples responsible for backscattering
the radar waves, are around 1 m/s. This results in a
highly non-linear effect which can lead to a complete
loss of information on waves travelling in the alongtrack
direction. Moreover, waves of length less than
about 100 m travelling in any direction are not imaged
by the SAR — because of smearing and decrease in the
signal-to-clutter ratio.
Thus SAR is more likely to provide useful data in
the open ocean rather than in enclosed seas such as
the North Sea where wavelengths tend to be less than
100 m, but even in mid-ocean the waves can sometimes
be so short that the SAR will fail to “see” them.
Given the directional wave spectrum, it is then
possible to obtain a good estimate of the spectrum from
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