GUIDE WAVE ANALYSIS AND FORECASTING - WMO

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Figure 8.3 — Bragg diffraction of radio waves from a sinusoidal
sea-wave. Conditions for constructively
interfering backscatter from successive crests
(after E. D. R. Shearman, 1983)
receding from the radar and of the correct wavelength.
For a microwave radar these would be ripples of a few
centimetres. However, the mechanisms illustrated in
Figure 8.4 ensure that the longer waves influence (modulate)
the ripples, by:
(a) “Straining” (stretching and compressing the ripples
by the orbital motion of the long waves), accompanied
by velocity modulation;
(b) Tilting the surface on which the ripples move and
so modifying the Bragg resonance condition
through ∆ in Equation 8.2; and
(c) Hydrodynamic interaction, which concentrates on
the leading edge of long waves, or through the
perturbed wind flow over the long-wave crests,
both of which produce a roughness modulation.
A radar observing backscatter from short waves by these
mechanisms sees exhibited, in the spatial variation of
backscatter intensity and in the Doppler spectrum, a
representation of the long waves.
Figure 8.4 — Mechanisms of spatial modulation by long
gravity waves of radar scattering from capillary
waves (a) Straining and velocity modulation of
short wind waves by longer wind waves and
swell; (b) Tilt modulation; and (c) Roughness
modulation (after E. D. R. Shearman, 1983)
(a)
(b)
(c)
wind wave
swell
λ s
λ r /2
∆
GUIDE TO WAVE ANALYSIS AND FORECASTING
8.5.2 Radar altimeter
The higher the waves then, as indicated in Figure 8.2, the
greater the time between the arrivals of returns from the
crests and from the troughs of the waves, and the more
spread out is the return pulse. From our knowledge of
the statistics of the sea surface, this stretching of the
shape of the return pulse can be related quantitatively to
the variance of the sea surface, and hence to the significant
wave height H m0 (using the spectral definition — see
Section 1.3.8). Satellite altimeters usually have a pulse
rate of 1 000 Hz; estimates of H m0 are derived on board
and values averaged over one second are transmitted.
Theoretically, there should be no need to calibrate these
values, but in practice it has been found necessary to do
so, comparing them with buoy measurements (Carter et
al., 1992).
Radar altimeters give near-global coverage and
hence provide wave height data for almost anywhere on
the world’s oceans, including areas where data were
previously very scarce. Four long-term satellite altimeter
missions have so far made global measurements. These
are GEOSAT (1985–89), ERS-1 (1991–), ERS-2
(launched April 1995) and Topex/Poseidon (1992–).
Raw data from each of these missions can be obtained
from the responsible space agencies. Fast delivery data
from the ERS satellites are also available in near-real
time (see also Section 6.4). Note that these data can be
significantly biased compared to reference buoy data and
should therefore be corrected before use. From these
data world maps of significant wave height can be
produced, see, for example, Chelton et al. (1981), and
Challenor et al. (1990) and the example in Figure 8.5. In
addition, specialized products are available from a
number of commercial companies (e.g. Oceanor in
Norway, Satellite Observing Systems in the UK).
The strength of the return pulse is also dependent
upon the statistics of the sea surface which are affected
by the wind speed over the surface. So there is a relationship
between the strength of the return pulse and the
wind speed, but this relationship is complex and not
fully understood. In general the stronger the wind, the
lower the strength of the return. Various algorithms have
been derived from observations which give estimates of
wind speed at 10 m above the sea surface from the
strength of the altimeter return, but further work needs to
be done to determine which is the most accurate, see, for
example, Witter and Chelton (1991) and the recent studies
based on the Topex/Poseidon data (Freilich and
Challenor, 1994; Lefevre et al., 1994).
8.5.3 Oblique platform-mounted sensors
An interesting sensor for measurements at fixed sites,
using the phenomena shown in Figure 8.4 to measure
wave characteristics, is the Norwegian MIROS
microwave wave radar (Grønlie et al., 1984), which uses
an oblique 6 GHz (5 cm wavelength) beam and a radar
range resolution of 7.5 m. The velocity of ripples is