GUIDE WAVE ANALYSIS AND FORECASTING - WMO

a Directional Waverider (Barstow et al., 1994(a)), and
of which there are about 25–30 operational worldwide.
In the USA, NOAA operates a number of directional
buoys in its extensive measurement network, including
large 6–20 m heave/pitch/roll buoys and a smaller 3 m
system operated closer to the coast. The large buoys
from which the UKOOA (United Kingdom Offshore
Operators Association) acquired directional wave data
also used the heave/pitch/roll system. For further information
on systems presently in use refer to Hamilton
(1990, presently under revision) and WMO (1985,
1991, 1994(a)). For more on analysis of these data see
Section 8.7.2.
8.4.2.2 Arrays of wave recorders
Arrays of recorders can provide directional detail and
have been used in several oceanographic and engineering
studies. The degree of directional resolution depends
on the number of wave recorders and their spacing. The
types of instruments which can be used include wave
staffs (Donelan et al., 1985) and pressure transducers
(O’Reilly et al., 1995). Wave directional information
collected from such systems are usually site specific as
the recorded waves are influenced by refraction and
dissipation in shallow water. The arrays can be either
mounted on an offshore platform or bottom mounted in
shallow water if the depth does not exceed 10–15 m.
Similarly, pressure transducer arrays on offshore platforms
should not be located deeper than 10–15 m below
the surface because of depth attenuation of the surface
wave signature.
8.4.2.3 Current meters
Ultrasonic or electromagnetic current meters which
measure the two horizontal components of wave orbital
velocity in conjunction with a pressure recorder or wave
staff can provide useful directional information. These
systems are usually deployed in shallow water but can
also be mounted on offshore platforms, provided the
influence of the platform is not too great. These systems
are directly analogous to the Directional Waverider or
pitch/roll buoy system discussed in Section 8.4.2.1.
Perhaps the most well-known, self-contained instrument
of this type is the “S4” current meter. Also commonly
used is the “UCM-40” current meter.
8.5 Remote sensing from large distances
In general, conventional wave recorders measure the
displacement with respect to time of the water surface at
a fixed point. Remote-sensing techniques (E. D. R.
Shearman, 1983) are not able to measure in this way.
Instead they interrogate an area or footprint which, in the
case of satellite-borne sensors, has typical dimensions of
several kilometres (except synthetic aperture radar,
which can have a footprint of a few tens of metres) and
report some measure of the average wave conditions
over the whole area. Surface-based radars also can have
a range resolution of a few metres, smaller than the
WAVE DATA: OBSERVED, MEASURED AND HINDCAST 93
wavelengths of the gravity waves being measured,
although the footprint is still broad in azimuth. In consequence,
low-resolution sensors are rarely used for
coastal or shallow water sites, because the wave field
there can vary considerably over distances of much less
than one kilometre. The strength of remote sensing is
that equipment can usually be kept in a safe place well
away from the ravages of the sea and be readily accessible
for maintenance and testing.
The satellite is the most spectacular mounting for a
sea-wave sensor. It is an extremely powerful tool
and can obtain vast quantities of oceanic wave data
worldwide.
Wave measurements made using remote sensing
generally use active microwave sensors (radar), which
send out electromagnetic waves. Deductions about wave
conditions are made from the return signal. Before
considering individual sensors, some features of the
interaction between electromagnetic waves and the sea
surface will be discussed.
8.5.1 Active sensing of the ocean surface with
electromagnetic waves
If we consider a radar sending energy directly down
towards a level, glassy, calm sea surface, the energy will
be reflected from a small region directly below the radar
as if from a mirror. If now the surface is perturbed by
waves, a number of specular reflections (glints) will
arise from facets within the radar beam and perpendicular
to it — essentially from horizontal facets for a
narrow beam radar (see Figure 8.2).
Now consider a radio beam pointing obliquely at
the sea surface at a grazing angle ∆ as in Figure 8.3. If
the sea wavelength, λs, is scaled to the radar wavelength,
λr, as shown, backscattered echoes returning to the
radar from successive crests will all be in phase and
reinforced. The condition for this “Bragg resonant
scatter” is:
λr
λs
cos ∆ = .
(8.2)
2
In the real sea, there will be a mix of many wavelengths
and wave directions, and the Bragg resonance
mechanism will select only those waves approaching or
Figure 8.2 — A radar altimeter normally incident on a rough
sea, illustrating the different path lengths for
energy reflected from facets (from Tucker,
1991)
Incident wave front
Reflections from facets