GUIDE WAVE ANALYSIS AND FORECASTING - WMO

98
Range in km
67.5
60.0
52.5
45.0
37.5
30.0
22.5
15.0
Ground-wave radars use vertically polarized radio
waves in the HF band and must be located on a sea
coast, island, platform or ship. Coverage of 0–200 km in
range is achievable. The potential for continuous seastate
monitoring is shown in Figure 8.8. Recent work
indicates that two stations some distance apart can triangulate
on particular sea areas and can yield maps of
directional spectra with a one-hour update.
Sky-wave radars are large installations, but can be
located well back from the sea coast as they employ
radio waves travelling by way of the ionosphere.
Coverage extends from 900 to 3 000 km. Surface wind
direction measurement is readily achieved, but ionospheric
variability limits wave-height measurement
(Barrick and Gower, 1986).
8.5.8 Comparison of remote-sensing methods
Platform-based microwave sensors fulfil a useful role for
point measurement in environments where buoys are a
hazard. HF ground-wave radars promise to provide maps
of spatial-temporal wave spectra (if necessary, at hourly
intervals), up to 200 km from the coast. Sky-wave radars
extend coverage to oceanic areas, particularly for tracking
fronts and hurricanes, but suffer ionospheric
limitations (Georges and Harlan, 1994).
Satellite altimeters, scatterometers and radiometers
provide worldwide coverage of wave height and surface
wind for modest telemetry and processing overheads. Data
close to coastlines can be suspect, either due to land in the
sensor footprint or to a delay in the sensor switching from
land to ocean mode as the satellite ground track moves
from land to sea. Spatial and temporal coverage of the
Earth is constrained by the satellite orbit. The satellite
usually has an exact repeat cycle of between three and 35
days. The spacing between tracks is inversely proportional
to the repeat cycle — from about 900 km (three-day cycle)
to 80 km (35-day cycle) at the Equator. Satellite data are
therefore particularly suitable for long-term wave climate
studies, but they can also be incorporated, with other
observations, into numerical forecast models or used for
validating hindcast data.
Satellite synthetic aperture radars require expensive
real-time telemetry and off-line processing. Difficulty is
GUIDE TO WAVE ANALYSIS AND FORECASTING
December 8 9 10
experienced in imaging waves travelling parallel to the
satellite track and waves travelling in any direction if the
wavelength is less than about 100 m. But research indicates
that SAR data can be used with wave models to give
improved estimates of directional wave spectra.
8.6 Wave data in numerical models
Wave models are being increasingly used, not only for
predicting wave conditions a few hours or days ahead,
but also for deriving wave climate statistics (see Section
9.6.2 et seq.). Wave data are needed to validate the
models, and they have recently (since 1992) also been
used in model initializations.
The assimilation of wave data into a model, reinitializing
the model at each time step, can improve its
performance, and considerable activity is under way to
develop techniques for doing this. Satellite data, with
their global coverage, are especially useful. Table 8.1
lists countries at present carrying out research into the
assimilation of satellite data into wave models, and
indicates those which are using the data in their operational
wave models.
Data from the satellite altimeter are most readily
and widely available, but the only wave parameter it
measures is significant wave height. How best to incorporate
this one value into a model’s directional wave
spectrum has been addressed in several studies and workable
solutions found (see, for example, Thomas, 1988).
8.7 Analysis of wave records
Figure 8.8 —
Time history
of significant
wave height
vs range, as
measured by
HF groundwave
radar
(after Wyatt
et al., 1985)
Nowadays, most wave data are collected digitally for
subsequent computer analysis. Data are commonly
either transmitted from the measurement site by radio
link or by satellite (e.g. via ARGOS), or recorded locally
(e.g. on board a buoy). A number of systems are available
for real-time presentation of data in the form of time
series plots or statistical presentations. In addition to
providing information suitable for operations, such
systems also give an immediate indication of the correct
functioning of the system. Traditionally, however, analysis
was carried out by a manual technique based on
analogue chart records (see also Section 1.3).