GUIDE WAVE ANALYSIS AND FORECASTING - WMO

in some applications, the zero up/downcrossing period)
and the direction in which waves are moving. It should
be noted that the convention of wave direction from
wave models varies (i.e. “from which” direction or “to
which” direction), whereas measured data are invariably
presented as the direction “from which” waves travel,
consistent with the meteorological convention for wind
direction.
The parameters for significant height, peak period
and direction can be further separated into their wind-sea
and swell components giving a total of six additional
wave parameters of operational interest. For the sake of
completeness, these six wave parameters are often
accompanied by three related atmospheric parameters,
namely surface pressure, wind speed and wind direction.
Either all, or a selected few of these nine parameters can
be suitably plotted on a map and disseminated to users.
The algorithms for separating wave trains may be quite
simple (see Section 3.6) or they may fully partition the
spectrum into the wind sea and primary and secondary
swell features (Gerling, 1992). In this case for each
feature integral parameters for the significant wave
height, mean period and mean direction are computed.
6.2 Wave charts
A map (or chart) showing spatial distribution of a
selected number of wind and wave parameters is called a
wave chart. For efficient transmission, a wave chart
should be simple and uncluttered. Almost all wave
charts show isopleths of significant wave height suitably
labelled and a few additional parameters like peak
period, wave direction, etc. The chart may provide seastate
information in a diagnostic (analysed) or in a
prognostic (forecast) form.
Examples of typical output from operational centres
The AES operational wave model CSOWM is run twice
daily at the Canadian Meteorological Center (CMC) in
Montreal and is driven by 10 m level winds generated by
the CMC regional weather prediction model. Figure 6.2
provides an example of a four-panel wave chart showing
wave fields at analysis time (zero-hour forecast) plus
forecast fields out to 12, 24 and 36 hours, respectively.
The wave height contours refer to the total wave height,
H, which is defined as:
H2 2 2 = Hwi + Hsw (6.1)
In Equation 6.1, H wi and H sw are wind-wave and swellwave
heights, respectively. The wave charts cover the
north-west Atlantic region extending from the Canadian
Atlantic provinces to about 20°W and refer to the “storm
of the century” in mid-March 1993 when significant
wave heights of 15 m and higher were recorded in the
Scotian Shelf region. A similar four-panel chart covering
the Canadian Pacific extending from the west coast of
Canada out to about the International Date Line is generated
by the Pacific version of the CSOWM, which is also
run twice a day at the CMC.
OPERATIONAL WAVE MODELS 69
Wave charts prepared and disseminated by other
national Services typically include wave height contours
and a few other selected parameters. For example:
• The GSOWM (Global Spectral Ocean Wave Model
of the US Navy), which was run operationally at
the Fleet Numerical Meteorology and Oceanography
Center in Monterey, California, until May
1994 (when it was replaced by an implementation
of the WAM model, see Section 5.5.4), produced
wave charts depicting contours of significant wave
height and primary wave direction by arrows (see
Clancy et al., 1986);
• The National Oceanographic and Atmospheric
Administration (NOAA) in Washington, D.C.
(USA) has been running a second generation deepwater
global spectral wave model since 1985; the
main output from the NOAA Ocean Wave (NOW)
model is a wave chart depicting contours of significant
wave height and arrows showing primary
wave direction. In addition to the global wave
model, NOAA also operates a regional wave model
for the Gulf of Mexico, which is also a second
generation spectral wave model but with shallowwater
physics included. A typical output from the
regional model is given in Figure 6.3;
• The Japan Meteorological Agency (JMA) in Tokyo
has been operating a second generation coupled
discrete spectral wave model (Uji, 1984) for the
north-west Pacific and a hybrid model for the
coastal regions of Japan. A typical output from the
coupled discrete model is the wave chart in
Figure 6.4.
In addition, directional wave spectrum output
charts for selected locations in the north-west
Pacific and in the seas adjacent to Japan are
prepared and disseminated — see Figure 6.5 which
shows two-dimensional spectra at 12 selected locations
near Japan. The wave spectra are displayed in
polar diagrams. Such detailed spectral information
at selected locations can be very useful for offshore
exploration and related activities.
For a general discussion on the various types
(classes) of wave models see Section 5.5.
6.3 Coded wave products
Measured wave data (perhaps from a Waverider buoy or
a directional wave buoy) can be reported or disseminated
using a code called WAVEOB. This code is constructed
to allow for wave parameters and one-dimensional or
two-dimensional spectral information.
For a one-dimensional spectrum (perhaps measured
by a heave sensor), the report consists of a maximum
spectral density value followed by ratios of individual
spectral densities to the maximum value. For a twodimensional
spectrum, the report consists of directional
wave functions in the form of mean and principal wave
directions together with first and second normalized
polar Fourier coefficients.