GUIDE WAVE ANALYSIS AND FORECASTING - WMO

8.1 Introduction
Wave data are often required by meteorologists for realtime
operational use and also for climatological
purposes. This chapter discusses the three types of wave
data that are available, namely, observed, measured and
hindcast. Sections 8.2 and 8.3 deal with visual wave
data. Section 8.4 is a brief description of instruments for
wave measurement and includes a discussion of the
measurement of wave direction. Section 8.5 contains a
review of remotely-sensed data which are becoming of
increasing importance in real-time and climatological
applications. These new data sources have made possible
the assimilation of real data into wave modelling procedures,
and an introduction to this subject is given in
Section 8.6. The analysis of measured data records is
briefly considered in Section 8.7 and some of the repositories
for wave data are indicated in Section 8.8.
8.2 Differences between visual and
instrumental data
Although the simplest method to characterize waves is to
make visual observations of height and period, this
produces data which are not necessarily compatible with
those from instrumental measurements.
It is generally accepted that visual observations of
wave height tend to approximate to the significant wave
height (see definitions in Section 1.3.3). Although there
are several formulae which have been used to convert
visual data to significant wave height more accurately,
for almost all practical meteorological purposes it is
unlikely to be worth the transformation.
Visually observed wave periods are much less reliable
than instrumentally observed ones, as the eye tends
to concentrate on the nearer and steeper short-period
waves, thereby ignoring the longer-period and more
gently sloping waves, even though the latter may be of
greater height and energy. This can be seen by examination
of joint probability plots (scatter diagrams) of
visually observed wave period and height. In many of
these the reported wave period is so short that the steepness
(height to length ratio) is very much higher than is
physically possible for water waves. It is almost certainly
the wave period which is in error, not the height.
8.3 Visual observations
Waves are generally described as either sea (wind sea) or
swell; in this context, sea refers to the waves produced
by the local wind at the time of observation, whereas
CHAPTER 8
WAVE DATA: OBSERVED, MEASURED AND HINDCAST
J. Ewing with D. Carter: editors
swell refers either to waves which have arrived from
elsewhere or were generated locally but which have
subsequently been changed by the wind.
Useful visual observations of wave heights can be
made at sea from ships. Visual observations from land is
meaningful only at the observation site because the
waves change dramatically over the last few hundred
metres as they approach the shore, and the observer is
too far away from the unmodified waves to assess their
characteristics. Shore-based observations normally apply
only to that particular location and although relevant to a
study of local climatology they are rarely meaningful for
any other meteorological purpose.
Mariners, by the very nature of their work, can be
regarded as trained observers. The observation of waves
is part of their daily routine, and a knowledge of changes
in sea and swell is vitally important for them, since such
changes affect the motion of the ship (pitching, rolling
and heaving) and can be the cause of late arrivals and
structural damage.
The observer on a ship can usually distinguish more
than one wave train, make an estimate of the height and
period of each train and give the directions of wave
travel. Waves travelling in the same direction as the wind
are reported as sea; all other trains are, by definition,
swell (although mariners often refer to well-developed
seas in long fetches, such as in the “trade winds”, as
swell). To a coastal observer, waves usually appear to
approach almost normal to the shore because of refraction.
8.3.1 Methods of visual observation
Visual observations should include measurement or
estimation of the following characteristics of the wave
motion of the sea surface in respect of each distinguishable
system of waves, i.e. sea and swell:
(a) Height in metres;
(b) Period in seconds;
(c) Direction from which the waves come.
There may be several different trains of swell
waves. Where swell comes from the same direction as
the sea, it may sometimes be necessary to combine the
two and report them as sea. The following methods of
observing characteristics of separate wave systems
should be used as a guide.
Figure 8.1 is a typical record drawn by a wave
recorder and is representative of waves observed on the
sea (see also Figures 1.14 and 1.15). However, it cannot
indicate that there are two or more wave trains or give
any information on direction. It shows the height of the