GUIDE WAVE ANALYSIS AND FORECASTING - WMO

VIII
has been included particularly in the chapters on wind
fields, wave data, climate and statistics. These data are
also now being used in conjunction with wave models to
provide improved initializations and to validate hindcast
data. Reference is given to some of the work being
carried out.
Notwithstanding such advances, it is recognized
that many wave products are based on visual observations
and manual analyses and forecasts are still widely
used. Hence, material on manual methods has been
retained in the present Guide. These manual methods are
linked with the numerical by demonstrating their joint
physical basis.
Terminology
Analysis of waves is used in this Guide to refer to a
broad range of procedures. The conventional meteorological
context of an analysis involves data assimilation
and this has had limited application in deriving wave
products since wave data have been too scarce. That is
until recently, when the large volume of data from satellites
has brought to the fore the problem of assimilating
wave data into numerical models.
In this Guide, wave analysis incorporates the procedures
for estimating, calculating or diagnosing wave
conditions. There is a direct relationship between wind
and waves which, in principle, is a matter of calculation
by manual or automatic means. Sometimes this relationship
is quite simple and can be represented by a table
showing the wave height for a given wind speed (and
possibly wind direction). In many cases, however, a
more complicated approach is required, depending on
(a) the amount of detail that is needed, i.e. information
on wave periods, wave steepness, etc.; (b) the environmental
conditions of the forecasting area, including the
geometry of coastlines, bathymetry and currents; and (c)
the nature of the wind which can sometimes be quite
variable, i.e. with changes taking place before a stationary
wave condition has been reached.
Forecasting has a meaning here which is slightly
different from that which is common in meteorology.
When issuing a wave forecast, one can forecast the propagation
of wave energy, but the evolution (growth) of the
wave energy is dependent on the wind and so a major
part of the procedure is actually referring to the forecast
of the winds that cause the waves. The wave growth is in
fact diagnosed from the forecast wind.
Hindcasting, on the other hand, refers to the diagnosis
of wave information based on historical wind data.
A computation based on present wind data is commonly
referred to as wave analysis. The term nowcast is
increasingly used in meteorology in a similar context.
A number of conventions have been adopted in the
formulations used in the Guide. In particular, vector
quantities are written in bold italic type (e.g. a) to distinguish
them from scalar quantities (e.g. a). A glossary of
variables, symbols and acronyms is included as Annex I.
GUIDE TO WAVE ANALYSIS AND FORECASTING
Climatological issues
Whatever the main objective of the wave analysis or
forecast, an appreciation of wave climatology is essential.
The aim of some readers may be to derive the wave
climate and estimate extremes, and for others to produce
operational wave forecasts. For the latter it must be
noted that verification of an estimate of wave conditions
is often not possible due to the lack of wave observations.
Hence, as with all computations, sensible results
are more likely if the local wave climate is known,
particularly the ranges and likelihood of various wave
parameters (for example, height and period) in the sea or
ocean area of interest. Furthermore, experience should
give a feeling for the probable values which might occur
under the given wind conditions. It is of great importance,
therefore, that any investigation, including training
for wave forecasting, starts with a detailed study of the
geography and climatology of the area of interest, in
order to appreciate the limitations of wind fetch for
certain wind directions, the existence of strong ocean
currents, the typical configuration of wind fields in the
weather patterns prevailing over the area and the
climatological probability of wind speeds and directions.
It is useful to know the range of wave heights and
periods which may occur at sea in general: individual
waves higher than 20 m are very rare — the highest
wave reported and checked being 33 m (North Pacific,
1933) — which implies that characteristic or significant
wave heights will rarely exceed 10 m; characteristic
wave periods usually vary between four and 15 seconds
and are seldom greater than 20 seconds. Furthermore,
one should be aware that waves can travel long distances
and still retain appreciable height and energy; for
instance, waves generated in mid-latitude storms in the
North Atlantic Ocean have been observed as swell in the
South Atlantic, and certain atolls in the equatorial
Pacific have been damaged by swell waves which must
have travelled several thousands of kilometres.
If regular wave forecasting is required for a fixed
position or area (e.g. in support of coastal or offshore
engineering or other marine operations such as ship
loading) it is preferable to arrange for regular wave
measurements at suitable points. This provides data for
verification of wave forecasts or validation of models for
hindcasting. In a few instances it may even be possible
to develop a sufficiently homogeneous set of measured
data for determining the wave climate from statistical
analysis. In many applications, the only way to obtain a
satisfactory data set is to hindcast the waves for a sufficiently
long time period, using wind fields derived from
historical weather charts or archived air-pressure data
from atmospheric models. However, the current availability
of upwards of 10 years of satellite altimeter data
globally allows wave height climatology to be accurately
described, at least in areas of the world’s oceans not
affected by tropical storms, down to the spatial resolution
of the satellite data.