GUIDE WAVE ANALYSIS AND FORECASTING - WMO

114
quality control and re-analysis of historical information
necessary to produce a high-quality database (particularly
of the winds needed to drive the hindcast wave
model).
One approach, which has been adopted in order to
minimize the costs associated with continuous hindcasts,
is to archive the analysis portion of operational wave
analysis and forecast programmes. This is a very costeffective
means of producing a continuous database,
being a by-product of an existing operational programme.
The disadvantages are that the operational time
constraints mean that not all available data are included
in the analysis, that time-consuming techniques such as
kinematic analysis cannot be performed, that the use of
backward as well as forward continuity in the development
of weather patterns is not possible, and that it will
take N years of operation to produce an N-year database.
There is also a danger that such archives may suggest
“climate change” that is not real but is a result of
changes in the characteristics of the models used.
Nevertheless, this approach represents a viable way to
develop a continuous database of wave information,
albeit of lesser quality. Attention also needs to be given
to the temporal homogeneity of the model data, especially
when different procedures to estimate ocean winds
are used at different times.
9.6.2.2 Storm hindcasts
In order to perform the extremal analysis necessary for
establishing design criteria for offshore operations, it is
necessary to have wave information for a period of at
least 20 years. In fact, recent experience suggests that
even 20 years may not be sufficient to produce stable
estimates of long return period wave heights. The cost
associated with producing a continuous hindcast for
more than 20 years is prohibitive in most instances. As a
result, an approach has been adopted in many countries
whereby a selection of the top-ranked wave-producing
storms over a period of 30 or more years is hindcast,
with the wave heights analysed using peak-overthreshold
techniques (see Section 9.4.1.2).
This approach has several advantages, not least of
which is that the cost is a fraction of that for a continuous
hindcast. Storms hindcast are typically of about five
days duration, so for a sample of 50 storms in a period of
30 years, for example, the total hindcast period would be
250 days. Another major advantage is that the storms
can be hindcast in considerably more detail, with full reanalysis
of each storm, including kinematic analysis, and
forward and backward continuity. All available data can
be used in the analysis, including those data abstracted
from ships’ logs as well as that originally obtained from
the Global Telecommunication System (GTS). There is
no doubt that this approach produces the highest quality
data of any hindcast procedure for the storm periods
selected.
On the negative side, the database produced, while
suitable for extremal analysis, is inappropriate for any
GUIDE TO WAVE ANALYSIS AND FORECASTING
analysis which requires a time series of data at one or
more locations (such as persistence analysis). It also
cannot give any information on frequency distributions
of waves, since only the extreme conditions are analysed.
One additional drawback is that one cannot
remove lingering doubts that the storms selected for the
detailed hindcasts are the most severe wave-producing
storms, since these must usually be selected in the
absence of direct wave measurements, by proxy criteria
such as the time history of pressure gradients over wavegenerating
areas.
9.6.2.3 Hybrid hindcasts
Hybrid hindcasts are being used increasingly to try to
combine the best features of the continuous and storm
hindcasts. They start from a continuous hindcast produced
as described above, and are then augmented by
hindcasts of the most severe storms and of periods where
the verification against measurements shows that the
continuous hindcast has produced significant errors. The
periods from the storm hindcast then replace the archive
in the continuous hindcast. This approach has been used
effectively in the Gulf of Mexico and off the west coast
of Canada. It is relatively cost effective, and continued
correction to the continuous hindcast can be made
depending on available resources. The database
produced is suitable for all types of statistical analysis,
including extremal analysis if the hindcast period is
sufficiently long.
9.6.3 Hindcast procedure
The following paragraphs describe a wave hindcast
storm procedure. Most aspects of the hindcast procedure,
other than storm selection, are the same whether the
continuous or storm approach is selected. If the hindcast
is produced using the analysis cycle of an operational
wave forecast programme, most of the decisions on
model domain, input wind fields, etc. will have been
made; the only remaining decision will involve the
archiving process. A detailed description of the steps
involved in a storm hindcast are given in the WMO
Guide to the applications of marine climatology (WMO,
1994(b)).
The application of the hindcast method includes the
following main steps:
(1) Given the point or area where hindcast wave data
are required, decide the area necessary to be
included to represent the wave conditions in the
region of interest (to catch distantly generated
swells, etc.).
(2) Select the time span for the hindcast; previous
experience with the historical marine meteorological
databases supports selection of storms from
about the past 30 years. The database for earlier
periods is much less extensive and wind fields may
not be specified as accurately. Therefore, the historical
period which should generally considered
extends from about the mid-1950s to the present.