GUIDE WAVE ANALYSIS AND FORECASTING - WMO

For a storm hindcast, select the most severe waveproducing
storms over the time span, using wave
and wind data where available, and archived analyses
of surface pressure.
(3) Choose a suitable wave model, such as one of those
listed in Table 6.2; the grid spacing and time step
should be appropriate for the application.
(4) Specify surface wind fields on discrete grids for the
time span selected (or for each selected historical
storm).
(5) Execute the numerical hindcast of the time history
of the sea state on a grid of points representing the
basin, for the time span (or for each storm).
(6) Archive input wind fields and hindcast wave
parameters and spectra at a large number of sites
for each model time step.
9.6.4 Wind field analysis
The most crucial point is the production of the wind
fields. Data are sparse over the oceans and it is difficult
to find grid data of a sufficiently fine resolution farther
back than the early 1970s. An example of a procedure
may be to:
(1) Procure all available surface data from ships, buoys,
synoptic and climatic stations, drilling vessels,
Ocean Weather Stations, satellite data and sea ice
cover data (where appropriate).
(2) Digitize historical surface pressure analyses (after
re-analysis using the available data if necessary), or
acquire digital gridded fields of sea-level pressure
or wind speed and direction.
(3) Apply a planetary boundary-layer model to the
isobaric analysis to approximate the near-surface
wind field (e.g. Cardone, 1969).
(4) Construct streamlines and isotachs using all synoptic
observations of wind speed and direction from
ships and land stations, using forward and backward
continuity which defines the movements of
storm centres and fronts and other significant
features of the surface wind field.
(5) Extract kinematic winds (speed and direction) from
the streamline/isotach analyses on the defined grid;
where kinematic analysis is performed over part of
the grid only, the kinematic winds replace the
winds derived from the planetary boundary-layer
model, with blending along the boundaries of the
kinematic area.
The kinematic winds are by far the most accurate
and least biased winds, primarily because the
method allows a thorough re-analysis of the evolution
of the wind fields. Kinematic analysis also
allows the wind fields to represent effects not well
modelled by pressure-wind transformation techniques,
such as inertial accelerations associated
with large spatial and temporal variations in surface
pressure gradients and deformation in surface
winds near and downstream of coasts.
(6) Define the resulting wind fields at a specific level.
WAVE CLIMATE STATISTICS 115
Some modellers have adopted the simple concept
of the “effective neutral” wind speed introduced by
Cardone (1969) to describe the effects of thermal stratification
in the marine boundary layer on wave
generation. The effective neutral wind speed is simply
the wind which would produce the same surface stress at
the sea surface in a neutrally stratified boundary layer as
the wind speed in a boundary layer of a given stratification.
Calculation of the effective wind at a reference
elevation from measured or modelled winds and air-sea
temperature differences requires a model of the marine
surface boundary wind profile which incorporates a
stability dependence and a surface roughness law.
A boundary-layer model is set up to provide the
effective neutral wind speed, for example at 10 m.
Reports of wind speed from ships and rigs equipped with
anemometers are then transformed into the effective
neutral 10 m values (see Dobson, 1982; Shearman and
Zelenko, 1989), using a file of anemometer heights of
ships in the merchant fleet. For ships which use estimated
wind speeds, values are adjusted according to the
scientific Beaufort scale (see Chapter 2, Table 2.2). A
revised table of wind speed equivalents is used to
retrieve the 10 m wind speed and then correct for
stability.
If winds must be interpolated in time for input to
the wave model, the recommended algorithm is linear
interpolation in time of zonal and meridional wind
components to compute wind direction, and interpolation
of the fourth power of wind speed, because wave
energy scales with this quantity. This scheme has been
found to provide sufficient resolution in wind fields
encompassing extra-tropical cyclones off the east coast
of North America.
9.6.5 Archiving of wind and wave fields
The fields archived depend on the user’s needs and
should be as comprehensive as possible. For example,
for a full spectral wave model it would be useful to
archive the following hindcast gridded wind and wave
fields at each model grid point:
• Wind speed in m/s (e.g. effective neutral 10 m
winds);
• Wind direction in degrees (meteorological
notation);
• Wind stress or friction velocity (u *);
• Significant wave height (H s) in metres and tenths of
metres;
• Peak period (T p) or significant period in seconds
and tenths of seconds;
• Vector mean direction, or spectral peak direction in
degrees;
• Directional (2-D) spectral variance in m 2 .
9.6.6 Verification of wave hindcasts
The validation of hindcasts against measurements using
comparisons reveal the skill achievable in the hindcasts.
Time series plots, error statistics and correlation should