GUIDE WAVE ANALYSIS AND FORECASTING - WMO

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2.1.1 Wind and pressure analyses — general
considerations
The wind field is usually analysed by indirect means,
starting with a surface pressure analysis in mid- and
higher latitudes, or a streamline analysis in low latitudes.
In routine analyses of weather charts, the analyst uses
the latest available analysis from which to obtain a first
guess. An isobaric pattern is drawn using pressure observations,
and hopefully some satellite imagery. The
observations of wind speed are used as a check on the
pressure gradient, and those of wind direction on the
orientation of the isobars. Both pressure and wind direction
are used for streamline analysis (see the end of this
section for more discussion on wind analysis in the
tropics). At best the analyses are coarse because of
sparseness of the surface data.
It takes time to carefully examine and fit the pressure
and wind data in the final analysis. Sanders (1990)
has shown that producing a good subjective analysis of
pressure, even when sufficient surface pressure data are
available, is a time consuming activity and requires careful
editing for quality control. Then, to prepare the wind
field analysis, an additional step is required. Wind observations
are used to specify the wind speed and direction
at observation locations. However, over large areas of
the ocean where no (or very few) observations are available,
one must resort to determining wind speed and
direction from parameters which can be obtained from
weather charts (i.e. pressure gradients, curvature of
isobars, and air-sea temperature differences). The corresponding
wind speeds and directions are analysed in such
a way that a consistent pattern emerges which shows the
characteristic features of wind fields associated with
weather disturbances over the ocean.
Thus, determining the wind field requires an analysis
of a weather chart in two consecutive steps. First, a
standard analysis of the isobaric pattern is performed to
determine the main locations of weather systems. Then
follows a close examination of the exact position of the
isobars, with a view to adjusting their position at places
where that is needed to arrive at a logical and consistent
wind-isotach field. In such reviews, one soon finds that,
even in areas where the network of ship observations is
comparatively dense, local adjustments to the isobar
spacing can be made which may easily result in
gradient-wind correction of about 2 m/s. Without observations
the potential for error is much higher. Hence, the
main errors in determining the wind field result from a
lack of pressure and wind observations over the oceans.
Today, operational meteorologists generally use
numerical forecast guidance as the starting point to
produce ocean wind charts. Figure 2.1(a) shows a
sample 48-hour forecast extracted from the NCEP
(National Center for Environment Prediction, USA)
global forecast model of 10 m surface winds and sealevel
pressures for the north-west Atlantic Ocean. The
forecaster studies the movement and development of
GUIDE TO WAVE ANALYSIS AND FORECASTING
weather systems through the forecast period (out to 72
hours at 12-hour increments), as well as the consistency
of the forecasts from run cycle to run cycle. Many forecast
centres also maintain statistics on their own forecast
model performance (i.e. the accuracy of movement and
intensity of weather systems) so that this information
can be used to improve the forecasts. Meteorologists can
either accept the guidance as is, or use the additional
information to modify the forecasts. Figure 2.1(b) shows
the final interpretation for the ocean surface weather
forecast chart using the information in Figure 2.1(a). Of
course, the work is all done within restrictive deadlines.
The forecast chart is “broadcast” in “real time” to the
marine community through various distribution systems
including marine radio facsimile and private companies.
In the tropics it is not possible to determine the
wind field directly from the pressure analyses. This is
because the geostrophic relationship is weaker in the
lower latitudes and breaks down completely at the
Equator. Also, errors in pressure measurements can
become significant compared to the pressure gradients
which are to be analysed. Direct analysis of the wind in
the form of streamlines and isotachs gives a useful
depiction of the low-level wind field.
The procedure for streamline analysis is similar to
pressure analysis in that all weather systems have
consistency over time and they need to be located and
tracked from chart to chart. Each system should be
followed from genesis, to maturity and decay, and its
movement tracked. A knowledge of conceptual models
of weather systems is required to carry this out so that
streamlines and isotachs can be correctly analysed where
there are few observations. It is particularly important
for the analyst to monitor the evolution of the anticyclones
in the subtropical ridge as sudden increases in
intensity can result in surges of wind penetrating well
into the tropics, with resultant increases in wave and
swell height. A useful reference for analysis in the
tropics is the Forecasters guide to tropical meteorology
(AWS, 1995).
2.2 Sources of marine data
There are three sources of surface observations that are
generally used to make analyses of pressure and winds
over the ocean. These are observations taken routinely at
six-hour synoptic intervals by ships, ocean buoys (fixed
and drifting) and land (coastal) weather stations. They
are disseminated worldwide via the Global Telecommunication
System (GTS) to be available in real
time for use in operational centres. In addition, remote
measurements of winds using active and passive
microwave sensors on board satellites are now available
and used by the operational meteorological world.
2.2.1 Ship weather reports
Today, ship weather reports provide the standard source
of marine data. They are prepared by deck officers as
part of their routine duties. Wind speed and direction are