GUIDE WAVE ANALYSIS AND FORECASTING - WMO

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Figure 2.2(a) (right) —
Usual way of computing
the true wind (3) from a
diagram plot of head wind
(1) and measured (apparent)
wind speed and
direction (2)
Figure 2.2(b) (far right) —
The computed true wind
may be erroneous both as
regards direction and
speed
Head
wind
(1)
Wilkerson and Earle (1990) shows that the quality of
wind reports from ships with anemometers is not much
better than those without. Pierson (1990) also reported
on the characteristics of ship wind observations made
with and without anemometers and concluded that ship
reports, with or without anemometers, were inferior to
buoy measurement. Errors from anemometer measurements
on board ships are generally introduced by poor
instrument exposure, improper reading of wind speed
and direction indicators, vessel motion and maintenance
problems.
Winds measured by an anemometer on board ship
will be the combined effect of the wind over the ocean
and the ship’s motion. Therefore, in order to obtain a
best estimate of the prevailing (true) wind over the sea,
the measured wind must be corrected to exclude the
ship’s movement. This is done by means of a vector
diagram as illustrated in Figure 2.2(a).
Whilst an anemometer may give an accurate
representation of the air flow at the location of the
anemometer, the main problem with measuring wind on
board ship is that a fixed anemometer cannot always be
properly exposed to all wind directions. The vessel’s
superstructure may interfere with the flow of air, and
consequently the measurement may not be representative
of the true air flow over the ocean surface. It should
be obvious that errors in the apparent wind, and/or the
incorrect application of the vector diagram will produce
poor quality wind reports, as illustrated in Figure 2.2(b).
On larger ships, anemometers are usually installed
at great heights: heights of 40 m above the surface of the
sea are by no means uncommon. The wind speed
normally increases with height, but the rate of speed
increase depends on stability of the air. However, routine
observations are not corrected for height, and that is yet
another source of error when comparing wind data from
many platforms. For a more detailed review of problems
GUIDE TO WAVE ANALYSIS AND FORECASTING
True
Apparent wind (2)
w
ind
( 3)
(1)
Directional
errors of
computed
true wind
of wind measurements at sea, see Dobson (1982) and
also Taylor et al. (1994).
2.2.2 Fixed buoy reports
Since 1967, moored buoys equipped with meteorological
instruments have provided surface atmospheric
and oceanographic data. Buoys can be expected to
provide better quality data than those reported by ships
for several reasons:
• The sensor’s location on the buoy is carefully
considered to avoid exposure problems;
• Sampling and averaging periods for the measurements
are determined after accounting for buoy
motion;
• Duplicate sensors are used for redundancy and each
is calibrated before deployment;
• Monitoring in near-real time allows detection of
instrument errors. (This is the practice in the USA
for all data gathered by buoys deployed by the
National Data Buoy Center.)
Buoys are presently providing measurements which
are within the original accuracy specification.
The National Data Buoy Center’s specified accuracy
requires that winds, averaged for 8.5 minutes, should
have a root-mean-square error for speed of less than
1.0 m/s (or 10 per cent) and for direction of less than
10°. This has been verified through field calibration
studies (Gilhousen, 1987).
2.2.3 Land (coastal) stations
Land (coastal) reporting stations provide data of variable
quality and applicability. In some cases they may
provide reliable data (NOAA, 1990). However, using
these reports requires knowledge of the exposure, local
topography, proximity to the coast and type of station:
whether it is a buoy, light tower, or coastguard station.
Consideration should be given also to the time of day
(2)
(3)
Error
ellipse
of measured
(apparent)
wind vector