EM 1110-2-1100 (Part II)
(Change 1) 31 July 2003
Figure II-2-5. Coefficient of drag versus wind speed
f.
Estimating marine and coastal winds.
(1) Wind estimates based on near-surface observations. Three methods are commonly used to estimate
surface marine wind fields. The first of these, estimation of winds from nearby measurements, has the appeal
of simplicity and has been shown to work well for water bodies up through the size of the Great Lakes. To
use this method, it is often necessary to transfer the measurements to different locations (e.g. from overland
to overwater) and different elevations. Such complications necessitate consideration of the factors given
below.
(a) Elevation correction of wind speed. Often winds taken from observations of opportunity (ships, oil
rigs, offshore structures, buoys, aircraft, etc.) do not coincide with the standard 10-m reference level. They
must be converted to the 10-m reference level for predicting waves, currents, surges, and other wind-
generated phenomena. Failure to do so can produce extremely large errors. For the case of winds taken in
near-neutral conditions at a level near the 10-m level (within the elevation range of about 8-12 m), the
"1/7" rule can be applied. This simple approximation is given as
1
10
7
U10 ' Uz
(II-2-9)
z
where z is measured in meters.
(b) Elevation and stability corrections of wind speed. Figure II-2-6 provides a more comprehensive
method to accomplish the above transformation, including both elevation and stability effects. The "1/7" rule
is given as a special case. In Figure II-2-6, the ratio of the wind speed at any height to the wind speed
Meteorology and Wave Climate
II-2-11
Previous Page
