EM 1110-2-1100 (Part II)
(Change 1) 31 July 2003
e. Characteristics of near-surface winds.
(1) Winds very close to a marine surface (within the constant-stress layer) generally follow some form
of the "law-of-the-wall" for near-boundary flows. At wind speeds above about 5 m/s (at a 10-m reference
level), turbulent transfers, rather than molecular processes, dominate air-sea interaction processes. Given a
neutrally stable atmosphere, the wind speed close to the surface follows a logarithmic profile of the form
U(
z
ln
Uz '
(II-2-3)
k
z0
where
Uz = wind speed at height z above the surface
k = von Krmn's constant (approximately equal to 0.4)
z0 = roughness height of the surface
(2) In this case, the rate of momentum transfer into a water column (of unit surface area) from the
atmosphere can be written in the parametric form
ρa 2
τ'
U(
ρw
(II-2-4)
ρ
2
' CD z a Uz
ρw
where
τ = wind stress
ρa = density of air
ρw = density of water
CDz = coefficient of drag for winds measured at level z
(3) The international standard reference height for winds is now taken to be 10 m above the surface. If
winds are taken from this level, the z is usually dropped from the subscript notation and the momentum
transfer is represented as
τ ' CD U 2
(II-2-5)
where CD specifically refers now to a 10-m reference level.
(4) Extensive evidence shows that the coefficient of drag over water depends on wind speed (Garratt
1977, Large and Pond 1981, Smith 1988).
(5) When surfaces (land or water) are significantly warmer or cooler than the overlying air, thermal
stability effects tend to modify the logarithmic profile in Equation II-2-3. If the underlying surface is colder
Meteorology and Wave Climate
II-2-9
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