EM 1110-2-1100 (Part II)
(Change 1) 31 July 2003
Figure II-2-4. Wind profile in atmospheric boundary layer
height. This results in wind directions at the top of the boundary layer which typically deviate about 10 to
15 deg to the right of near-surface wind directions over water and about 25 to 35 deg to the right of near-
surface winds over land. Above the Ekman layer, the so-called geostrophic level is (asymptotically)
approached. Winds in this level are assumed to be outside of the influence of the planetary surface;
consequently, variations in winds above the Ekman layer are produced by different mechanisms than exist
in the atmospheric boundary layer.
(3) Estimates of near-surface winds for wave prediction have historically been based primarily on two
methods: direct interpolation/extrapolation/transformation of local near-surface measurements and
transformation of surface winds from estimates of winds at the geostrophic level. The former method has
mainly been applied to winds in coastal areas or to winds over large lakes. The latter method has been the
main tool for estimating winds over large oceanic areas. A third method, termed "kinematic analysis" has
received little attention in the engineering literature. All three of these methods will be discussed following
a brief treatment of the general characteristics of winds within the atmospheric boundary layer.
c. Winds in coastal and marine areas.
(1) Background.
(a) Winds in marine and coastal areas are influenced by a wide range of factors operating at different
space and time scales. Two potentially important local effects in the coastal zone, caused by the presence of
land, are orographic effects and the sea breeze effect. Orographic effects are the deflection, channeling, or
blocking of air flow by land forms such as mountains, cliffs, and high islands. A rule of thumb for blocking
of low-level air flow perpendicular to a land barrier is given by the following:
Meteorology and Wave Climate
II-2-7
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