EM 1110-2-1100 (Part II)
(Change 1) 31 July 2003
termed the isallobaric wind. In certain situations, the isallobaric wind can attain magnitudes nearly equal to
those of geostrophic wind.
(h) Due to the factors discussed above, winds at the geostrophic level can be quite complicated.
Therefore, it is recommended that these calculations be performed with numerical computer codes rather than
manual methods.
(i) Once the wind vector is estimated at a level above the surface boundary layer, it is necessary to
relate this wind estimate to wind conditions at the 10-m reference level. In some past studies, a constant
proportionality was assumed between the wind speeds aloft and the 10-m wind speeds. Whereas this might
suffice for a narrow range of wind speeds if the atmospheric boundary layer were near neutral and no
horizontal temperature gradients existed, it is not a very accurate representation of the actual relationship
between surface winds and winds aloft. Use of a single constant of proportionality to convert wind speeds
at the top of the boundary layer to 10-m wind speeds is not recommended.
(j) Over land, the height of the atmospheric boundary layer is usually controlled by a low-level
inversion layer. This is typically not the case in marine areas where, in general, the height of boundary layer
(in non-equatorial regions) is a function of the friction velocity at the surface and the Coriolis parameter, i.e.
U(
h.λ
(II-2-12)
f
where
λ = dimensionless constant
(k) Researchers have shown that, within the boundary layer, the wind profile depends on latitude (via
the Coriolis parameter), surface roughness, geostrophic/gradient wind velocity, and density gradients in the
vertical (stability effects) and horizontal (baroclinic effects). Over large water bodies, if the effects of wave
development on surface roughness are neglected, the boundary-layer problem can be solved directly from
specification of these factors. Figure II-2-13 shows the ratio of the wind at a 10-m level to the wind speed
at the top of the boundary layer (denoted by the general term Ug here) as a function of wind speed at the top
of the boundary layer, for selected values of air-sea temperature difference. Figure II-2-14 shows the ratio
of friction velocity at the water's surface to the wind speed at the upper edge of the boundary layer as a
function of these same parameters. It might be noted from Figure II-2-14 that a simple approximation for U*
in neutral stratification as a function of Ug is given by
U( . 0.0275 Ug
(II-2-13)
This approximation is accurate within 10 percent for the entire range of values shown in Figure II-2-14.
(l) Measured wind directions are generally expressed in terms of azimuth angle from which winds
come. This convention is known as a meteorological coordinate system. Sometimes (particularly in relation
to winds calculated from synoptic information), a mathematical vector coordinate or Cartesian coordinate
system is used (Figure II-2-15). Conversion from the vector Cartesian to meteorological convention is
accomplished by
Meteorology and Wave Climate
II-2-21
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