EM 1110-2-1100 (Part II)
(Change 1) 31 July 2003
at the 10-m height is given as a function of measurement height for selected values of air-sea temperature
difference and wind speed. Air-sea temperature difference is defined as
∆T ' Ta & Ts
(II-2-9)
where
∆T = air-sea temperature difference, in deg C
Ta = air temperature, in deg C
Ts = water temperature, in deg C
As can be seen in Figure II-2-6, the "1/7" rule should not be used as a general method for transforming wind
speeds from one level to another in marine areas. The ACES software package (Leenhnecht, Szuwalski, and
Sherlock 1992) contains algorithms, based on planetary boundary layer physics, which compute the values
shown in Figure II-2-6; so it is recommended that ACES be used if at all possible for individual situations.
EXAMPLE PROBLEM II-2-3
FIND:
The estimated wind speed at a height of 10 m.
GIVEN:
The wind speed at a height of 25 m is 20 m/s and the air-sea temperature difference is +3EC.
SOLUTION:
From Figure II-2-6 (a), the ratio U/U10 is about 1.18 for a 20-m/s wind at a height of 25 m. So
the estimated wind speed at a 10-m height U10 is equal to U at 25 m (20 m/s) divided by U/U10 (1.18),
which gives U10 = 16.9 m/s.
(c) Simplified estimation of overwater wind speeds from land measurements. Due to the behavior of
water roughness as a function of wind speed, the ratio of overwater winds at a fixed level to overland wind
speeds at a fixed level is not constant, but varies nonlinearly as a function of wind speed. Figure II-2-7
provides guidance for the form of this variation. The specific values shown in this figure are from a study
of winds in the Great Lakes and care should be taken in applying them to other areas. Figure II-2-8
indicates the expected variation with air-sea temperature difference (calculated with ACES). Although air-sea
temperature difference can significantly affect light and moderate winds, it has only a small impact (5 percent
or less) on high wind speeds typical of design. If at all possible, it is advisable to use locally collected data
to respecify the exact form of Figures II-2-7 and II-2-8 for a particular project. One concern here would be
the use of wind measurements from aboveground elevations that are markedly different from those used in
the Resio and Vincent study (9.1 m or 30 ft).
(d) Wind speed variation with fetch. When winds pass over a discontinuity in roughness (e.g., a land-
sea interface), an internal boundary layer is generated. The height of such a boundary layer forms a slope in
the neighborhood of 1:30 in the downwind direction from the roughness discontinuity. This complication
can make it difficult to use winds from certain locations at which winds from some directions fall within the
Meteorology and Wave Climate
II-2-13
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