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| Titel |
Wavelength Scaling of Ocean Surface Friction Coefficient in Wind Sea and Mixed Sea |
| VerfasserIn |
Paul Hwang, Hector García-Nava, Francisco (Paco) Ocampo-Torres |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250048948
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| Zusammenfassung |
| The momentum exchange across the air-sea interface is represented by the wind stress.
Over the ocean, the wind stress is rarely measured directly but calculated from
wind speed, with additional modification on the air-sea stability and surface wave
conditions when such information is available. Because of the boundary layer effect,
wind speed varies as a function of elevation. After about 1980s, the neutral wind
speed at 10 m elevation, U10, is generally used as the reference and the friction
coefficient is C10. Extensive research has been conducted on the dependence of C10, or
alternatively, the dynamic roughness, on environmental parameters including wind speed,
stability and surface waves. Somewhat surprisingly, after the stability correction
or under neutral stability condition, the most practical expressions of the friction
coefficient seem to be in the form of dimensionally inconsistent polynomial functions
of wind speed, C10(U10). Sometimes wave parameters such as significant wave
height and wave steepness are also included to produce regression equations of
C10.
Hwang (2004) suggests that the source of the difficulty in finding a dimensionally
consistent similarity relation of the ocean surface friction coefficient may be the
arbitrary 10-m reference elevation, which is selected as a matter of convenience or
practical necessity rather than as a dynamically significant height in the wave-induced
boundary layer (WBL). From fluid dynamics consideration, the proper reference
velocity is the free-stream velocity, U-, outside the boundary layer. Although a full
understanding of the WBL in the open ocean may not be available at this stage, it is well
established that the dynamic effects of waves decay exponentially with distance from the
interface and the rate of decay is proportional to the wavelength. At a distance of
one half wavelengths, the wave dynamic effects are attenuated to 0.043 so that
Uλ-2 is a good candidate for U- in problems involving WBL such as the ocean
surface friction coefficient. Subsequent application of wavelength scaling to an
assembly of data sets under wind-generated wave conditions has produced positive
outcome of a similarity relationship of dimensionally consistent ocean surface friction
coefficient.
Here we presents an analysis of wavelength scaling for the ocean surface friction
coefficient in mixed sea using the data from a recent field experiment of air-sea interaction in
the Gulf of Tehuantepec (IntOA). For wind sea with mono-peak wave spectra, the natural
choice of the scaling wavelength is that of the spectral peak component. For mixed sea with
multi-peak spectra, the peak component in the wind sea portion of the wave spectrum is not a
good reference wavelength. A better scaling wavelength is the weighted average of swell
and wind sea following the consideration of equivalent momentum in the wave
field.
While referencing wind speed at a fixed elevation such as U10 is of practical
necessity,Uλ-2 is physically significant as the free-stream velocity in wave modulated
boundary layer flows. A simple procedure to apply the similarity relation of Cλ-2 to obtain
C10 is described. An application of the similarity relation of ocean surface friction
coefficient to the growth functions of wind generated surface waves is discussed. |
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