Professor S. A. Hsu Coastal Studies Institute,

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Wind Stress over Water Surfaces: Comparisons of Various Estimation Methods Professor S. A. Hsu Coastal Studies Institute, Louisiana State University Email: sahsu@lsu.edu

Wind stress τ = ϱ U. 2 = ϱ Cd U102 Where ϱ is the density of air, U Wind stress τ = ϱ U*2 = ϱ Cd U102 Where ϱ is the density of air, U* is the friction velocity, Cd is the drag coefficient, and U10 is the wind speed at 10m above the water surface.

www.gfdl.noaa.gov/

1. Wind-wave Interaction Method When U10 is less than 7.5 m/s, surface tension and thermal effects dominate the air-water interaction; When U10 > 7.5 m/s, mechanical turbulence takes over, so that the logarithmic wind profile prevails, U10 = (U*/k) Ln (10/Zo) (1) Where k (=0.4) is the von Karman constant, Zo is the aerodynamic roughness length.

According to Taylor and Yelland (2001, JPO), Zo / Hs = 1200 (Hs/Lp) 4.5 (2) Lp = g Tp 2 / (2π) = 1.56 Tp2 (3) Where Hs is significant wave height, Lp is peak wave length, g is gravitational acceleration, and Tp is dominant wave period.

2. Wave Method According to Csanady (2001) and JONSWAP Wave Spectra (Carter, 1982), for U10 > 20m/s after wave breaker saturation (Amorocho and DeVries,1980; and Geernaert et al.,1987), g Hs/U*2 = 0.053 (g Tp /U*) 3/2 (4) Therefore, U* = 36 Hs2/ Tp 3 (5)

3. Turbulence Intensity or Gust Factor Method According to Hsu (1988, Coastal Meteorology) And Hsu (2003, Journal of waterway, Port, Coastal, and Ocean Engineering), U* = k p U10 (6) G = 1 + 2P (7) U2 / U1 = (Z2 / Z1) p (8) P = σu / U10 (9) G = Ugust / Usustained (10)