Limited validity of the logarithmic wind law for neutral conditions over the sea

Data covering the period 1995 – 2001 from the air-sea interaction station Östergarnsholm in the Baltic Sea have been used to identify a subset of 1077 hours with truly neutral conditions, wind-following waves and undisturbed fetch. In previous studies it has been demonstrated that the measurements of the turbulence instruments on the tower “see” areas so far removed from the shore that shoaling effects are not expected. It is demonstrated that the state of the sea strongly influences the wind profile in the lowest 30 m above the water. To characterize the state of the sea, it is demonstrated that two parameters are needed, wave age cp/u* and the spectral ratio E1/E2, where E1 is the energy of waves travelling faster than the wind at 10 m (‘long waves’) and E2 is the corresponding energy of the waves travelling slower than the 10 m wind (‘short waves’). A logarithmic wind profile is found only when E1/E2 is sufficiently small, i.e. for growing sea conditions. In fact, for E1/E2 < 0.2, (where z0 is roughness length and s is significant wave height) fits a function of cp/u* which agrees very well with corresponding results reported by other groups from open ocean sites. When the relative amount of ‘long’ waves increases, the wind profile in the layer of measurements, 8 – 30 m, deviates gradually more and more from the logarithmic form. For pronounced swell, when E1/E2 > 5, the wind speed is constant with height in this layer. Logarithmic profiles occur at the Östergarnsholm site only during about 20 percent of the time with neutral stratification.