P5.6
The effect of sea state on the momentum exchange over the sea during neutral conditions
Xiaoli Guo Larsen, Uppsala University, Uppsala, Sweden; and U. Högström and A. S. Smedman
The dependence of drag on the ocean of parameters representing wave state has been studied for near-neutral conditions with the aid of an extensive data set taken from the air-sea interaction station Östergarnsholm in the Baltic Sea. The measurements include turbulence flux, slow response, 'profile', data at several levels in the lowest 30 m above the sea surface and wave data from a Waverider Buoy anchored 4 km outside the island. During conditions with developing sea, the drag is found to depend on wave age, expressed with the parameter u*/cp (where u* is friction velocity and cp is the phase speed of the dominant waves) in agreement with recent findings over the world ocean. This result strongly supports the claim that the Östergarnsholm station indeed can be relied upon to give results representative of open ocean conditions. It also suggests that this relation has very general validity for the developing wave case. For such conditions, it is also demonstrated that the logarithmic wind law is indeed valid. For the case of mixed sea/swell the situation is shown to be much more complicated. First, in spite of the occurrence of near-neutral conditions, the data clearly show that the logarithmic wind law is not valid any more. Second, the drag coefficient, CD is found to depend on two parameters representing the wave state: the wave age, u*/cp and a wave spectral ratio E1/E2, where E1 is a measure of the energy of the relatively long waves (those having a phase velocity larger than the wind speed at 10m) and E2 a corresponding measure of the short wave energy. Thus, plotting CD as a function of u*/cp gives a clear ordering of the data in parallel, sloping bands according to the value of E1/E2. A tentative interpretation of the results suggest that, whereas very young and slow waves affect the atmospheric flow similar to rigid roughness elements, with the occurrence of longer waves, an entirely different mechanism gains successively more importance. For such waves dynamical coupling with the atmospheric turbulence is bound to occur. For those cases, it may be speculated that the often observed kink in the wind profile represents the upper bound of a wave-boundary -layer, which is thus, in the general case, an order of magnitude deeper that predicted and observed during growing sea conditions.
Poster Session 5, ABL Parameterizations; Marine BLs
Thursday, 18 July 2002, 2:00 PM-2:00 PM
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