A case study of air-sea interaction during swell conditions

Air-sea interaction data from a situation with pronounced unidirectional sw ell have been analyzed. Measurements of turbulence at three levels (10, 18, and 26 m above mean sea level) together with directional wave buoy data fr om the site Ostergarnsholm in the Baltic Sea were used. The situation, whic h lasted for similar to 48 hours, appeared in the aftermath of a gale. The wind direction during the swell situation turned slowly within a 90 degrees sector. Both during the gale phase and the swell phase the over-water fetc h was: >150 km. The wind speed during the swell phase was typically 4 m s(- 1). During the swell phase a wind maximum near or below the lowest wind spe ed measuring level 10 m was observed. The net momentum flux was very small, resulting in C-D values similar to 0.7 x 10(-3). Throughout the lowest 26 m, covered by the tower measurements, turbulence intensities in all three c omponents remained high despite the low value of the kinematic momentum flu x <(-u'w')over bar>, resulting in a reduction of the correlation coefficien t for thr longitudinal and vertical velocity from its typical value around -0.35 to between -0.2 and 0 (and with some positive values at the higher me asuring levels), appearing abruptly at wave age c(0)/U-10 equal to 1.2. Tur bulence spectra of the horizontal components were shown not to scale with h eight above the water surface, in contrast to vertical velocity spectra for which such a variation was observed in the low-frequency range. In additio n, spectral peaks in the horizontal wind spectra were found at a frequency as low as 10(-3) Hz. From a comparison with results from a previous study i t was concluded that this turbulence is of the "inactive" kind, being broug ht down from the upper parts of the boundary layer by pressure transport.