A case study of air‐sea interaction during swell conditions

Air‐sea interaction data from a situation with pronounced unidirectional swell have been analyzed. Measurements of turbulence at three levels (10, 18, and 26 m above mean sea level) together with directional wave buoy data from the site Östergarnsholm in the Baltic Sea were used. The situation, which lasted for ∼48 hours, appeared in the aftermath of a gale. The wind direction during the swell situation turned slowly within a 90° sector. Both during the gale phase and the swell phase the over‐water fetch 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 speed measuring level 10 m was observed. The net momentum flux was very small, resulting in C D values ∼0.7 × 10 −3 . Throughout the lowest 26 m, covered by the tower measurements, turbulence intensities in all three components remained high despite the low value of the kinematic momentum flux‐ u ′ w ′ ¯resulting in a reduction of the correlation coefficient for the 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 measuring levels), appearing abruptly at wave age c 0 / U 10 equal to 1.2. Turbulence spectra of the horizontal components were shown not to scale with height above the water surface, in contrast to vertical velocity spectra for which such a variation was observed in the low‐frequency range. In addition, 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 it was concluded that this turbulence is of the “inactive” kind, being brought down from the upper parts of the boundary layer by pressure transport.