Wind stress structure in the unstable marine surface layer detected by SAR

The wind stress in the marine surface layer under unstable conditions and low wind speed has been studied using a Synthetic Aperture Radar (SAR) image of the sea surface and time series of the horizontal and vertical wind velocities and of the wind stress recorded on board the C.N.R. research platform, in the northern Adriatic Sea, during a SAR overflight. A conditional sampling technique has been used on the wind stress time series and on the SAR image to detect downward (sweep) and upward (ejection) bursts of the momentum flux, as well as the two-dimensional structure of the radar backscatter. From the ensemble average of both the wind stress and the backscatter structures, it has been possible to estimate the mean duration of the upward (≈11 s) and the downward (≈15 s) wind stress bursts and the mean size of the bright patches of the SAR image (≈120 m). The front of the mean backscatter structure, associated with the downward wind stress bursts, has been related to the time length of the mean sweep stress structure to get, after accounting for a threshold of the wind stress for the generation of the sea surface wavelets, the translation velocity Ut of the mean wind stress of sweep, very close to the mean wind speed. The vertical coherence of the wind stress structures has permitted to refer the translation velocity to a level very close to the sea surface, but above the viscous sublayer. The variability of Ut with height has been studied through comparison with the mean wind speed at different heights z calculated by a boundary-layer model. Accounting for the results reported in the literature, there is an indication that Ut is constant with height in the range 0.5 m ≤ z ≤ 15 m. The two-dimensional pattern of the wind stress structures has been derived from the SAR image. The structures appear elongated crosswind, as with microfronts, with an average cross- to down-wind ratio of ≈ 4. The area covered by the downward wind stress structures represents 13% of the total area.