The spatial structure of the downward momentum flux on sea derived from a SAR image

The spatial structure of the atmospheric surface layer over the sea has been investigated through a radar image taken by an airborne Synthetic Aperture Radar (SAR) at C-band (5.3 GHz) and wind stress data recorded on board of the C.N.R. research platform located in the northern Adriatic Sea. The SAR image shows quasi-ordered structures of radar backscatter hundreds of meters long, while the wind stress time series are characterized by the well known intermittent bursts of tenth of seconds. A conditional sampling technique has been applied both to the wind stress time series and the SAR image, to get the mean structure of the downward momentum flux and the one and two dimensional shape of the structures of the image. It has been shown that a part of the backscatter structures represents the imprint of the downward wind stress. They appear like microfronts, 50 m wide in the up-downwind direction, elongated in the crosswind direction. The crosswind size is in average four times larger than the up-downwind one. The downward wind stress structures cover the 13% of the total area.