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Sea surface slicks measured by SAR
Full text linkThe Synthetic Aperture Radar (SAR) system capability to detect and characterise marine surface slicks was tested during the SAR-580 experiment in the northern Adriatic Sea, offshore the Venice coast, in October 1990. Two small artificial slicks of oleyl alcohol were produced in an area around the oceanographic platform of the Italian National Research Council (CNR). The oleyl alcohol produces a damping of the sea centimetric waves, which has been measured by an airborne two band (C and X) SAR, by a tower based 3 band (L, S and C) scatterometer and by a wave gauge, installed on board the platform, which measures the instantaneous sea surface elevation in the range from gravity up to capillary waves. The good agreement among measures proves that multi-frequency SAR is able to detect and characterise sea surface films. Slicks in SAR images taken during SIR-C/X-SAR mission in 1994 have been analysed on the basis of these results and L-band measurements of spatial attenuation near the borders of the slicks have been done, in order to test the slicks detectability using single-band SAR images
"The impact of higher-order Bragg terms on radar sea return"
No full textThe possibility that radar sea return observed using a Ku band fan beam Doppler airborne scatterometer flown over crude oil artificial spills might have been back-scattered via the second-order Bragg interaction is surmised. An attempt is made to justify the absence of the first-order Bragg term
MODULATION OF THE SPECTRA OF SHORT GRAVITY-WAVES BY SEA-SURFACE FILMS - SLICK DETECTION AND CHARACTERIZATION WITH A MICROWAVE PROBE
No full textComparison between algebraical and numerical solutions of the characteristic equation for ripple motion in presence of a surface film
No full textNumerical solutions of the characteristic equation for ripple motion in the presence of a surface film due to a tensioactive substance are obtained. A comparison between numerical solutions and algebraical approximate ones, obtained by several authors, is made. The role of tensioactive substances in increasing the vorticity of waves in the gravito-capillary frequency region is evidentiated by the dynamics of water under the surface. In particular the main dynamical differences between the two possible wave modes of water in such conditions (Laplace and Marangoni waves) are examined
Ocean surface films measured by interferential microwave probe
No full textMeasurements on sea surface films were conducted during five separate experimental periods in the following marine areas: the Sicilian Channel, the Gulf of Maine (spring and fall periods), Bermuda, and the Pacific Ocean West of Southern California. The measurements consisted in surface elevation sampling using an interferential microwave probe with frequency spectra evaluation. Wave spectra were performed for both clean and film-covered sea surface conditions to determine the wave attenuation ratio within the 2-20 Hz spectral range. The method is able to detect, chart and characterize sea surface films. Theoretical analyses of the results yield several viscoelastic film parameters: the modulus of elasticity, the relaxation frequency and the maximum of the damping ratio as a function of wave frequency. The analysed data are interpreted to infer film weathering effects, surface concentration of film-forming constituents, and compactness of the organic sea surface film
SAR detection and characterization of sea surface slicks
No full textIn this paper we demonstrate SAR system capability for detecting and characterizing marine surface slicks. During an aircraft measurement campaign over the Gulf of Genoa (Italy), a multi-frequency SAR system, operating in P-, L- and C-bands, explored a sea area heavily covered by slicks. At the same time in situ measurements were performed with an interferential microwave probe, installed on board a small boat, capable of measuring high resolution sea spectra up to frequencies of capillary waves. By plotting SAR pixel intensity versus sea wave Bragg frequency we obtained wide portions of the sea spectrum region affected by the surface film damping. Spectra derived from SAR imagery and from gauge data present comparable slopes and furthermore the ratio between clean to slicked water spectrum obtained with the two techniques were surprisingly similar. This demonstrates the multi-frequency SAR systems ability to detect and characterize sea surface films assuming the Bragg mechanism in the radar back-scatter. The outlined analysis suggests a simple methodology to monitor coastal water quality by using airborne SAR
Wind stress structure in the unstable marine surface layer detected by SAR
No full textThe 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
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