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Myasoedov A.,Russian State Hydrometeorological University | Johannessen J.A.,Nansen Environmental and Remote Sensing Center | Kudryavtsev V.,Russian State Hydrometeorological University | Collard F.,Direction of Radar Applications | Chapron B.,French Research Institute for Exploitation of the Sea
2012 2nd International Conference on Remote Sensing, Environment and Transportation Engineering, RSETE 2012 - Proceedings | Year: 2012

A method for retrieval of the spatial variations of the sea surface mean square slope (MSS) in sun glitter imagery is proposed. Observed sun glitter brightness anomalies are converted to the MSS anomalies with use of a transfer function determined from the smoothed shape of the sun glitter brightness. The method is applied to MODIS and MERIS sun glitter imagery of natural oil seeps and the catastrophic Deepwater Horizon oil spill in the Gulf of Mexico. The results clearly demonstrate a highly feasible approach for investigation of surface signatures of the oil slicks, as well as other ocean phenomena. © 2012 IEEE. Source


Kudryavtsev V.,Saint Petersburg State University | Myasoedov A.,Russian State Hydrometeorological University | Chapron B.,French Research Institute for Exploitation of the Sea | Johannessen J.A.,Nansen Environmental and Remote Sensing Center | And 2 more authors.
Journal of Geophysical Research: Oceans | Year: 2012

A synergetic approach for quantitative analysis of high-resolution ocean synthetic aperture radar (SAR) and imaging spectrometer data, including the infrared (IR) channels, is suggested. This approach first clearly demonstrates that sea surface roughness anomalies derived from Sun glitter imagery compare very well to SAR roughness anomalies. As further revealed using these fine-resolution (∼1 km) observations, the derived roughness anomaly fields are spatially correlated with sharp gradients of the sea surface temperature (SST) field. To quantitatively interpret SAR and optical (in visible and IR ranges) images, equations are derived to relate the "surface roughness" signatures to the upper ocean flow characteristics. As developed, a direct link between surface observations and divergence of the sea surface current field is anticipated. From these satellite observations, intense cross-frontal dynamics and vertical motions are then found to occur near sharp horizontal gradients of the SST field. As a plausible mechanism, it is suggested that interactions of the wind-driven upper layer with the quasi-geostrophic current field (via Ekman advective and mixing mechanisms) result in the generation of secondary ageostrophic circulation, producing convergence and divergence of the surface currents. The proposed synergetic approach combining SST, Sun glitter brightness, and radar backscatter anomalies, possibly augmented by other satellite data (e.g., altimetry, scatterometry, ocean color), can thus provide consistent and quantitative determination of the location and intensity of the surface current convergence/divergence (upwelling/downwelling). This, in turn, establishes an important step toward advances in the quantitative interpretation of the upper ocean dynamics from their two-dimensional satellite surface expressions. © 2012 by the American Geophysical Union. Source


Hansen M.W.,Nansen Environmental and Remote Sensing Center | Kudryavtsev V.,Nansen Environmental and Remote Sensing Center | Kudryavtsev V.,Marine Hydrophysical Institute | Chapron B.,French Research Institute for Exploitation of the Sea | And 5 more authors.
Remote Sensing of Environment | Year: 2012

A radar imaging model including a Doppler shift module is presented for quantitative studies of radar observations of wave-current interaction in a strong tidal current regime. The model partitions the Doppler shift into the relative contribution arising from the motion of the backscattering facets including Bragg waves, specular points, and breaking waves that are advected by and interact with the underlying surface current. Simulated and observed normalized radar cross sections and Doppler shifts for different environmental conditions and radar parameters are compared and discussed. © 2012 Elsevier Inc. Source


Rouault M.J.,Council for Scientific and Industrial Research | Rouault M.J.,University of Cape Town | Mouche A.,Direction of Radar Applications | Collard F.,Direction of Radar Applications | And 3 more authors.
Journal of Geophysical Research: Oceans | Year: 2010

Over 2 years of surface current information collected in the Agulhas Current region and derived from the Doppler centroid anomalies of Envisat's advanced synthetic aperture radar (ASAR) are examined. The sources of errors and potential use of ASAR surface current velocities for oceanographic research are assessed. ASAR surface current velocities are compared to surface drifter data and merged altimetry observations. Maps of sea surface temperature are used to establish the ASAR's capacity to capture the synoptic circulation. Discrepancies between observed and predicted ASAR velocities result predominantly from inadequate wind corrections combined with radar incidence angles below 30. Occasionally observed wind-induced outliers cause a bias in the estimated ASAR velocities but do not affect the ability of the ASAR to systematically image regions of strong surface current flow and shear. Time-averaged maps of ASAR-derived surface current velocity seem able to accurately capture the position as well as the intensity of the Agulhas Current. The ability of the ASAR to pick up the smaller features of the circulation along the shelf break also shows that variability along the Agulhas Bank is of the same order of magnitude as that observed in the Agulhas retroflection. ASAR surface current velocities offer a very good complement to altimetry in regions where the mean dynamic topography is poorly resolved. The quasi-synoptic nature of ASAR acquisitions combined with the relatively high resolution of ASAR surface current velocities also make it attractive for studies of submesoscale processes and western boundary current dynamics. Copyright 2010 by the American Geophysical Union. Source


Mouche A.A.,Direction of Radar Applications | Collard F.,Direction of Radar Applications | Chapron B.,French Research Institute for Exploitation of the Sea | Dagestad K.-F.,Nansen Environmental and Remote Sensing Center | And 4 more authors.
IEEE Transactions on Geoscience and Remote Sensing | Year: 2012

The synthetic aperture radar (SAR) Doppler centroid has been used to estimate the scatter line-of-sight radar velocity. In weak to moderate ocean surface current environment, the SAR Doppler centroid is dominated by the directionality and strength of wave-induced ocean surface displacements. In this paper, we show how this sea state signature can be used to improve surface wind retrieval from SAR. Doppler shifts of C-band radar return signals from the ocean are thoroughly investigated by colocating wind measurements from the ASCAT scatterometer with Doppler centroid anomalies retrieved from Envisat ASAR. An empirical geophysical model function (CDOP) is derived, predicting Doppler shifts at both VV and HH polarization as function of wind speed, radar incidence angle, and wind direction with respect to radar look direction. This function is used into a Bayesian inversion scheme in combination with wind from a priori forecast model and the normalized radar cross section (NRCS). The benefit of Doppler for SAR wind retrieval is shown in complex meteorological situations such as atmospheric fronts or low pressure systems. Using in situ information, validation reveals that this method helps to improve the wind direction retrieval. Uncertainty of the calibration of Doppler shift from Envisat ASAR hampers the inversion scheme in cases where NRCS and model wind are accurate and in close agreement. The method is however very promising with respect of future SAR missions, in particular Sentinel-1, where the Doppler centroid anomaly will be more robustly retrieved. © 2012 IEEE. Source

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