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Several studies have shown the capabilities of Synthetic Aperture Radar to map sea currents in ocean regions mainly characterized by large flows. We consider the well known method based on the analysis of the Doppler Centroid. The Doppler, as, in general, the scattering from the sea, is sensitive to several phenomena, occurring between the upper ocean and atmospheric boundary layers. To investigate such phenomena, we considered the combined use of both ENVISAT calibrated amplitude and Doppler data in conjunction with hindcast wind information provided by atmospheric models as well as Wind and Doppler Geophysical Model Functions (W/D-GMF) developed, in the literature, for C-Band systems. This integrated analysis for the interpretation of the Doppler surface currents measurements was carried out on a case study located in the Mediterranean Sea which is characterized by a general low circulation regime: specifically, the coastal region around the city of Naples. In this case study, we show that generally, wind plays a direct significant role in the observed Doppler surface current. The availability of an oceanographic numerical model for one of the analyzed cases also allowed us to attempt to interpret the effect of the typical thermohaline circulation pattern on the Doppler anomaly.

Young sea ice types, namely grease-pancake ice (GPI), and thin ice floes as well, mainly compose the marginal ice zone (MIZ) during the freezing period. Such types of sea ice are considered the primary source of the sea ice fringing the Antarctica continent during winter season through the process called the "pancake cycle", and are massively produced in the Arctic as a result of the sea ice extent and volume decline. The properties of such sea ice types are exceedingly difficult to measure autonomously, due to their size, transient nature, and the hostile environment where they form and grow. However, some internal properties of the ice cover, such as the thickness, can in principle be inferred by measuring the modifications induced by the presence of the sea ice cover on the propagation of gravity waves coming from the open ocean. In this regard, the available spaceborne SAR imagery can be operated as a synoptic wave buoy and measure the attenuation and the shortening of incoming ocean waves inside GPI fields. The SAR inversion procedure is based on knowledge of the open sea wind-generated waves spectrum. It is essential that such waves come from the open ocean before traveling inside the GPI field. The mapping between ocean wave spectrum and SAR image spectrum is provided by the closed nonlinear integral transformation by Hasselmann and Hasselmann (Hasselmann and Hasselmann 1991) and later developments, which account for the SAR image cross-spectrum (Engen and Johnsen 1995). The wind wave spectrum is thus changed according to the rheological properties and to the thickness of the ice layer crossed in order to achieve the best fit between the observed SAR image spectrum and the simulated one. Finally, swell waves are estimated directly from the observed SAR image spectrum regardless of their provenience. The ability of different viscous layer models to describe the attenuation of gravity waves propagating in GPI covered ocean has been already investigated (De Santi et al. 2018). In particular, the Keller's model (Keller, 1998), the two-layer viscous model (De Carolis and Desiderio 2002) and the close-packing model (De Santi and Olla 2017) have been extensively validated by using wave attenuation data collected during two different field campaigns (Weddell Sea, Antarctica, April 2000; Western Arctic Ocean, autumn 2015). For thin GPI, good retrievals of ice thickness have been obtained by considering the ice layer as the only source affecting the wave dynamics, so that the wind input can be disregarded. Validation of these viscous layer models is now extended by inverting different ERS and Sentinel 1 SAR images acquired in the Arctic.

The ability of viscous layer models to describe the attenuation of waves propagating in grease-pancake ice covered ocean is investigated. In particular, the Keller's model (Keller,; https://doi.org/10.1029/97JC02966), the two-layer viscous model (De Carolis & Desiderio,; https://doi.org/10.1016/S0375-9601(02)01503-7) and the close-packing model (De Santi & Olla,; arXiv:1512.05631) are extensively validated by using wave attenuation data collected during two different field campaigns (Weddell Sea, Antarctica, April 2000; western Arctic Ocean, autumn 2015). We use these data to inspect the performance of the three models by minimizing the differences between the measured and model wave attenuation; the retrieved ice thickness is then compared with measured data. The three models allow to fit the observation data but with important differences in the three cases. The close-packing model shows good agreement with the data for values of the ice viscosity comparable to those of grease ice in laboratory experiments. For thin ice, the Keller's model performance is similar to that of the close-packing model, while for thick ice much larger values of the ice viscosity are required, which reflects the different ability of the two models to take into account the effect of pancakes. The improvement of performance over the Keller's model achieved by the two-layer viscous model is minimal, which reflects the marginal role in the dynamics of a finite eddy viscosity in the ice-free layer. A good ice thickness retrieval can be obtained by considering the ice layer as the only source in the wave dynamics, so that the wind input can be disregarded.

International audience; Wave attenuation coefficients (α, m À1) were calculated from in situ data transmitted by custom wave buoys deployed into the advancing pancake ice region of the Weddell Sea. Data cover a 12 day period as the buoy array was first compressed and then dilated under the influence of a passing low-pressure system. Attenuation was found to vary over more than 2 orders of magnitude and to be far higher than that observed in broken-floe marginal ice zones. A clear linear relation between α and ice thickness was demonstrated, using ice thickness from a novel dynamic/thermodynamic model. A simple expression for α in terms of wave period and ice thickness was derived, for application in research and operational models. The variation of α was further investigated with a two-layer viscous model, and a linear relation was found between eddy viscosity in the sub-ice boundary layer and ice thickness.

The aim of the research reported here is to evaluate Synthetic Aperture Radar (SAR) capability to estimate the wind vector and associated directional wave spectrum. Two ERS–2 SAR images of the Mediterranean Sea, one over the Sicily Channel and one over the Ligurian Sea, were selected as case studies. Wind speed was estimated using SAR calibrated backscatter response, in conjunction with empirically derived ERS scatterometer models such as CMOD4 and CMOD–IFREMER. The predictions of these models were then compared with the actual sea surface wave spectra either provided by in situ measurements or resulting from the inversion of the SAR image spectrum. SAR detected effects of both wind and wave features, induced either by atmospheric boundary layer instability or by land shadowing, were also used as reliable indicators of wind direction.

A theoretical model describing the propagation of gravity waves on ice-covered waters is developed. The ice–water system is approximated as a two-layer viscous and immiscible Newtonian fluid. The model is validated against existing viscous models by using laboratory and in situ collected wave data.

This paper is focused on the analysis of SAR imagery of the Mediterranean Sea to estimate the directional wave spectrum and the wind vector. It is discussed the potential of using fetch-limited wave spectral parameterisation, which is currently used to represent wind generated waves, in the case of SAR imaging of swells. As the interest is also focused on the role of the SAR-estimated wind vector to get reliable estimates of the wave spectra, the spectral form used in this study is that due to Donelan et al.. ERS-1 SAR imagery, which is co-located with instrumented directional buoys belonging to the Italian Buoy Network (RON, Rete Ondametrica Nazionale), was exploited. The main purpose is to study SAR capability to discriminate between locally generated wind waves and old wave systems. Three case studies were selected and analysed. They are devoted to compare wave spectral properties typical of swell systems observed at buoy stations located off-shore La Spezia, Ponza island and Alghero with SAR inverted directional wave spectra.

Ocean waves properties propagating in grease ice composed of frazil and pancakes as observed by SAR images are discussed. An ERS-2 SAR scene relevant to the Greenland Sea in an area where the Odden ice tongue developed in 1997 is considered as case study. The scene includes open sea and ice covered waters where a wave field is traveling from the open sea region. Wind induced features known as "wind rolls" can be distinguished, allowing the estimation of the wind vector. Hence the related wind generated ocean waves can be retrieved using a SAR spectral inversion procedure. The wave field is tracked while it propagates inside the ice field, thus allowing the estimation of the wave changes. Under the assumption of continuum medium, physical ice properties are then retrieved using a special SAR inversion procedure in conjunction with a recently developed wave propagation model in sea ice. The model assumes both the ice layer and the water beneath it as a system of viscous fluids. As a result, the changes suffered by the ocean wave spectrum in terms of wave dispersion and energy attenuation are related to sea ice properties such as concentration and thickness. Although the free parameters to be inverted are the ice thickness and viscosity and the water viscosity, the ice thickness is the only parameter of geophysical interest. Results are finally compared with external ice parameters information.

The retrieval of ocean wave spectra from ERS SAR image cross spectra is addressed in order to assess their potential to estimate the thickness of thin sea ice such as frazil and pancake ice. The inversion procedure based on the gradient descent algorithm, already demonstrated for airborne SAR data, is exploited and the capability of this method when applied to satellite SAR sensors is investigated. In fact the major differences between the two imaging situations lie in the illumination geometry and azimuth integration time. The SAR- ERS SLC image acquired on April 10, 1993 over the Greenland Sea was selected as test image. A couple of windows that include open sea only and sea ice cover, respectively, were selected. The inversions were carried out using different guess wave spectra taken from SAR image cross spectra. Moreover, care was taken to properly handle negative values eventually occurring during the inversion runs. This results in a modification of the gradient descending technique that is required if a non-negative solution of the wave spectrum is searched for. Results are discussed in view of the possibility of SAR data to detect ocean wave dispersion as a means for the retrieval of ice thickness.