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Gérard Caudal

Bio: Gérard Caudal is an academic researcher from Centre national de la recherche scientifique. The author has contributed to research in topics: Radar & Wind wave. The author has an hindex of 12, co-authored 30 publications receiving 950 citations.

Papers
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Journal ArticleDOI
TL;DR: In this article, a physical model that takes into account not only the Bragg mechanism, but also the non-Bragg scattering mechanism associated with wave breaking was developed to explain the background behavior of the NRCS and the wave radar Modulation Transfer Function (MTF) at HH and VV polarization.
Abstract: [1] Multiscale composite models based on the Bragg theory are widely used to study the normalized radar cross-section (NRCS) over the sea surface. However, these models are not able to correctly reproduce the NRCS in all configurations and wind wave conditions. We have developed a physical model that takes into account, not only the Bragg mechanism, but also the non-Bragg scattering mechanism associated with wave breaking. A single model was built to explain on the same physical basis both the background behavior of the NRCS and the wave radar Modulation Transfer Function (MTF) at HH and VV polarization. The NRCS is assumed to be the sum of a Bragg part (two-scale model) and of a non-Bragg part. The description of the sea surface is based on the short wind wave spectrum (wavelength from few millimeters to few meters) developed by Kudryavtsev et al. [1999] and wave breaking statistics proposed by Phillips [1985]. We assume that non-Bragg scattering is supported by quasi-specular reflection from very rough wave breaking patterns and that the overall contribution is proportional to the white cap coverage of the surface. A comparison of the model NRCS with observations is presented. We show that neither pure Bragg nor composite Bragg model is able to reproduce observed feature of the sea surface NRCS in a wide range of radar frequencies, wind speeds, and incidence and azimuth angles. The introduction of the non-Bragg part in the model gives an improved agreement with observations. In Part 2, we extend the model to the wave radar MTF problem.

248 citations

Journal Article
TL;DR: In this paper, a physical model that takes into account not only the Bragg mechanism, but also the non-Bragg scattering mechanism associated with wave breaking was developed to explain the background behavior of the NRCS and the wave radar Modulation Transfer Function (MTF) at HH and VV polarization.
Abstract: [1] Multiscale composite models based on the Bragg theory are widely used to study the normalized radar cross-section (NRCS) over the sea surface. However, these models are not able to correctly reproduce the NRCS in all configurations and wind wave conditions. We have developed a physical model that takes into account, not only the Bragg mechanism, but also the non-Bragg scattering mechanism associated with wave breaking. A single model was built to explain on the same physical basis both the background behavior of the NRCS and the wave radar Modulation Transfer Function (MTF) at HH and VV polarization. The NRCS is assumed to be the sum of a Bragg part (two-scale model) and of a non-Bragg part. The description of the sea surface is based on the short wind wave spectrum (wavelength from few millimeters to few meters) developed by Kudryavtsev et al. [1999] and wave breaking statistics proposed by Phillips [1985]. We assume that non-Bragg scattering is supported by quasi-specular reflection from very rough wave breaking patterns and that the overall contribution is proportional to the white cap coverage of the surface. A comparison of the model NRCS with observations is presented. We show that neither pure Bragg nor composite Bragg model is able to reproduce observed feature of the sea surface NRCS in a wide range of radar frequencies, wind speeds, and incidence and azimuth angles. The introduction of the non-Bragg part in the model gives an improved agreement with observations. In Part 2, we extend the model to the wave radar MTF problem.

205 citations

Journal ArticleDOI
TL;DR: In this paper, a self-consistent axisymmetric stationary MHD model of the Jovian magnetosphere is presented, in which both the centrifugal force and the pressure gradients exerted on the plasma are taken into account.
Abstract: This paper describes a self-consistent axisymmetric stationary MHD model of the Jovian magnetosphere, in which both the centrifugal force and the pressure gradients exerted on the plasma are taken into account. The outer boundary condition is chosen so as to represent the dayside (noon meridian). The plasma distribution within the magnetosphere is represented by two Maxwellian components: a cold plasma (several tens of eV) produced at the orbit of Io and diffusing outward under centrifugally driven interchange instability, and a hot (30 keV) low-density plasma, originating from the outer regions of the magnetosphere and diffusing inward in response to it. Outside the Io torus, this process is assumed to be sufficiently rapid that the amount of particles of each population per unit flux tube is independent of distance. The self-consistent solution to the model, with input parameters suitable for the Voyager 1 encounter, is presented and compared to observations. It is found to reproduce very satisfactorily the large-scale features of the dayside magnetic field, plasma density, and plasma pressure, thus supporting the relevance of the plasma distribution that was assumed as input to the model. Among the main conclusions, the pressure gradients are found to constitute the major source of azimuthal currents, thus producing, at all distances, at least 68% of the total current (integrated over latitude) per unit radial distance. The centrifugal current is of secondary importance, although its distribution is more closely confined to the equator. Several runs of the model made with different outer boundary conditions have also permitted study of the compressibility of the magnetosphere, thus showing that due to the presence of high beta plasma within it, the Jovian magnetosphere is much more sensitive to variations of the solar wind pressure than is the terrestrial magnetosphere, in accordance with the observed large variability of the Jovian magnetopause location.

111 citations

Journal ArticleDOI
TL;DR: In this article, a two-scale sea surface emissivity model was used to simulate brightness temperature at L band (1.4 GHz) and the influence of wind speed on Tb with various parameterizations of the sea wave spectrum was explored.
Abstract: [1] In order to prepare the sea surface salinity (SSS) retrieval in the frame of the Soil Moisture and Ocean Salinity (SMOS) mission we conduct sensitivity studies to quantify uncertainties on simulated brightness temperatures (Tb) related to uncertainties on sea surface and scattering modeling. Using a two-scale sea surface emissivity model to simulate Tb at L band (1.4 GHz), we explore the influence on estimated SSS of the parameterization of the seawater permittivity, of the sea wave spectrum, of the choice of the two-scale cutoff wavelength, and of adding swell to the wind sea. Differences between Tb estimated with various existing permittivity models are up to 1.5 K. Therefore a better knowledge of the seawater permittivity at L band is required. The influence of wind speed on Tb simulated with various parameterizations of the sea wave spectrum differs by up to a factor of two; for a wind speed of 7 m s−1 the differences on estimated SSS is several psu depending on the sea wave spectral model taken, so that sea spectrum is a major source of uncertainty in models. We find no noticeable effect on simulated Tb when changing the two-scale cutoff wavelength and when adding swell to the wind sea for low to moderate incidence angles. The dependence of the wind-induced Tb on SST and SSS being weak, we assess the error in SSS estimated assuming that the wind speed influence is independent of SST and SSS. We find errors on estimated SSS up to 0.5 psu for 20°C variation in SST. Therefore this assumption would induce regional biases when applied to global measurements.

82 citations

Journal ArticleDOI
TL;DR: In this paper, the authors developed a physical model that takes into account not only the Bragg mechanism but also the non-Bragg scattering associated with radio wave scattering from breaking waves.
Abstract: [1] Multiscale composite models based on the Bragg theory are widely used to study the normalized radar cross section (NRCS) over the sea surface. However, these models are not able to correctly reproduce the NRCS in all configurations. In particular, even if they may provide consistent results for vertical transmit and receive (VV) polarization, they fail in horizontal transmit and receive (HH) polarization. In addition, there are still important discrepancies between model and observations of the radar modulation transfer function (MTF), which relates the modulations of the NRCS to the long waves. In this context, we have developed a physical model that takes into account not only the Bragg mechanism but also the non-Bragg scattering associated with radio wave scattering from breaking waves. The same model was built to explain both the background NRCS and its modulation by long surface wave (wave radar MTF problem). In part 1, the background NRCS model was presented and assessed through comparisons with observations. In this part 2, we extend the model to include a third underlying scale associated with longer waves (wavelength ∼10–300 m) to explain the modulation of the NRCS. Two contributions are distinguished in the model, corresponding to the so-called tilt and hydrodynamic MTF. Results are compared to observations (already published in the literature or derived from the FETCH experiment). As found, taking into account modulation of wave breaking (responsible for the non-Bragg mechanism) helps to bring the model predictions in closer agreement with observations. In particular, the large MTF amplitudes for HH polarization (much larger than for VV polarization) and MTF phases are better interpreted using the present model.

72 citations


Cited by
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Journal ArticleDOI
12 Apr 2010
TL;DR: The SMOS satellite was launched successfully on November 2, 2009, and will achieve an unprecedented maximum spatial resolution of 50 km at L-band over land (43 km on average over the field of view), providing multiangular dual polarized (or fully polarized) brightness temperatures over the globe.
Abstract: It is now well understood that data on soil moisture and sea surface salinity (SSS) are required to improve meteorological and climate predictions. These two quantities are not yet available globally or with adequate temporal or spatial sampling. It is recognized that a spaceborne L-band radiometer with a suitable antenna is the most promising way of fulfilling this gap. With these scientific objectives and technical solution at the heart of a proposed mission concept the European Space Agency (ESA) selected the Soil Moisture and Ocean Salinity (SMOS) mission as its second Earth Explorer Opportunity Mission. The development of the SMOS mission was led by ESA in collaboration with the Centre National d'Etudes Spatiales (CNES) in France and the Centro para el Desarrollo Tecnologico Industrial (CDTI) in Spain. SMOS carries a single payload, an L-Band 2-D interferometric radiometer operating in the 1400-1427-MHz protected band . The instrument receives the radiation emitted from Earth's surface, which can then be related to the moisture content in the first few centimeters of soil over land, and to salinity in the surface waters of the oceans. SMOS will achieve an unprecedented maximum spatial resolution of 50 km at L-band over land (43 km on average over the field of view), providing multiangular dual polarized (or fully polarized) brightness temperatures over the globe. SMOS has a revisit time of less than 3 days so as to retrieve soil moisture and ocean salinity data, meeting the mission's science objectives. The caveat in relation to its sampling requirements is that SMOS will have a somewhat reduced sensitivity when compared to conventional radiometers. The SMOS satellite was launched successfully on November 2, 2009.

1,553 citations

Journal ArticleDOI
TL;DR: In this article, a two-dimensional wave spectral model is proposed for the high and low-wavenumber regimes, which is based on the Joint North Sea Wave Project (JONSWAP) in the long-wave regime and on the work of Phillips [1985] and Kitaigorodskii [1973] at the high-wavenumbers.
Abstract: Review of several recent ocean surface wave models finds that while comprehensive in many regards, these spectral models do not satisfy certain additional, but fundamental, criteria. We propose that these criteria include the ability to properly describe diverse fetch conditions and to provide agreement with in situ observations of Cox and Munk [1954] and Jahne and Riemer [1990] and Hara et al. [1994] data in the high-wavenumber regime. Moreover, we find numerous analytically undesirable aspects such as discontinuities across wavenumber limits, nonphysical tuning or adjustment parameters, and noncentrosymmetric directional spreading functions. This paper describes a two-dimensional wavenumber spectrum valid over all wavenumbers and analytically amenable to usage in electromagnetic models. The two regime model is formulated based on the Joint North Sea Wave Project (JONSWAP) in the long-wave regime and on the work of Phillips [1985] and Kitaigorodskii [1973] at the high wavenumbers. The omnidirectional and wind-dependent spectrum is constructed to agree with past and recent observations including the criteria mentioned above. The key feature of this model is the similarity of description for the high- and low-wavenumber regimes; both forms are posed to stress that the air-sea interaction process of friction between wind and waves (i.e., generalized wave age, u/c) is occurring at all wavelengths simultaneously. This wave age parameterization is the unifying feature of the spectrum. The spectrum's directional spreading function is symmetric about the wind direction and has both wavenumber and wind speed dependence. A ratio method is described that enables comparison of this spreading function with previous noncentrosymmetric forms. Radar data are purposefully excluded from this spectral development. Finally, a test of the spectrum is made by deriving roughness length using the boundary layer model of Kitaigorodskii. Our inference of drag coefficient versus wind speed and wave age shows encouraging agreement with Humidity Exchange Over the Sea (HEXOS) campaign results.

1,093 citations

Journal ArticleDOI
TL;DR: The dual technique magnetometer system onboard the Cassini orbiter is described in this paper, which consists of vector helium and fluxgate magnetometers with the capability to operate the helium device in a scalar mode.
Abstract: The dual technique magnetometer system onboard the Cassini orbiter is described. This instrument consists of vector helium and fluxgate magnetometers with the capability to operate the helium device in a scalar mode. This special mode is used near the planet in order to determine with very high accuracy the interior field of the planet. The orbital mission will lead to a detailed understanding of the Saturn/Titan system including measurements of the planetary magnetosphere, and the interactions of Saturn with the solar wind, of Titan with its environments, and of the icy satellites within the magnetosphere.

544 citations

Journal ArticleDOI
TL;DR: In this paper, a spherical harmonic model of the magnetic field of Jupiter was derived from in situ magnetic field measurements and remote observations of the position of the foot of the Io flux tube in Jupiter's ionosphere.
Abstract: Spherical harmonic models of the planetary magnetic field of Jupiter are obtained from in situ magnetic field measurements and remote observations of the position of the foot of the Io flux tube in Jupiter's ionosphere. The Io flux tube (IFT) footprint locates the ionospheric footprint of field lines traced from Io's orbital radial distance in the equator plane (5.9 Jovian radii). The IFT footprint is a valuable constraint on magnetic field models, providing “ground truth” information in a region close to the planet and thus far not sampled by spacecraft. The magnetic field is represented using a spherical harmonic expansion of degree and order 4 for the planetary (“internal”) field and an explicit model of the magnetodisc for the field (“external”) due to distributed currents. Models fitting Voyager 1 and Pioneer 11 magnetometer observations and the IFT footprint are obtained by partial solution of the underdetermined inverse problem using generalized inverse techniques. Dipole, quadrupole, octupole, and a subset of higher-degree and higher-order spherical harmonic coefficients are determined and compared with earlier models.

426 citations

Journal ArticleDOI
01 May 2010
TL;DR: In this article, an L-band microwave interferometric radiometer with aperture synthesis (MIRAS) is used to generate brightness temperature images, from which both geophysical variables are computed.
Abstract: Soil Moisture and Ocean Salinity, European Space Agency, is the first satellite mission addressing the challenge of measuring sea surface salinity from space It uses an L-band microwave interferometric radiometer with aperture synthesis (MIRAS) that generates brightness temperature images, from which both geophysical variables are computed The retrieval of salinity requires very demanding performances of the instrument in terms of calibration and stability This paper highlights the importance of ocean salinity for the Earth's water cycle and climate; provides a detailed description of the MIRAS instrument, its principles of operation, calibration, and image-reconstruction techniques; and presents the algorithmic approach implemented for the retrieval of salinity from MIRAS observations, as well as the expected accuracy of the obtained results

382 citations