1,721,028 research outputs found
A new interpretation of multifrequency/multipolarization radar signatures of the Gulf Stream front
No full textRadar signatures which are observed on SIR-C/X-SAR multifrequency/multipolarization synthetic aperture radar images of the Gulf Stream off the U.S. east coast are compared with results of simulations with a numerical radar imaging model. Based on in situ data, current and wind variations are included into the model as well as a variation of the thermal stability of the marine atmospheric boundary layer across the Gulf Stream front. According to our model predictions, all of these parameter variations can cause radar signatures of similar shape and modulation depth. But, due to specific dependencies of radar signatures on variations of surface currents and winds, we show that it is possible to distinguish between radar signatures of oceanic and atmospheric origin in multifrequency/multipolarization images and to estimate the corresponding current and wind variations independently. For one set of radar images we derive a most likely scenario of oceanic and atmospheric parameters during the time of the image acquisition for which good overall agreement between observed and simulated radar signatures is obtained at most radar channels
Numerical study on signatures of atmospheric convective cells in radar images of the ocean
No full textCurrent and wind variations at the ocean surface can give rise to a modulation of the sea surface roughness and thus become visible in radar images. The discrimination between radar signatures of oceanic and atmospheric phenomena can be quite difficult, since signatures of different origin can have very similar shapes and magnitudes and are often superimposed upon each other. In this work we employ a numerical radar imaging model for an investigation of typical properties of radar signatures of atmospheric convective cells and of theoretical differences between such atmospherically induced radar signatures and those of oceanic phenomena. We show that main characteristics of observed multifrequency/multipolarization radar signatures of atmospheric convective cells over the Gulf Stream are reproduced quite well by the proposed model. This encourages us to vary wind and radar parameters systematically in order to get a general overview of the dependency of atmospherically induced radar signatures on these parameters. Finally, we compare typical characteristics of radar signatures of atmospheric and oceanic phenomena, and we present simulated radar images of a scenario of superimposed atmospheric convective cells and oceanic internal waves. We show that the proposed model supports the experimental finding that radar signatures of oceanic phenomena are stronger at horizontal (HH) than at vertical (VV) polarization, while atmospherically induced radar signatures are better visible at VV polarization
On the determination of characteristics of the interior ocean dynamics from radar signatures of oceanic internal solitary waves
No full textIn this paper we discuss two different methods of inferring characteristics of the interior ocean dynamics from radar signatures of internal solitary waves visible on synthetic aperture radar (SAR) images. The first one consists in the recognition and the interpretation of sea surface patterns of internal solitary waves; the second one consists in the analysis of the modulation depth of the normalized radar backscattering cross section (NRCS) associated with internal solitary waves. For this purpose we consider a data set composed of SAR and in situ measurements carried out from 1991 to 1997 in the region of the Strait of Messina. The recognition and the interpretation of sea surface patterns of internal solitary waves in the Strait of Messina can be used to study characteristics of the density distribution in the area: The internal wave field varies with seasonal variations in the vertical density stratification and with remotely induced variations, i.e., variations induced by the larger-scale circulation, in the horizontal density distribution. In order to inquire into the possibility of inferring parameters of the interior ocean dynamics by analyzing the modulation of the NRCS associated with internal solitary waves, several numerical simulations are carried out using a radar imaging model. These simulations are performed by assuming different wind conditions and internal wave parameters. It is shown that an accurate knowledge of wind conditions is crucial for deriving internal wave parameters and hence parameters of the interior ocean dynamics from the modulation of measured NRCS associated with internal solitary waves
Remote sensing of oceanic current features by synthetic aperture radar - achievements and perspectives
No full textIt is generally accepted that synthetic aperture radar (SAR) images can be quite useful for a better understanding of hydrodynamic processes in the ocean, because they provide valuable information on the location and spatial scales of oceanic features such as fronts, internal waves, and eddies. However, the retrieval of actual surface current fields from the shape and modulation depth of radar signatures is a much more challening problem, since the imaging mechanism is a complex and nonlinear two-step mechanism which cannot be inverted easily. In this article we review the state-of-the-art in modeling radar signatures of current features and we present the concept of an iterative scheme for inverting radar images into current fields, which will be implemented within the framework of the European project MARSAIS. We estimate the accuracy and spatial resolution of the proposed remote sensing system on the basis of findings from recent case studies and some dedicated simulations. According to the results of our analyses, it should be possible to retrieve spatial surface current variations and current gradients from a typical spaceborne C band SAR image with an accuracy on the order of 20% and a spatial resoution on the order of 50m
On the dependence of radar signatures of oceanic internal solitary waves on wind conditions and internal wave parameters
No full textOn August 22, 1997, a large amplitude internal solitary wave was detected simultaneously by using in-situ and remote sensing observations south of the Strait of Messina, in the Mediterranean Sea. In-situ observations consisted of temperature and conductivity, hence salinity, measurements with a towed chain; remote sensing observations in radar backscatter measurements with a spaceborne synthetic aperture radar (SAR). A theoretical velocity field inferred from the observed density field associated with the internal solitary wave and the wind speed and direction measured during the experiment were inserted into a wave-current interaction model to calculate the spatially varying surface wave spectra over the internal solitary wave. Using a composite surface scattering model, the modulated wave spectra were converted into theoretical radar signatures. It is found that the observed and simulated variations of the normalized radar cross section (NRCS) agree quite well. This agreement results from an exceptionally accurate knowledge of the whole set of parameters which determine the theoretical radar signature of the internal solitary wave in the frame of the above mentioned radar imaging model suite. Several numerical simulations performed with the radar imaging model suite elucidate the dependence of the simulated radar signatures on wind conditions, namely wind speed and direction, and on internal wave parameters, namely wave amplitude, undisturbed interface depth, and relative density difference. It is found that the dependence of the modulation depth of the NRCS over internal waves on interior ocean parameters and on wind conditions over the ocean, which are, in general, not well known, is of the same orde
Status report on the remote sensing of current features by spaceborne synthetic aperture radar
No full textSpatial variations in ocean surface currents can become visible in synthetic aperture radar (SAR) images via hydrodynamic modulation of the surface roughness. The interpretation of such SAR signatures is a challenging problem, since the imaging mechanism is quite complex and nonlinear and cannot be inverted easily. Furthermore, the distinction between SAR signatures of current features and other phenomena can be difficult. However, SAR is the only existing technique for the observation of current variations on spatial scales of tens of meters from satellites. There is a vital demand for such information, particularly in coastal regions. A variety of algorithms have been developed for the retrieval of information on current features from SAR images for different purposes. We give an overview of the state of the art, existing and potential applications, and future perspectives and requirements
On the complexity of the inversion of radar signatures of oceanic internal solitary waves into characteristics of the interior ocean
No full textOn August 22, 1997, a large-amplitude internal solitary wave was detected simultaneously by using in-situ and spaceborne synthetic aperture radar observations south of the Strait of Messina. The observed surface velocity field, together with wind speed and direction measured in-situ during the experiment, were inserted into a radar imaging model to calculate theoretical radar signatures. Good agreement was found between model results and observations. Several numerical simulations performed with the radar imaging model using different wind directions indicate that the simulated radar signatures depend strongly on this parameter which, in general, is not known accurately over the ocean. Inserting the measured undisturbed density stratification into an internal wave model, theoretical density and velocity fields associated with an internal solitary wave having the same amplitude as the observed one were calculated. Also in this case, good agreement was found between model results and observations. To study the dependence of the surface velocity field associated with an internal solitary wave on strength of the pycnocline and internal wave amplitude, several numerical simulations were performed with the internal wave model using three different density stratifications. The obtained model results can be seen as a measure of the complexity related to the inversion of sea surface manifestations of internal solitary waves into characteristics of the interior ocea
On the remote sensing of oceanic and atmospheric convection in the Greenland Sea by synthetic aperture radar
No full textIn this paper we discuss characteristic properties of radar signatures of oceanic and atmospheric convection features in the Greenland Sea. If the water surface is clean (no surface films or ice coverage), oceanic and atmospheric features can become visible in radar images via a modulation of the surface roughness, and their radar signatures can be very similar. For an unambiguous interpretation and for the retrieval of quantitative information on current and wind variations from radar imagery with such signatures, theoretical models of current and wind phenomena and their radar imaging mechanisms must be utilized. We demonstrate this approach with the analysis of some synthetic aperture radar (SAR) images acquired by the satellites ERS-2 and RADARSAT-1. In once case, an ERS-2 SAR image an a RADARSAT-1 ScanSAR image exhibit pronounced cell-like signatures with length scales on the order of 10-20 km and modulation depths of about 5-6 dB and 9-10 dB, respectively. Simulations with a numerical SAR imagaing model and various input current and wind fields reveal that the signatures in both images can be expained consistently by wind variations on the order of±2.5 ms, but not by surface current variations on realistic orders of magnitude. Accordingly, the observed features must be atmospheric convection cells. This is confirmed by visible typical cloud patterns in a NOAA AVHRR image of the test scenario. In another case, the presence of an oceanic convective chimney is obvious from in situ data, but no signatures of it are visible in an ERS-2 SAR image. We show by numerical simulations with an oceanic convection model and our SAR imaging model that this is consistent with theoretical predictions, since the current gradients associated with the observed chimney are not sufficiently strong to give rise to significant signatures in an ERS-2 SAR image under the given conditions. Further model results indicate that it should be generally difficult to observe oceanic convection features in the Greenland Sea with ERS-2 or RADARSAT-1 SAR, since their signatures resulting from pure wave-current interaction will be too weak to become visible in the noisy SAR images in most cases. This situation will improve with the availability of future high-resolution SARs such as RADARSAT-2 SAR in fine resolution mode (2004) and TerraSAR-X (2005) which will offer significantly reduced speckle noise fluctuations at comparable spatial resolutions and thus a much better visibility of small image variations on spatial scales on the order of a few hundred meters
Interpretation of convection cell signatures in radar images of the Greenland Sea
No full textWe present a comprehensive analysis of a convection cell pattern in two SAR images of a site in the Greenland Sea from the satellites ERS-2 and RADARSAT. It is not obvious whether the observed radar signatures can be attributed to oceanic or atmospheric convection phenomena. We show by numerical simulations that they are, on the on hand, not consistent with the theory of oceanic convection features and their radar imaging mechanism, while, on the other hand, they could very well be caused by atmospheric convection cells. The conclusion that the observed features must be atmospheric convection cells is supported by in-situ data and an AVHRR image of the scenario. We discuss the significance of our findings in view of the general problem of discriminating between radar signatures of oceanic and atmospheric phenomena
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