It can be delineated from these results that Bragg scattering as well as specular reflection contribute to the backscattered radar signal at low incidence angles (up to 30ø). It is concluded that at low to moderate wind speeds, multifrequency radar techniques seem to be capable of discriminating between the different surface films, whereas at high wind conditions a discrimination seems to be difficult.
Abstract. Radar signatures of rain cells are investigated using multifrequency/multipolarization synthetic aperture radar (SAR) images acquired from the space shuttle Endeavour during the spaceborne imaging radar-C/X-band SAR (SIR-C/X-SAR) missions in April and October 1994. In SIR-C/X-SAR images, radar signatures of rain cells over the ocean usually consist of irregularly shaped bright and dark patches that strongly depend on radar frequency and polarization.The radar signatures of rain cells observed in SIR-C/X-SAR imagery of the ocean originate from (1) the scattering and attenuation of the microwaves by raindrops and ice particles in the atmosphere and (2) the modification of the sea surface roughness induced by the impact of raindrops and by wind gusts associated with rain cells. Raindrops impinging on the sea surface generate ring waves, which enhance the sea surface roughness, but they also generate turbulence in the upper water layer, which reduces the sea surface roughness. Depending on the radar wavelength, ocean areas struck by rain can have higher or lower normalized radar cross section (NRCS) than the surrounding rain-free area; in ocean areas where heavy rain is impinging on the sea surface, the X-and C-band NRCS is usually enhanced, and the L-band NRCS is reduced. From the phase difference between the horizontally and vertically copolarized signals, estimates of the rain rate are obtained. The present analysis shows further that the presently used wind speed retrieval algorithms for the scatterometers aboard the ERS and ADEOS satellites may yield biased wind fields if several rain cells lie within a scatterometer resolution
Results from the analyses of several spaceborne imaging radar-C/X-band synthetic aperture radar (SIR-C/X-SAR) images are presented, which were acquired during the two SIR-C/X-SAR missions in April and October 1994 by the L-, C-, and X-band multipolarization SAR aboard the space shuttle Endeavour. The images showing natural (biogenic) surface slicks as well as man-made (anthropogenic) mineral oil spills were analyzed with the aim to study whether or not active radar techniques can be applied to discriminating between these two kinds of surface films. Controlled slick experiments were carried out during both shuttle missions in the German Bight of the North Sea as well as in the northern part of the Sea of Japan and the Kuroshio Stream region, where surface films of different viscoelastic properties were deployed within the swath of the shuttle radars. The results show that the damping behavior of the same substance is strongly dependent on wind speed. At high wind speed (8-12 m/s) the ratio of the radar backscatter from a slick-free and a slick-covered water surface (damping ratio) is smaller than at low to moderate wind speeds (4-7 m/s). At 12 m/s, only slight differences in the damping behavior of different substances were measured by SIR-C/X-SAR. Furthermore, several SAR scenes from various parts of the world's oceans showing radar signatures of biogenic as well as anthropogenic surface films at low to moderate wind speeds are analyzed. The damping behavior of these different kinds of oceanic surface films varies particularly at Lband where the biogenic surface films exhibit larger damping characteristics. Results of polarimetric studies from multipolarization SAR images showing various surface films are presented. It can be delineated from these results that Bragg scattering as well as specular reflection contribute to the backscattered radar signal at low incidence angles (up to 30ø). It is concluded that at low to moderate wind speeds, multifrequency radar techniques seem to be capable of discriminating between the different surface films, whereas at high wind conditions a discrimination seems to be difficult. 1988, and references therein]. Since these waves are responsible for the backscattering of microwaves of comparable wavelengths (Bragg scattering, see Valenzuela [1978]), oil spills are visible as dark patches on radar images of the ocean surface. Further advantages of the use of radar techniques are that the microwaves are mostly not influenced by clouds and that active microwave techniques are independent of daylight. For these reasons the 5.3 GHz (C-band) synthetic aperture ra-Paper number 97JC01915. 0148-0227/98/97JC-01915509.00 dar (SAR) aboard the first and second European Remote Sensing Satellites (ERS-1/2) is already used for operational oil surveillance (reported, e.g., by Wahl et al. [1994] and by Pellemans et al. [1995]). However, not only anthropogenic surface films consisting of mineral oil dampen the surface waves and can be delineated on SAR images. Also, biogenic surface slicks, which a...
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