On the remote sensing of oceanic and atmospheric convection in the Greenland Sea by synthetic aperture radar

In 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

Synergy between synthetic aperture radar and other sensors for the remote sensing of the ocean

Over the last decades, satellite remote sensing has proved to be a valuable and effective tool for monitoring physical and biological ocean processes. However there are cases where data from one remote sensor alone cannot be interpreted unambiguously. In these situations the combination of data from different sensors can help to understand the observed processes due to the combined benefits of the various strengths and advantages of individual instruments. This paper illustrates the potential of synergy between synthetic aperture radar data and data from thermal and optical satellite sensors. Different aspects of oceanic and atmospheric fronts, eddies, upwelling, internal waves and surface films are imaged by the sensors and combined data give a broader picture of the physical processes involved. While the strengths of synergy are demonstrated in several examples, more frequent coincidence of data from existing and future sensors will be necessary before the benefits of synergy occur on an operational basis

Remote sensing of oceanic current features by synthetic aperture radar — achievements and perspectives

It 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 challenging 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 resolution on the order of 50 m.Il est généralement admis que les images de radars à synthèse d’ouverture (rso) peuvent être très utiles pour la compréhension des processus hydrodynamiques dans l’océan, car elles fournissent des informations de valeur sur la position et l’importance des caractéristiques de surface océaniques, tels les fronts, les ondes internes ou les tourbillons. Cependant, la récupération des champs de courants de surface à partir de la forme et de la modulation des signatures radars est un problème bien plus délicat, car le mécanisme d’imagerie se fait en deux étapes complexes et non-linéaires qui ne peuvent être inversées facilement. Dans cet article nous présentons l’état de l’art de la modélisation de la signature radar des caractéristiques de courant et nous présentons le concept d’un schéma itératif pour l’inversion des images radars en champs de courants. Ce schéma sera implanté dans le cadre du projet européen marsais. Nous estimons la précision et la résolution spatiale du système de télédétection proposé sur la base des découvertes provenant de cas d’études récents et de quelques simulations qui leur étaient dédiées. D’après les résultats de nos analyses, il devrait être possible de retrouver les variations spatiales des courants de surface et les gradients de courant à partir d’images RSO bande C typiques avec une précision de l’ordre de 20 % et une résolution spatiale d’environ 50 m

Marine SAR Analyses and Interpretation System— MARSAIS

There is a growing need for better monitoring and managing of the marine coastal zone environment. The parameters and features most frequently required for their relevance and importance include•surface waves and high-resolution wind fields•surface current strength and variability•identification and location of pollutant material including toxic algae bloom and oil spill Remote sensing observations play a valuable role in this context. Mature algorithms and synthetic aperture radar (SAR) imaging models are integrated and supplemented by complementary thermal and visible sensor data. This paper outlines how a system like MARSAIS can guide and help non-experts in exploring SAR imaging data for coastal ocean monitoring