Multisensor, multifrequency, and multipolarization SAR imagery of oceanic upwelling off the coast of England: a case study of spacecraft and aircraft imagery

Abstract

During a joint U.S.-Russian experiment designed to detect signatures of oceanic processes with synthetic aperture radar, an upwelling event was imaged off the west coast of England. The event was observed on 5 July 1991 at 6:41:46 GMT, and was centered near 48/spl deg/N, 8.9/spl deg/ W. The images were acquired nearly simultaneously, with radar look-direction and swaths being as closely aligned as geometry and platform speed allowed. The upwelling was thus observed on 13 channels, with wavelengths of 68, 24, 10 and 5.7 cm, and polarization combinations of /spl nu//spl nu/, hh, /spl nu/h, and h/spl nu/. The differences between the images made with the various frequencies and polarizations give information on important processes occurring at the sea surface. The upwelling appears to have been caused by tidal flow against the continental shelf in a region of rapidly varying bathymetry. The tidal forcing thus drives the deep, cold water upwards toward the surface in a process known in Russian as a soloy, or a mushroom-shaped blossom that spreads out from the deep across the surface. At the surface it modifies the wind stress over the regions of cooler water, because of lowered evaporation and thus reduced turbulent fluctuations in the atmospheric boundary layer. Areas of reduced roughness appear dark relative to the surrounding fluid, even though the wind speed is uniform over the entire region. However, the morphology of the dark regions in the lower-stress regions varies greatly with frequency and polarization. What appears to be rough at one wavelength and polarization is smoother at another combination of these parameters.<>