Imaging of oceanic features by ERS 1 synthetic aperture radar

Abstract

Data from the European Space Agency's ERS 1 satellite synthetic aperture radar (SAR) covering the region of the East Australian Current during late 1991 have been analyzed in conjunction with ship and airborne data and infrared imagery. Apart from wind effects, the largest differences in backscatter seem to be associated with different water masses, these differences probably being caused by wave damping due to natural surfactants. Slicklike features in the Tasman Sea are common, even in the deep ocean, when the winds are light. Ocean currents are often visible but as relatively subtle features. It is unlikely that changes in atmospheric boundary layer stability due to changing sea surface temperature are the dominant mechanism for radar imaging of ocean currents. It is suggested that C band radar backscatter will prove to be more directly correlated with biological activity (and ocean color) than with sea surface temperature, particularly at higher latitudes. Surfactants could cause errors of 20% or more in wind speeds derived from satellite scatterometers.

Atmospheric front over the East China Sea studied by multisensor satellite and in situ data

“…On radar images, oceanic fronts may show up as bright lines (increase of the NRCS relative to the gray level background), bright‐and‐dark lines (alternating NRCS along a linear feature), or dark lines (decrease of the NRCS relative to the gray level background). Bright or bright‐and‐dark lines usually can result from the hydrodynamic interaction of surface waves with a varying surface current [see, e.g., Lyzenga , 1991; Johannessen et al , 1994; Nilsson and Tildesley , 1995; Johannessen et al , 1996; Beal et al , 1997; Chubb et al , 1999; Grodsky et al , 2000]. Accumulation of surface films (or floating algae) in regions of surface convergence can result in dark lines on the radar image.…”

Internal wave refraction observed from sequential satellite images

“…Thus, SAR images are ideal for observing features in detail and monitoring cloudy coastal regions. The ability of a SAR sensor to provide valuable information on the type, condition and motion of sea ice and ships, and surface signatures of swells, wind fronts, oil slicks and eddies has been amply demonstrated (Liu et al 1994, Joughin and Winebrenner 1995, Nilsson and Tildesley 1995, Rignot et al 1995, Smith et al 1995, Thompson and Beal 2000, Liu and Wu 2001, Wu and Liu 2003, as has its ability to provide information on internal waves (Apel and Gonzalez 1983, Fu and Holt 1984, Gasparovic et al 1985, 2004, Hsu and Liu 2000, Zheng et al 2001). However, their high spatial resolutions also result in quite long temporal sampling intervals.…”

Ocean feature detection using altimetry backscatter: The Gulf Stream

“…In many cases the mechanism responsible for such radar features has been deduced quantitatively via the elimination of alternative mechanisms. Among those mechanisms identi® ed as being important in detecting ocean-radar features are: changes in wind regimes (Liu 1984); atmospheric instability (air-sea temperature diVerences) and wind regimes (Topliss et al 1994); a sea surface temperature (SST) bias (Liu 1984); atmospheric instability processes and wind direction (La Violettte 1983); wind direction with respect to the Gulf Stream current (Askari et al 1993); large oceanic surface slicks (Nilsson and Tildesley 1995 ) and the complex patterns resulting from inshore natural slicks combined with physical mechanisms (Thompson et al 1988 ).…”