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Oceanic Phenomena Introduction

Mesoscale oceanic phenomena become visible on SAR images because they are associated with a variable surface current which modulates the sea surface roughness and thus the normalized radar cross section (NRCS). The variable surface current also gives rise to a velocity-induced modulation of the SAR image intensity, called velocity bunching. This modulation is often - but not always - small compared to the NRCS modulations (see, e. g., Alpers and Hennings, 1984).

First-order theories describing the modulation of the NRCS by a variable surface current induced by tidal flow over underwater bottom topography have been developed by Alpers and Hennings (1984) and Shuchman et al. (1985). Since then more advanced theories have been developed, among others, by Thompson and Gasparovic (1986), Holliday et al. (1986), Lyzenga and Bennett (1988), Romeiser and Alpers (1997), and Lyzenga (1998). These theories also take into account the perturbation of intermediate scale waves by the surface current which in turn modulate the Bragg waves. It turns out that the strength of the modulation, also called modulation depth, is strongly dependent on the wind vector. The higher the wind speed, the smaller is the modulation depth (see, e. g., Brandt et al., 1999). This implies that in the tropics and subtropics where, on the average, the wind speed is lower than at higher latitudes, the mesoscale oceanic features are better visible on ERS SAR images as compared with images acquired over higher lattitude ocean areas.

References
  • Alpers, W. & Hennings, I., A theory of the imaging mechanism of underwater bottom topography by real and synthetic aperture radar, J. Geophys. Res., 89, 10529-10546 (1984).
  • Brandt, P., Romeiser, R. & Rubino, A., On the determination of characteristics of the interior ocean dynamics from radar signatures of internal solitary waves, J. Geophys. Res., 104, 30039-30045 (1999).
  • Holliday, D., St-Cyr, G. & Woods, N.E., A radar ocean imaging model for small to moderate incidence angles, Int. J. Remote Sens., 7, 1809-1834 (1986).
  • Lyzenga, D.R., Effects of intermediate-scale waves on radar signatures of ocean fronts and internal waves, J. Geophys. Res., 103, 18759-18768 (1998).
  • Lyzenga, D.R. & Bennett, J.R., Full-spectrum modeling of synthetic aperture radar internal wave signatures, J. Geophys. Res., 93, 12345-12354 (1988).
  • Romeiser, R. & Alpers, W., An improved composite surface model for the radar backscattering cross section of the ocean surface. 2. Model response to surface roughness variations and the radar imaging of underwater bottom topography, J. Geophys. Res., 102, 25251-25267 (1997).
  • Shuchman, R.A., Lyzenga, D.R. & Meadows, G.A., Synthetic aperture radar imaging of ocean-bottom topography via tidal-current interactions: Theory and observations, Int. J. Remote Sens., 6, 1179-1200 (1985).
  • Thompson, D.R. & Gasparovic, R.F., Intensity modulation in SAR images of internal waves, Nature, 320, 345-348 (1986).

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