Radar Imaging of Current Features:
A Synergy of Backscatter Power and Doppler Shift

Recently two new independent methods for quantitative studies of SAR imaging of current features were published in the July 2005 issue of Journal of Geophysical Research-Ocean. They are the Doppler shift approach (Chapron et al. 2005), and the radar imaging model (RIM), (Kudryavtsev et al. 2005; Johannessen et al. 2005). The study presented here combines these two approaches in a unified DopRim model. This model simulates modulation of radar cross-section and Doppler shift caused by various sea surface phenomena, like: homogeneous or spatially varying currents and near surface winds, temperature fronts and contaminants.

The role of different scattering facets (Bragg waves, specular reflected wave slopes and breakers), as well as intermediate scale waves tilting and modulating facets for different geometry of radar observation (radar wavelength, incidence angle and polarization) is analysed. It is shown that the modulation of the wind wave spectrum by surface current leads to a redistribution of the return power between the scattering facets. This, in turn, leads to modulation of the Doppler velocity making it distinct from the pure surface current. Results of these model simulations are presented for a range of surface current phenomena.

A comparison and assessment of these DopRim model simulations with available observations on background dependence of backscatter power and Doppler shift on wind speed and sea state, as well as their modulations by internal waves, surface slicks and coastal current are given. Overall the agreement is promising, although a number of uncertainties still remain suggesting that further development and refinements are needed.