On the origin of bright lines and other features in X-band radar imagery of frontal surface velocity fields

Enhanced radar backscatter occurs in radar imagery of strongly convergent ocean currents, but the origin of this phenomenon is not well-understood. Although the Alpers and Hennings (AH) relaxation model indicates that variations in radar cross-section (RCS) intensity at convergent fronts are proportional to the magnitude of the local current convergence, at higher radar frequencies, this model significantly underpredicts RCS enhancement and exhibits a non-physical look-angle dependence. Full-wave spectral modeling incorporating composite Bragg backscatter does not remedy the situation. In developing a procedure for extracting estimates of the near frontal 2-dimensional surface velocity fields of convergent ocean frontal features from radar imagery, the authors have identified a plausible explanation, empirically, that accounts for the correlation between the presence of bright lines in X-band radar imagery near convergent fronts and the magnitude of the local current convergence. The authors are able to quantitatively infer a 2-dimensional model of the currents, using radar data simulations of wave spectra, and radar cross-section RCS. In the resulting model, enhanced radar intensity occurs where the local convergence increases because of enhanced wave-steepening and wave-breaking. Other effects associated with current structure (for example, shear) are discussed