DETECTION OF OCEANIC FRONTS AT LOW GRAZING ANGLES USING AN X-BAND REAL APERTURE RADAR

We examine the radar signatures and changes in the surface roughness a ssociated with oceanic features in the low grazing angle (LGA) scatter ing regime. The X band (HH) radar signatures consist of high-amplitude sea spikes, step changes in the normalized radar cross-section (NRCS) modulations, and bright narrowbanded frontal structures. Using in sit u observations coupled with airborne precision radiation thermometer ( PRT-5) data, we show that the step changes in radar cross-section modu lations are associated with either thermal stability-induced stress va riations or current velocity variations, Superimposed on the step chan ges are additional modulations that result from wave breaking and hydr odynamic straining. The amplitudes of the NRCS LGA measurements are co mpared with the predictions of four backscattering models: the Bragg, the tilted-Bragg, the wedge, and the plume model. It is shown that whi le the simple Bragg model can describe the measurements to a limited d egree, it generally tends to underpredict the results. Agreement is im proved by including the tilt contribution from the longwave surface wa ves in the context of the composite scattering model. We use the wedge and plume models as the basis for explaining the cross sections assoc iated with the high-amplitude sea spikes. The wedge model is used to d escribe scattering from sharply crested waves, and the plume model is used to. describe the extreme cross sections that are associated with breaking waves near the fronts. In describing the probability density function characteristics we show that the backscattering statistics ex hibit ''K distribution'' behavior for the Gulf Stream current region a nd near-frontal regions, while the cooler shelf waters have characteri stics of an exponential distribution.