A MODEL FOR MICROWAVE DOPPLER SEA RETURN AT HIGH-INCIDENCE ANGLES - BRAGG SCATTERING FROM BOUND, TILTED WAVES

We show that if ocean surface waves of the order of a few meters long are frequently steep enough to generate bound centimetric waves, then composite surface scattering theory can account for many anomalous pro perties of microwave backscatter from the sea at high incidence angles . The model proposed here postulates that these intermediate waves are made sufficiently steep to generate bound centimetric waves because o f their modulation by longer, dominant ocean surface waves. The bound centimetric waves have a nonzero mean tilt because they are located on the steep forward face of the intermediate waves, and they move at th e speed of the intermediate waves. Applying composite surface scatteri ng theory to this sea surface model, we show that much of the apparent ly anomalous behavior of microwave sea return measured at incidence an gles between 50 degrees and 80 degrees during the Synthetic Aperture R adar and X Band Ocean Nonlinarities-Forschungsplatform Nordsee (SAXON- FPN) experiment can be explained using reasonable parameters to charac terize the surface waves. In the SAXON-FPN measurements the mean value s of the first moments of microwave Doppler spectra for horizontally p olarized backscatter differ from those for vertically polarized backsc atter by an amount which varies with the incidence angle and with the azimuthal angle between the radar look direction and the direction of the dominant wave. The modulation of this first moment by surface wave s tens of meters in length is the same for the two polarizations at lo w to moderate incidence angles and can be interpreted in terms of the advection of free centimetric waves by the long waves. At higher incid ence angles, however, this modulation is different for the two polariz ations and cannot be explained by simple advection of free waves. Fina lly, microwave cross sections measured at high incidence angles using horizontal polarization are much larger than can be explained by a com posite surface theory that includes only freely propagating centimetri c waves. Most of these effects can be explained by the composite surfa ce model presented here, which includes Bragg scattering from both fre e and bound, tilted waves.