The normalized radar cross section of the sea at 10 degrees incidence

Measurements of the normalized radar cross section of the sea at K-u band at an incidence angle of 10 degrees were performed from a manned airship off the Oregon coast in September and October of 1993. The cross section at this incidence angle is often assumed to hare little dependence on windspeed and direction, Our measurements, however, indicate that at windspeeds below 6-7 mis, the cross section is in fact dependent on these quantities, and the azimuthal modulation can reach values on the order of 5-8 dB. Comparisons of the measured values with the predictions of the quasispecular scattering model are presented. The theory is shown to be accurate in predicting the azimuthal modulation and the strength of the backscatter if the effects of swell are included or if measured wind directions are ignored and the upwind direction is forced to be near the maximum cross section. Values of mean-square wind-wave slope and effective-reflection coefficient required to obtain these fits are very close to those obtained by previous investigators. In particular, mean-square wind-wave slopes are about 70-80 % of those of Cox and Munk (1953) because the radar responds only to facets larger than about 10 cm, with smaller ripples acting to reduce the reflection coefficient. If swell is included, we find that mean-square slopes in the direction of the swell, that are as much as ten times the measured smell slopes, are required to fit the model to the cross-section data at low windspeeds. We suggest that this may be due to high-order effects of the hydrodynamic modulation of short waves by the swell, We believe that this explanation is more likely than assuming that wind directions were incorrectly measured.