Normalized radar cross section of the sea for backscatter: 1. Mean levels

[1] The normalized radar cross section of the sea for backscatter, σo, is investigated for incidence angles between 0° and 89° using data collected over more than two decades. The most recent measurements were made from several ships using a coherent, dual-polarized, X band radar. These measurements show that vertically polarized transmit and receive signals, σo(VV), at high incidence angles exhibit wind speed and azimuth angle dependence similar to those at lower incidence angles. They are nearly as large looking downwind as they are looking upwind and minimize near the crosswind direction. Horizontally polarized transmit and receive signals, σo (HH), behave differently at high incidence angles. They are largest looking upwind and smallest looking downwind. Fits of the multiscale model of microwave backscatter from the ocean to these data along with data collected previously at lower incidence angles show that over the whole range of incidence angles from 0° to 89°, σo(VV) is explained by the model, while measured σo(HH) values are generally higher than the model predicts at incidence angles above about 45°. Thus scattering phenomena exist on the ocean surface that affect HH backscatter very strongly at the higher incidence angles while impacting VV-polarized backscatter only slightly. This conclusion is strengthened by our observation of high-incidence-angle backscatter from the ocean where mean σo(HH) exceeds mean σo(VV) by as much as 15 dB. We examine phenomena that might account for this behavior and suggest that multipath dihedral-type features are likely to be important scatterers since they produce large σo(HH)/σo(VV) owing to Brewster damping of the first VV bounce.