Calculation of Microwave Doppler Spectra from the Ocean Surface with a Time-Dependent Composite Model

Doppler spectra are computed using a time-dependent composite model. This model reduces to the proper specular and small-perturbation limits for V-V and H-H polarization, and its time dependence is based on the assumption of linear surface evolution. The width of the computed spectra increases with increasing radar frequency and decreasing incidence angle due primarily to the motion of the long-wave surface. This motion also produces a shift in the spectral peak away from the Bragg frequency. This shift, which is largest at lower incidence angles and higher radar frequencies, is also due to the effect of long-wave motion on the backscattered field. At incidence angles ≥ 45 deg, where most of the Doppler measurements exist, the calculations are in good general agreement with the data.