An analysis of the effects of swell and surface roughness spectra on microwave backscatter from the ocean

The spectrum of ocean surface roughness is significantly modified by the presence of background long waves not generated by local wind. Active radar scattering and passive microwave emission from the ocean surface are therefore modified by swell conditions. Here we investigate predictions of the normalized radar cross section ( NRCS) of the sea by a multiscale radar scattering model using four different spectral functions, one of which accounts for swell effects. Variations in predicted NRCS using the different spectral functions are quantified. As a result, the effect of swell on microwave backscatter can be separated from uncertainty due to the form of the spectrum without swell. The tilting effects of swell are also examined, and their effect on backscatter is calculated using the model. We find that changes in the ocean surface roughness spectrum due to swell reduce the wind speed dependence of the NRCS at low and moderate incidence angles while tilting effects produce changes in both the incidence angle and wind speed behavior of the NRCS. In general C band NRCS measurements are better explained by the multiscale model and less sensitive to choice of roughness spectral model than are Ku band NRCS values.