Surface film effects on the radar cross section of the ocean surface

Ocean surface films, biogenic and/or hydrocarbon in nature, damp the gravity-capillary waves important for radar backscatter at microwave frequencies. Although petroleum and biogenic slicks have been observed by imaging radars on many occasions, there is no satisfactory model for ocean RCS with surface films present. The author reports results for a candidate model. Wave damping effects for the surface films are introduced into the calculation of the surface waveheight spectrum by a damping term calculated according to the approximate method of Cini and Lambardini and with viscoelastic data on various films drawn from the work, of Huhnerfuss and his colleagues at Hamburg University. The wave height spectrum model is a modification of the model of Durden and Vesecky. The effect of sea surface temperature on the bulk viscosity of the water is included. The resulting waveheight spectra are used to note the impact on ocean radar backscatter from L- to K_u-band. Model results indicate significant radar cross section reductions for a wind speed of 5 m/s. For this simple model the σ° reduction increases monotonically with radar frequency for constant film type and environmental and observational conditions. Predicted reductions vary over a 0 to 10 dB range depending on surface film type and water temperature. Results are compared with ocean observations of several types of artificial surface films by a five frequency, helicopter mounted, radar operated by Wismann and his colleagues from Hamburg University. The author finds that surface-film-wave damping, acting only as a loss term in the spectral model, can not account properly for the observed radar cross section reductions. Thus, it is concluded that a better model for surface film effects on σ° must include surface film effects in the wind driven source term for the wave spectrum and/or wave-wave interaction