AN IMPROVED MODEL OF THE OCEAN SURFACE-WAVE VECTOR SPECTRUM AND ITS EFFECTS ON RADAR BACKSCATTER

Images of the ocean surface taken with active microwave sensors often contain much information on near-surface and subsurface processes. How ever, their interpretations may depend on a detailed understanding of the physics of electromagnetic scatter. In scattering theory, the surf ace hydrodynamics enters the equations via (1) the probability distrib ution function for either wave heights or slopes, and (2) the two-dime nsional wave height/slope autocovariance or its Fourier transform, the wave vector spectrum. This paper advances an improved model for the o cean surface wave vector spectrum based on recent work by M. A. Donela n, by M. L. Banner, and by B. Jahne and their collaborators. The model addresses the range of surface wavelengths from fully developed wind waves to the gravity-capillary region. For gravity-capillary waves, th e spectral equation satisfactorily represents the observational data o f Jahne et al. taken in tanks at large fetches, in the range from appr oximately 50 to 1500 rad/m to within the accuracy of the data. From th e spectrum, the two-dimensional autocovariance of the sea surface is c omputed and correlation lengths and curvatures obtained. When used wit h a modification of Holliday's formulation of microwave radar backscat ter from a Gaussian sea, it quantitatively reproduces observational cr oss section data taken at vertical polarization from aircraft and spac ecraft over the open ocean, with differences from the field data havin g a mean of -0.2 dB and a standard deviation of 1.7 dB. The range of p arameters for which satisfactory fits are obtained includes: wind spee ds from 1.5 to 24 m/s; frequencies from approximately 5.5 to 35 GHz; a nd incidence angles from 0-degrees to greater than 60-degrees. For hor izontal polarization, the scattering calculations fail rather badly fo r larger incidence angles, as do all theories based on the Kirchhoff a pproximation. Additionally, in spite of the incorporation of an anisot ropic angular distribution of wave energy, the observed azimuthal vari ation of radar scatter is not captured, indicating that the source of that variation lies elsewhere.