A Note on the Ocean Surface Roughness Spectrum

In a recent study, the dimensionless surface roughness spectrum has been empirically parameterized as a power-law function of the dimensionless wind speed expressed as the ratio of wind friction velocity and phase speed of the surface roughness wave component. The wave-number-dependent proportionality coefficient, A. and exponent, a, of the power-law function are derived from field measurements of the short-wave spectrum. To extend the roughness spectrum model beyond the wavenumber range of field data, analytical functions are formulated such that A and a approach their asymptotic limits: A(0) and a(0) toward the lowest wavenumber, and A(infinity), and a(infinity) toward the highest wavenumber. Of the four asymptotic values, A(infinity) is considered most questionable for the lack of reference information. When applied to the normalized radar cross-section (NRCS) computation, the results are in good agreement (within about 2 dB) with field data or geophysical model functions (GMFs) for incidence angles between 20 degrees and 40 degrees but significant underestimation occurs for higher incidence angles. The comparison study of NRCS computation offers helpful guidelines for adjusting the asymptotic factors, especially the numerical value of A(infinity). Improved agreement between the computed NRCS (vertical polarization) using the new roughness spectrum with GMF is expanded to incidence angles between 20 degrees and 60 degrees. The wind speed range of good agreement between calculation and GMF is below about 15 m s(-1) for Ku band and about 30 m s(-1) for C band.