COMPARISON OF OPTICALLY-DERIVED SPECTRAL DENSITIES AND MICROWAVE CROSS-SECTIONS IN A WIND-WAVE TANK

The most popular model of microwave backscatter from rough water surfa ces at mid-incidence angles (20 degrees < theta(i) < 70 degrees) is co mposite surface theory. This theory holds that the backscattered retur n is directly proportional to the spectral density of centimetric, Bra gg-resonant water waves which are tilted and advected by longer waves. A stringent test of this theory is to measure, independently and from the same surface area, the normalized microwave cross section (sigma( 0)) and the Bragg wave spectral density, and compare them using the th eory. In this paper, we use a calibrated optical slope imaging system in a wind-wave tank to measure the two-dimensional wavenumber spectrum of short waves. From these spectra, we calculate both the pure Bragg scattering sigma(0) which neglects longwave effects and the more compl ex composite surface sigma(0). The results are compared with ao obtain ed from backscatter measurements at X band (10 GHz) and K-a band (35 G Hz) made between 28 degrees and 68 degrees incidence angle. We find th at composite surface theory generally shows better agreement with expe riment at both frequencies than pure Bragg scattering theory. The agre ement seems best for friction velocities above 40 cm s(-1). For all fr iction velocities up to 70 cm s(-1), however, composite surface theory somewhat underpredicts the actual sigma(0) in a majority of the cases . This is especially true for horizontal polarization at large inciden ce angles. We conclude that while composite surface theory accounts fo r much of the backscatter at both frequencies in the incidence angle r ange we examined, the discrepancy between the predicted and measured c ross sections is sufficiently large that contributions from other scat tering processes cannot be ruled out.