Comparison of microwave backscatter with optically-derived cross sections from wind-generated waves in a wavetank

Microwave backscatter from wind-generated waves at intermediate incidence angles has been theoretically shown to be proportional to the spectral density of Bragg-resonant gravity-capillary and capillary waves. Explicit proof would be provided by direct measurement of the shortwave spectrum combined with calculations of the expected returns for a given microwave frequency and incidence angle. To obtain shortwave spectra independently, the authors used a calibrated two dimensional optical slope imaging system capable of resolving slopes from waves of wavelengths between 25 cm and 3 mm. The measured slope spectra were used with both simple Bragg and composite surface theory to derive cross section values for comparison against microwave measurements at X band (10 GHz) for vertical and horizontal polarization and K_a band (35 GHz) for V-pol. Overall, the more complex composite surface theory generally improved the fit between the optical and microwave measurements. The agreement seems to be best far friction velocities below 20 cm s^-1 and between 40 and 70 cm s^-1. For all friction velocities, however, composite surface theory somewhat underpredicts the actual cross sections in a majority of the cases. The authors conclude that while composite surface theory accounts for much of the backscatter at both frequencies in the incidence angle range examined, the discrepancy between the measured and predicted cross sections is sufficiently large that contributions from other scattering processes cannot be ruled out