Radio Science

Microwave scattering and the straining of wind-generated waves


  • W. C. Keller,

  • J. W. Wright


The modulation in backscattered power from wind-generated waves due to the presence of a 0.575-Hz plunger-generated wave has been measured in a wave tank as a function of air friction velocity and plunger wave amplitude. The measurements were made at 9.375 GHz, a depression angle of 45° and vertical polarization. The straining of the wind waves is treated by a first-order perturbation of the Boltzmann transport equation and the scattering is calculated from a simple application of composite surface scattering theory utilizing first-order Bragg scattering. The theory predicts a characteristic relaxation behavior for the wind-speed dependence of the components of the modulation amplitude in phase and out of phase with the horizontal component of orbital velocity of the plunger wave. This relaxation behavior is closely followed by the observed modulation amplitudes for air friction velocities less than about 40 cm sec−1, i.e., winds less than about 7 m sec−1.