Bound waves and Bragg scattering in a wind-wave tank

• Published in: Journal of Geophysical Research 1999-02, Vol.104 (C2), p.3243-3263
• Main Authors: Plant, William J., Keller, William C., Hesany, Vahid, Hara, Tetsu, Bock, Erik, Donelan, Mark A.
• Format: Article
• Language: English
• Subjects: Earth, ocean, space; Exact sciences and technology; External geophysics; Marine; Marine optics and radiation energy; Marine optics and underwater sound; Physics of the oceans
• ISSN: 0148-0227; 2169-9275; 2156-2202; 2169-9291
• DOI: 10.1029/1998JC900061

We present optical and microwave measurements that show the presence of bound waves traveling at the speed of the dominant wave in a wind‐wave tank. We suggest that when these bound waves are much shorter than the dominant waves, they are preferentially located on the leeward face of the dominant wave and hence have a mean tilt. We hypothesize that the turbulence associated with these bound waves suppresses freely propagating, wind‐generated waves where bound waves are present so that we may divide the rough water surface into patches containing free and patches containing bound waves. This model is shown to account for the observed histograms of slope measured in the tank and, at least qualitatively, for the observed decrease in the probability of finding bound waves with increasing wind speed. Furthermore, if we add these bound, tilted waves to the free waves of the standard Bragg/composite‐surface scattering model for microwave scattering from rough water surfaces, then the model can account for many otherwise unexplained features of the scattering. Principal among these features are the rapid decrease in polarization ratio and rapid increase in the first moment of the microwave Doppler spectrum with increasing wind speed when the antenna is directed upwind, features that occur to a much lesser extent when the antenna looks downwind.