Normalized radar cross section of the sea for backscatter: 2. Modulation by internal waves

We report measurements of microwave surface signatures of internal waves with dual‐polarized, coherent, X band radars mounted on three ships and an airplane. In shipboard measurements in the South China Sea, internal waves generally increased the backscattering cross section near the peaks of the internal waves with little detectable decrease afterward. The peak of the cross‐section signature shifted its location relative to the internal wave crest depending on the maximum strain rate of the internal wave. We show that a similar shift is produced in the modulation of short‐gravity waves by internal waves. We suggest that this modulation of “intermediate‐scale” waves causes small‐scale radar scatterers to maximize at this location. In shipboard measurements off the New Jersey coast, the range resolution of the radar was sufficiently small to allow us to detect significant modulation of gravity waves on the order of 15–30 m long by the internal waves. At the high incidence angles of the shipboard measurements, the cross section for horizontally polarized transmit and receive signals, σo(HH), regularly exceeded that for vertically polarized transmit and receive signals, σo(VV), near the internal wave crest by 5–10 dB. At the more moderate incidence angles observed from the aircraft, maximum values of σo(HH) and σo(VV) are more nearly equal, with σo(HH) often being comparable to σo(VV) and only occasionally exceeding it. These observations suggest that roughness due to breaking short‐gravity waves plays a significant role in producing microwave signatures of internal waves, even at moderate incidence angles where it competes with the modulation of wind‐generated waves. The intensity of modulation of the cross section caused by internal waves observed from the plane depended little on the direction of observation. Internal wave surface signatures from the aircraft became less visible with increasing wind speed, being very difficult to observe at 9 m/s.