Observations of internal waves at X and Ka bands, and comparison with model calculations

Internal waves were observed with X and K_a-band shipboard mounted scatterometers operated at mid incidence angles during the Joint U.S./Russia Experiment conducted in the New York Bight during July 1992. The X-band scatterometer was dual polarized (H and V) and operated at 58° incidence. The K_a-band system was vertically polarized and operated predominantly at 70° incidence. Both scatterometers were fully coherent, and baseband data was recorded digitally at a rate sufficient to allow the unaliased Doppler spectrum to be computed. Cross sections were computed by integrating the pertinent portion of the Doppler spectrum. This technique avoided contamination from antenna sidelobes which were pointed near the ship structure. Many crossings of tidally generated internal waves were made under a variety of wind speeds and stability conditions. Increases and decreases in the X and K_a-band backscatter cross sections were well correlated with in-situ measurements made by a profiling CTD fish. The modulations observed in both bands were comparable, but did not always agree in detail. In addition, a side-looking incoherent X-band radar provided a spatial picture of the surface manifestations of internal waves up to several km from the ship. The observations are compared with model calculations based on a wave-current interaction and scattering code which has been developed over the past decade.