Synthetic Aperture Radar Imaging of Internal Waves

Over the past 10 years, synthetic aperture radars (SAR) have frequently detected pronounced signatures of nonlinear, coastal internal waves 1-6 that are produced by the interaction of short-wavelength surface waves with internal wave-induced surface currents. A qualitative representation of the signature generation process is shown in Fig. 1 for a group of nonlinear internal waves. At the leading edge of each wave, there is a region of convergent flow that produces increased surface roughness; flow divergence at the trailing edge diminishes the surface roughness. The radar image consists of adjacent bright and dark bands, coincident with the rough and smooth regions, respectively, on a nearly uniform background associated with the mean local roughness. In principle, it should be possible to combine a theoretical description for the internal wave/surface wave interaction process 7 with a model for radar backscatter from the sea surfaces in order to compute the intensity of the SAR signatures and their dependence on environmental and imaging conditions. Unfortunately, most existing SAR images of internal waves have been obtained without the coincident in situ observations needed to characterize the internal wave properties and the surface wave conditions. Without such supporting information, it is not possible to assess the degree to which existing models can account for the features observed in the radar images. To rectify the situation, the Office of Naval Research is sponsoring research efforts to investigate quantitatively the SAR imaging of oceanic internal wave surface effects. The centerpiece of the program is the SAR Internal Wave Signature Experiment (SARSEX), conducted in the New York Bight off the coast of Long Island in the late summer and early fall of 1984 using tidal-generated, nonlinear internal waves. This article summarizes the activities during SARSEX and presents some initial results.