Observations of ship-generated internal waves in SAR images from Loch Linnhe, Scotland, and comparison with theory and in situ internal wave measurements

In the 1989 Loch Linnhe experiment, ship-generated internal waves were imaged from airborne synthetic aperture radars (SARs) operating at various frequencies. In this paper, we examine K/sub a/, K/sub u/, X, and C-band images at moderate incidence angles (21-30/spl deg/) where peak-to-peak modulations of 3.3, 1.6, 1.3, and 0.7 dB, respectively, were observed. In situ time-series measurements of the internal-wave induced currents were made from a spar buoy. The SAR and current-meter data sets were compared by using the images to estimate the equivalent image intensity time series at the location of the spar. This was accomplished using linear internal wave theory with the Keller-Munk formulation of the wave-wake kinematics. The validity of this approach is confirmed by the good agreement obtained between predicted and observed wake geometries. Quantitative comparisons were made between measured radar modulations and predictions from wave-current interaction and radar scattering models. These comparisons indicate that the observed phase relationships between the radar modulation and the current are well predicted for each of the four radar frequencies, but that the magnitude of the predicted modulations is too low at the higher frequencies.