BLOCKING AND TRAPPING OF WAVES IN AN INHOMOGENEOUS FLOW

The blocking and trapping of waves and wave packets by inhomogeneous flow fields were studied in a laboratory setting. Evidence for multiple reflections and thus trapping within the current gradient zone near the blocking point are the main results obtained, and lead to the following conclusions: (1) a strong current gradient zone near the blocking point can trap wave energy, which will shift the wavenumbers into the capillary region, to match the microwave wavelength of many remote sensing instruments; (2) this mechanism provides a direct link between strong current gradient zones (bathymetric features in tidal flows, eddies, current boundaries, etc.) and the surface wave structure, which in turn allows instruments such as the Synthetic Aperture Radar (SAR) to form images of bottom features based solely on the small wave conditions; (3) an indication of direct energy transfer from the carrier waves to low-frequency waves was also observed, and resolved by wavelet analysis. The instantaneous spectra from wavelet analysis reveal that a large portion of wave energy transfers directly into the low-frequency band from the carrier waves at the trapping zone. Subharmonics may play a critical role in the energy transfer process, but details are still to be established.