The Synthetic Aperture Radar (SAR) around the western entrance of the Tsugaru Strait has yielded well-defined images of an internal-wave packet accompanied by two or three surface-streaks (surface convergence within the same water mass), which is active during the stratified season, and whose wavelength is in the order of several hundred meters. Most of the waves in the packet were observed near the topographically shallow parts of the sill. Temporal changes were repeatedly observed in the internal waves confined to the sill using a high-frequency echosounder profiler within one-day in the summer 2017. The acoustic images suggest that a wave packet of extraordinary amplitude (> 150 m) has developed transiently around the downstream side of the sill at the ascending passage flow. This wave packet consists of two or three successive streak bands, with very disturbed sea surface conditions overlying wave troughs, i.e., strong downwelling areas. The dynamics of such waves developing over the sill is studied through a fully nonlinear nonhydrostatic numerical model. The vertical fluid stratification and temporal change of the barotropic passage flow were adjusted to approximate our observation conditions. The results suggest that the wave packet is effectively amplified near the downstream side of the sill, where the Froude number becomes a critical point, because upstream propagating waves on the sill slope stagnate and overlap efficiently. In this dynamical process, however, even if the wave grows to large amplitude, it does not form a well-organized solitary wave, but is rather scattered due to the strong dispersion of waves.

The progress of studies on ocean waves using high-frequency (HF) ocean radars is re viewed. All of the free-wave components are related to the Doppler spectrum of the HF radar. Ocean wave spectra can be estimated using HF radars by various methods, such as the semiempirical method, parameter fitting method, linear inversion method, and nonlinear inver sion method . The semiempirical method is widely used for estimating wave parameters by evaluating unknown factors in the equation relating the Doppler spectrum and wave parame ters. The parameter fitting method is adopted in broad beam radar systems. The linear inver sion method is a well-known method to estimate a directional spectrum by approximating the nonlinear equation that relates the Doppler spectrum and wave spectrum to a linear equation. The nonlinear inversion method has been extensively studied in Japan. The accuracy of wave estimation using HF radars, and thus, coastal wave prediction, will be improved by selecting high signal-to-noise ratio Doppler spectra for analyses.