Experimental study on the variation characteristics of focused wave packets with wave spectra scaled from different stages of wave growth

This study investigated the relationship between wave breaking and spectral evolution, focusing on the variations in wave spectra caused solely by breaking under different initial spectral shapes. Laboratory experiments utilizing a controlled wave focusing technique were conducted to generate breaking wave trains with input spectra derived from SWAN model simulations representing 12 distinct stages of wave growth under varying wind durations. Experiment data were combined with HOS simulations to isolate the breaking spectral changes from nonlinearity. Results reveal that the Benjamin-Feir instability dominates the nonlinear interactions for young waves, exhibiting an asymmetric shift towards higher wavenumbers. The characteristics of breakinginduced spectral changes depend on the initial spectral shape and consistent with the "two-phase behavior" of wind wave breaking. The study reveals new insights into the increase in the low-frequency energy, finding that this gain was primarily associated with the breaking of dominant waves near the spectral peak, whereas highfrequency breaking played a minimal role in transferring energy towards lower frequencies. A strong correlation between Qp and the proportion of spectral dissipation in different frequency ranges was revealed, suggesting that Qp can serve as a valuable parameter for characterizing the spectral evolution of breaking waves at various stages of development.