Laboratory Observations of Short Wave Hydrodynamic Modulation by Gravity Waves

The modulating effect of gravity waves on shorter, wind-generated gravity-capillary waves has been the focus of inquiry in the wave and remote sensing research communities for some time. This phenomenon allows for the radar remote sensing of swell due to the periodic steepening and shallowing of Bragg scatterers orders of magnitude smaller than the swell itself. In order to more fully understand the specific hydrodynamic relationship between young, wind-generated gravity-capillary waves and longer gravity waves, a laboratory experiment was devised to observe changes in short wave spectral behavior over the phase of the long wave. Measurements were made in the University of Miami's SUSTAIN (SUrge-STructure-Atmosphere INteraction) facility in the ASIST (Air-Sea Interaction Saltwater Tank) wind-wave tank, with wind speeds ranging between 4 and 14 m/s and paddle-generated wave steepnesses "ak" varying between O(0.01-0.1). A polarimetric camera was used to capture high sampling frequency maps of wave slope, yielding spatiotemporal information about short wind wave behavior (provided as temporal variations in the wavenumber spectrum, where k ~O(10-1000) rad/m). The simultaneous and co-located long-wave phase was measured via a side-looking camera. Results are presented to show the modulation of gravity-capillary and pure capillary waves as variations in mean square slope and wave slope spectral density over the long wave phase. Conditions with low long-wave steepness and slight wind speed show modulation that is in-phase with the elevation time series, with roughening occuring on the long wave crest and smoothing occuring in the long wave trough. The behavior for high wave steepness and high wind conditions is starkly nonlinear, with variations in short wave spectral density in excess of one order of magnitude for the former and rendered almost nonexistent for the overpowering, storm-scale wind conditions in the latter case. Ultimately, these results offer an expansion of similar past work [Donelan et al. 2010] towards phase-coherent variations in the short wave spectra and build on the available body of data in support of the radar remote sensing of ocean surface waves.