Wind Wave Generation : Turbulent Windprint below the Wave Onset and its Link with O. M. Phillips 1957 Theory

When the wind blows on a liquid surface above a speed threshold of few meter per second, it generates surface waves that could be amplified either by shear instability [Miles 1957] or resonance between turbulent pressure fluctuations and surface waves [Phillips 57]. In the low wind speed regime, surface waves are not amplified and the surface remains almost flat. The surface deformation, however, does not vanish. A. Paquier et al. have identified in 2015 a regime below the onset of wave generation called wrinkle. In that regime, desorganized patterns of small amplitude (1 ~ 10 µm) propagate on the liquid surface. We combine theory, numerics and experiments to investigate this wrinkle regime and disentangle their generation from pressure or shear sources. I will show that these wrinkles are the remniscence of pressure fluctuations in the air boundary layer that are {\emph windprint} on the liquid surface. These wrinkles are a base state for the wind wave generation and we show that they correspond to a Phillips type regime saturated by viscous dissipation. The existence of this wrinkle regime and our underlying theory may shed a new light on the possible role of turbulent fluctuations on the wind wave onset.