On the interaction between partially-reflected waves and an opposing wind

Air-sea interaction is a relevant topic with still many open questions, also in consideration of the numerous and complex scenarios that occur in nature. In this manuscript we report an experimental study of partially-reflected regular (long) waves under the action of an opposite wind, which generates short wind-waves (wind generated water waves, WGW). Wind blows against the incident wave, and therefore it is in the same direction as the reflected wave. Regular waves are generated by a paddle and are defined as mechanically generated water waves (MGW). MGW and WGW are separated by a spectral filtering technique, and free surface statistics of wave height, period and steepness are studied. The spatial variation induced by partial reflection of MGW (generated by paddles) is experimentally observed and theoretically described, and the effects of wind on reflection parameters are observed and examined. A phase-conditional analysis of wind-waves statistics, separating crest and trough, and upwind and downwind sides, reveals the importance of the local wind forcing, and captures features of the wind-waves modulated by the longer waves which are not detected with standard statistical analyses. The phase celerity of the wind-waves is analysed including the effects of the wind-induced shear current and the noninertial effects of the MGW; to the best of our knowledge, this last effect experimental analysis is novel. The study is relevant to characterize the wave field in front of coastal structures and beaches under swell and wind waves (co-linear in the same, and in the opposite direction) conditions, affecting run-up, overtopping and mean quantities modulated by the dynamics of group of waves.