Rogue waves are unexpected, individual ocean surface waves that are disproportionately large compared to the background sea state. They present considerable risk to mariners and offshore structures when encountered in large seas. Rogue waves have gone from seafarer’s folktales to an actively researched and debated phenomenon. In this work an easily derived spectral parameter, as an indicator of rogue wave risk, is presented, and further evidence for the generation mechanism responsible for these abnormal waves is provided. With the additional goal of providing a practical rogue wave forecast, the ability of a standard wave model to predict the rogue wave probability is assessed. Current forecasts, like those at the European Centre for Medium-Range Weather Forecasts (ECMWF), rely on the Benjamin Feir Index (BFI) as a rogue wave predictor, which reflects the nonlinear process of modulation instability as the generation mechanism for rogue waves. However, this analysis finds BFI has little predictive power in the real ocean. From the analysis of long term sea surface elevation records in nearshore areas and hourly bulk statistics from open ocean and coastal buoys in the Northeast Pacific, crest-trough correlation shows the highest correlation with rogue wave probability. These results provide evidence in support of a probabilistic prediction of rogue waves based on random linear superposition and should replace forecasts based on modulation instability. Crest-trough correlation was then forecast by a regional WAVEWATCH III ® wave model with moderate accuracy compared with the high performance of forecasting significant wave height. Results from a case study of a large fall storm October 21-22, 2021, are presented to show that the regional wave model produces accurate forecasts of significant wave height at high seas and presents a potential rogue wave probability forecast. / Graduate

La physique de la couche limite atmosphérique en domaine océanique est principalement régie par les processus couplés liés au vent, à l’état de mer local, et à des effets de flottabilité. Leur compréhension reste néanmoins parcellaire et leurs descriptions théoriques et stochastiques sont pour le moins lacunaires, lorsqu’elles ne sont tout simplement pas mises à mal par les rares observations. Dans un contexte d’exploitation croissante de la ressource éolienne offshore, la mise en place de méthodes numériques visant à une description plus fine des propriétés turbulentes de cette couche limite sera une étape déterminante dans la réduction des coûts et l’optimisation des structures pour des rendements de récupération d’énergie améliorés. Ainsi, un outil numérique a été mis en place afin d’étudier le couplage entre un écoulement atmosphérique et l’état de mer. Un code Large-Eddy Simulation massivement parallèle pour la simulation des écoulements atmosphériques incompressibles développé par P. Sullivan au National Center for Atmospheric Research est couplé à un code spectral d’états de mer non-linéaires développé au Laboratoire de recherche en Hydrodynamique, Energétique et Environnement Atmosphérique. De nombreuses configurations de vents et d’états de mer sont modélisées. On montre que les lois semi empiriques souvent utilisées pour représenter la distribution verticale de la vitesse moyenne du vent sont une bonne approximation dans les situations où un petit état de mer est soumis à un fort vent. Néanmoins, dans le cas de houles très rapides se propageant dans des zones de faible vent, la création d’un jet de vent par la houle invalide ces lois semi-empiriques. / Modelling the dynamic coupling of ocean-atmosphere systems requires a fundamental and quantitative understanding of the mechanisms governing the windwave interactions: despite numerous studies, our current understanding remains quite incomplete and, in certain conditions, sparse field observations contradict the usual theoretical and stochastic models. Within the context of a growing exploitation of the offshore wind energy and the development of met ocean models, a fine description of this resource is a key issue. Field experiments and numerical modelling have revealed that atmospheric stability and wave effects, including the dynamic sea surface roughness, are two major factors affecting the wind field over oceans. A numerical tool has been implemented in order to study the coupling between an atmospheric flow and the seastate. A massively parallel large-eddy simulation developed by P. Sullivan at the National Center for Atmospheric Research is then coupled to a High-Order Spectral wave model developed at the Hydrodynamics,Energetics & Atmospheric Environment Laboratory in Ecole Centrale de Nantes. Numerous configurations of wind and sea states are investigated. It appears that, under strongly forced wind conditions above a small sea state, the semi-empirical laws referred to as standards in the international guidelines are a good approximation for the vertical profile of the mean wind speed. However, for light winds overlying fast-moving swell, the presence of a wave induced wind jet is observed, invalidating the use of such logarithmic laws.