Geometric and kinematic indicators of breaking inception in surface gravity waves

Abstract: The process of breaking in surface gravity waves can be characterized by two distinct stages. Breaking onset, defined as the first visible surface manifestation of breaking, is preceded by breaking inception, which is characterized by the initiation of an irreversible process within the crest that leads inevitably to breaking. Breaking inception diagnostics formulated using the local energetic, kinematic and geometric properties of the wave crest have recently been proposed that appear to provide generic parametric threshold estimates and may facilitate the effect of wave breaking on larger scale processes to be parameterized. In a recent numerical study [McAllister et al, J. Fluid Mech. 974, A14 (2023)], a breaking inception diagnostic was proposed based on a threshold value for the maximum local interface angle \thetamax{}. We extend these findings to include surface tension effects, which are an inescapable feature of ocean surface waves and are known to have a non-negligible effect on wave geometry. We conduct a suite of numerical breaking wave packet simulations, which incorporate surface tension, to show that $\Theta_\textrm{th}=60^{\circ}$ is a robust threshold value for gravity waves of varying packet size, water depth and wind speed forcing. This value is twice the magnitude of that for waves in the absence of surface tension. We explore this result in the context of the kinematic inception parameter $B$ [Barthelemy et al, J. Fluid Mech. 841, 463 (2018)] and show that the kinematic and geometric methods are in fact equivalent and incorporate the same aspects of the underlying physical processes leading to wave breaking.