Boundary Layer Turbulence over Surface Waves in a Strongly Forced Condition: LES and Observation

Abstract

The impact of sea state on air–sea momentum flux (or wind stress) is a poorly understood component of wind–wave interactions, particularly in high wind conditions. The wind stress and mean wind profile over the ocean are influenced by the characteristics of boundary layer turbulence over surface waves, which are strongly modulated by transient airflow separation events; however, the features controlling their occurrence and intensity are not well known. A large-eddy simulation (LES) for wind over a sinusoidal wave train is employed to reproduce laboratory observations of phase-averaged airflow over waves in strongly forced conditions. The LES and observation both use a wave-following coordinate system with a decomposition of wind velocity into mean, wave-coherent, and turbulent fluctuation components. The LES results of the mean wind profile and structure of wave-induced and turbulent stress components agree reasonably well with observations. Both LES and observation show enhanced turbulent stress and mean wind shear at the height of the wave crest, signifying the impact of intermittent airflow separation events. Disparities exist particularly near the crest, suggesting that airflow separation and sheltering are affected by the nonlinearity and unsteadiness of laboratory waves. Our results also suggest that the intensity of airflow separation is most sensitive to wave steepness and the surface roughness parameterization near the crest. These results clarify how the characteristics of finite-amplitude waves can control the airflow dynamics, which may substantially influence the mean wind profile, equivalent surface roughness, and drag coefficient.

Impact of Shoaling Ocean Surface Waves on Wind Stress and Drag Coefficient in Coastal Waters: 1. Uniform Wind

“…Babanin et al (2007) suggested that the growth rate should be enhanced to model the observed enhancement. However, Husain et al (2019) found that the wave growth rate actually decreases with wave steepness under strong wind forcing. This result is consistent with previous studies from the field (Donelan et al, 2006) and in the laboratory (Peirson & Garcia, 2008).…”

Wind-induced changes to surface gravity wave shape in deep to intermediate water

“…All results are plotted using the full, O(ε 2 )-accurate expressions (A 57) and (A 58). The grey lines are the results for a fourth-order unforced Stokes wave, and the green dotted line represents ψ P = 3π/4 used in many of the other plots and supported by numerical simulations from Hara & Sullivan (2015) and Husain et al (2019). O(ε 2 )-accurate generalized Miles a 2 /(a 2 1 k) G (A 57) is also plotted in figures 3(b), 4(b) and 5(b).…”

“…In the dynamic boundary condition (2.5), we incorporated the normal stress (surface pressure) but neglected the shear stress as it is usually significantly smaller than the normal stress (e.g. Kendall 1970;Hara & Sullivan 2015;Husain et al 2019). Additionally, we note that surface shear stresses cause a slight thickening of the boundary layer, which is equivalent to a pressure phase shift on the remainder of the water column (Longuet-Higgins 1969).…”

“…where, c ∞ 0 = √ g/k is the unforced, linear, deep-water phase speed, ρ w is the water density and (2.11) is used to define P G . Using the value ψ P = 3π/4 from Hara & Sullivan (2015) and Husain et al (2019) gives P G k/(ρ w g) = 0.23 γ /f ∞ 0 . Furthermore, we use empirical data relating wind speed U to growth rate to constrain the P G pressure magnitude constant in deep water.…”

A Laboratory Study of the Effect of Surface Waves on Heat and Momentum Transfer at High Wind Speeds

“…The problem of dependency of the exchange coefficients in MABL on wind speed is closely connected with the question of the influence of surface waves and spray, that is, the products of wind‐forcing on the exchange of the momentum and mass between the atmosphere and ocean. The effect of waves on the momentum transfer has been studied intensively (see, e.g., Ayet et al, 2020; Husain et al, 2019; Janssen, 1989; Makin et al, 1995; Makin & Mastenbroek, 1996; Makin & Kudryavtsev, 1999; Moon et al, 2004; Takagaki et al, 2012; Takagaki, Komori, & Suzuki, 2016; Takagaki, Komori, Suzuki, et al, 2016; Troitskaya et al, 2012, 2017; Troitskaya, Druzhinin, et al, 2018; Troitskaya, Kandaurov, et al, 2018; Troitskaya & Rybushkina, 2008, and references therein). It has been shown that the momentum transfer by wave disturbances, or the form drag, increases with increasing wind speed due to the widening of the wave spectra.…”