Wave Boundary Layer Turbulence over Surface Waves in a Strongly Forced Condition

AbstractAccurate predictions of the sea state–dependent air–sea momentum flux require a thorough understanding of the wave boundary layer turbulence over surface waves. A set of momentum and energy equations is derived to formulate and analyze wave boundary layer turbulence. The equations are written in wave-following coordinates, and all variables are decomposed into horizontal mean, wave fluctuation, and turbulent fluctuation. The formulation defines the wave-induced stress as a sum of the wave fluctuation stress (because of the fluctuating velocity components) and a pressure stress (pressure acting on a tilted surface). The formulations can be constructed with different choices of mapping. Next, a large-eddy simulation result for wind over a sinusoidal wave train under a strongly forced condition is analyzed using the proposed formulation. The result clarifies how surface waves increase the effective roughness length and the drag coefficient. Specifically, the enhanced wave-induced stress close to the water surface reduces the turbulent stress (satisfying the momentum budget). The reduced turbulent stress is correlated with the reduced viscous dissipation rate of the turbulent kinetic energy. The latter is balanced by the reduced mean wind shear (satisfying the energy budget), which causes the equivalent surface roughness to increase. Interestingly, there is a small region farther above where the turbulent stress, dissipation rate, and mean wind shear are all enhanced. The observed strong correlation between the turbulent stress and the dissipation rate suggests that existing turbulence closure models that parameterize the latter based on the former are reasonably accurate.

Download Full-text

NEW HIGH-RESOLUTION MEASUREMENTS OF WAVE BOUNDARY LAYER FLOW UNDER FULL-SCALE SURFACE WAVES

Coastal Engineering 2008 ◽

10.1142/9789814277426_0129 ◽

2009 ◽

Author(s):

Jolanthe J. L. M. Schretlen ◽

Jebbe J. van der Werf ◽

Jan S. Ribberink ◽

Maarten Kleinhans ◽

W. Michel Zuijderwijk ◽

...

Keyword(s):

Boundary Layer ◽

High Resolution ◽

Surface Waves ◽

Boundary Layer Flow ◽

Full Scale ◽

Wave Boundary Layer ◽

Layer Flow ◽

Wave Boundary ◽

Scale Surface

Download Full-text

Wave-Driven Wind Jets in the Marine Atmospheric Boundary Layer

Journal of the Atmospheric Sciences ◽

10.1175/2007jas2562.1 ◽

2008 ◽

Vol 65 (8) ◽

pp. 2646-2660 ◽

Cited By ~ 49

Author(s):

Kirsty E. Hanley ◽

Stephen E. Belcher

Keyword(s):

Boundary Layer ◽

Atmospheric Boundary Layer ◽

Surface Waves ◽

Momentum Transfer ◽

Momentum Flux ◽

Ocean Surface ◽

Wave Age ◽

Marine Atmospheric Boundary Layer ◽

Induced Stress ◽

Wave Induced

Abstract The interaction between ocean surface waves and the overlying wind leads to a transfer of momentum across the air–sea interface. Atmospheric and oceanic models typically allow for momentum transfer to be directed only downward, from the atmosphere to the ocean. Recent observations have suggested that momentum can also be transferred upward when long wavelength waves, characteristic of remotely generated swell, propagate faster than the wind speed. The effect of upward momentum transfer on the marine atmospheric boundary layer is investigated here using idealized models that solve the momentum budget above the ocean surface. A variant of the classical Ekman model that accounts for the wave-induced stress demonstrates that, although the momentum flux due to the waves penetrates only a small fraction of the depth of the boundary layer, the wind profile is profoundly changed through its whole depth. When the upward momentum transfer from surface waves sufficiently exceeds the downward turbulent momentum flux, then the near-surface wind accelerates, resulting in a low-level wave-driven wind jet. This increases the Coriolis force in the boundary layer, and so the wind turns in the opposite direction to the classical Ekman layer. Calculations of the wave-induced stress due to a wave spectrum representative of fast-moving swell demonstrate upward momentum transfer that is dominated by contributions from waves in the vicinity of the peak in the swell spectrum. This is in contrast to wind-driven waves whose wave-induced stress is dominated by very short wavelength waves. Hence the role of swell can be characterized by the inverse wave age based on the wave phase speed corresponding to the peak in the spectrum. For a spectrum of waves, the total momentum flux is found to reverse sign and become upward, from waves to wind, when the inverse wave age drops below the range 0.15–0.2, which agrees reasonably well with previously published oceanic observations.

Download Full-text

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

Journal of Physical Oceanography ◽

10.1175/jpo-d-19-0070.1 ◽

2019 ◽

Vol 49 (8) ◽

pp. 1997-2015 ◽

Cited By ~ 11

Author(s):

Nyla T. Husain ◽

Tetsu Hara ◽

Marc P. Buckley ◽

Kianoosh Yousefi ◽

Fabrice Veron ◽

...

Keyword(s):

Boundary Layer ◽

Surface Roughness ◽

Surface Waves ◽

Wind Stress ◽

Wind Profile ◽

Wave Crest ◽

Turbulent Stress ◽

Boundary Layer Turbulence ◽

The Mean ◽

The Impact

AbstractThe 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.

Download Full-text

Wave boundary layer model in SWAN revisited

Ocean Science ◽

10.5194/os-15-361-2019 ◽

2019 ◽

Vol 15 (2) ◽

pp. 361-377 ◽

Cited By ~ 1

Author(s):

Jianting Du ◽

Rodolfo Bolaños ◽

Xiaoli Guo Larsén ◽

Mark Kelly

Keyword(s):

Boundary Layer ◽

Source Function ◽

Source Terms ◽

Layer Model ◽

Wave Boundary Layer ◽

Boundary Layer Model ◽

Wind Input ◽

Wave Induced ◽

The Impact ◽

Wave Boundary

Abstract. In this study, we extend the work presented in Du et al. (2017) to make the wave boundary layer model (WBLM) applicable for real cases by improving the wind-input and white-capping dissipation source functions. Improvement via the new source terms includes three aspects. First, the WBLM wind-input source function is developed by considering the impact of wave-induced wind profile variation on the estimation of wave growth rate. Second, the white-capping dissipation source function is revised to be not explicitly dependent on wind speed for real wave simulations. Third, several improvements are made to the numerical WBLM algorithm, which increase the model's numerical stability and computational efficiency. The improved WBLM wind-input and white-capping dissipation source functions are calibrated through idealized fetch-limited and depth-limited studies, and validated in real wave simulations during two North Sea storms. The new WBLM source terms show better performance in the simulation of significant wave height and mean wave period than the original source terms.

Download Full-text