Wind Profile and Drag Coefficient over Mature Ocean Surface Wave Spectra

Abstract The mean wind profile and the Charnock coefficient, or drag coefficient, over mature seas are investigated. A model of the wave boundary layer, which consists of the lowest part of the atmospheric boundary layer that is influenced by surface waves, is developed based on the conservation of momentum and energy. Energy conservation is cast as a bulk constraint, integrated across the depth of the wave boundary layer, and the turbulence closure is achieved by parameterizing the dissipation rate of turbulent kinetic energy. Momentum conservation is accounted for by using the analytical model of the equilibrium surface wave spectra developed by Hara and Belcher. This approach allows analytical expressions of the Charnock coefficient to be obtained and the results to be examined in terms of key nondimensional parameters. In particular, simple expressions are obtained in the asymptotic limit at which effects of viscosity and surface tension are small and the majority of the stress is supported by wave drag. This analytical model allows us to identify the conditions necessary for the Charnock coefficient to be a true constant, an assumption routinely made in existing bulk parameterizations.

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Parameterization of Wave Boundary Layer.

Atmosphere ◽

10.3390/atmos10110686 ◽

2019 ◽

Vol 10 (11) ◽

pp. 686 ◽

Cited By ~ 1

Author(s):

Chalikov ◽

Babanin

Keyword(s):

Boundary Layer ◽

Drag Coefficient ◽

Wind Velocity ◽

Prediction Models ◽

Wave Spectrum ◽

Direct Wave ◽

Wave Boundary Layer ◽

Ocean Atmosphere ◽

Cloud Of Points ◽

Wave Boundary

It is known that drag coefficient varies in broad limits depending on wind velocity and wave age as well as on wave spectrum and some other parameters. All those effects produce large scatter of the drag coefficient, so, the data is plotted as a function of wind velocity forming a cloud of points with no distinct regularities. Such uncertainty can be overcome by the implementation of the WBL model instead of the calculations of drag with different formulas. The paper is devoted to the formulation of the Wave Boundary Layer (WBL) model for the parameterization of the ocean-atmosphere interactions in coupled ocean-atmosphere models and wave prediction models. The equations explicitly take into account the vertical flux of momentum generated by the wave-produced fluctuations of pressure, velocity and stresses (WPMF). Their surface values are calculated with the use of the spectral beta-functions whose expression was obtained by means of the 2-D simulation of the WBL. Hence, the model directly connects the properties of the WBL with an arbitrary wave spectrum. The spectral and direct wave modeling should also take into account the momentum flux to a subgrid part of the spectrum. The parameterization of this effect in the present paper is formulated in terms of wind and cut-off frequency of the spectrum.

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Implementation of the wave boundary layer model in the OpenIFS model

10.5194/egusphere-egu2020-3605 ◽

2020 ◽

Author(s):

Nefeli Makrygianni ◽

Jean R. Bidlot ◽

Michaela Bray ◽

Shunqi Pan

Keyword(s):

Boundary Layer ◽

Drag Coefficient ◽

Roughness Length ◽

Coupled Model ◽

Extreme Conditions ◽

Layer Model ◽

Wave Boundary Layer ◽

Boundary Layer Model ◽

Wind Input ◽

Wave Boundary

For more than 30 years, many studies have been carried out to improve the understanding of the air-sea interaction and its impact on the predictions of atmospheric and the oceanic processes. It is well understood that the accuracy in predictions of the wind-driven waves is highly dependent on the source input and dissipation terms. The Wave Boundary Layer (WBL) approach for the estimation of surface stress has previously been used to improve the wind and wave simulations under extreme conditions. However, until recently the WBL was only used to determine the roughness length (z0) and drag coefficient (Cd), but not to alter the wind input source function in wave models. In this study, the wave boundary layer model (WBLM) was implemented in the OpenIFS coupled model as source functions as suggested by Du et al. (2017, 2019). The new wind input and dissipation terms are then tested using numerical model simulations, with a particular focus on the contribution of the high frequency tail in the source input function.&#160; The comparison of the results of this study with published results hints at better performance of the model on the estimation of the roughness length and drag coefficient. This should improve predictions of the significant wave height and wind speeds, especially under extreme conditions.Corresponding Author: Nefeli Makrygianni ([email protected])

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