On the parameterization of the drag coefficient in mixed seas

García–Nava, Héctor; Ocampo‐Torres, Francisco J.; Hwang, Paul A.

doi:10.3989/scimar.03615.19f

Cited by 10 publications

(4 citation statements)

References 26 publications

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“…Moreover, there are situations with accelerated extreme wind conditions that may be maintained several hours and reach wind speed up to 20 m s −1 . Two well-known examples of wind wave fields generated under accelerated wind conditions are the ones associated with Tehuano winds (Gulf of Tehuantepec, Mexico [17]) and Mistral winds (Gulf of Lyon, France [18]). Furthermore, the track and intensity forecast of tropical cyclones requires detailed knowledge of the air-sea interface fluxes also under accelerated wind conditions.…”

Section: Introductionmentioning

confidence: 99%

On the early stages of wind-wave generation under accelerated wind conditions

Robles-Díaz

Ocampo‐Torres

Branger

et al. 2019

European Journal of Mechanics - B/Fluids

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a b s t r a c tThe energy and momentum transfer between the atmosphere and the ocean has typically been studied for conditions where the waves have almost or already reached a local equilibrium with a uniform wind. The purpose of this work is to investigate the early stages of the generation of waves under non-stationary wind conditions and to describe the momentum and energy exchange at the airwater interface for non-equilibrium wind conditions. Some experiments with a characteristic wind acceleration were conducted in a large wind-wave facility at the Institut Pythéas (Marseille-France). Momentum fluxes were estimated through hot-wire anemometry and, the free surface displacement was measured along the wave tank by resistance and capacitance wire probes. Wind speed and water elevation measurements were acquired at a high sampling rate. During the experiments, the wind speed was increased with a constant acceleration over time, reaching a constant maximum intensity of 13 ms −1 . Under accelerated wind conditions, the degree of wave field development associated with a certain value of wind speed depends on the wind acceleration. Accordingly, once the rough flow regime is established, the drag coefficient values associated with a certain wind speed also vary depending on wind acceleration. It was observed that higher wind speed is needed to reach a rough flow regime as the wind acceleration increases. Also, the momentum transfer is reduced as wind acceleration increases. Under the rough flow regime, a less developed wave field induces a higher increase of drag coefficient with wind speed.

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Section: Introductionmentioning

confidence: 99%

On the early stages of wind-wave generation under accelerated wind conditions

Robles-Díaz

Ocampo‐Torres

Branger

et al. 2019

European Journal of Mechanics - B/Fluids

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“…Potter (2015) argued that the observed increase in C D was possibly due to wind gustiness inducing departure from the MO theory, combined with increased viscous effects. Nevertheless, the data reported by Garcia-Nava et al (2012) and Ocampo-Torres et al (2011) show even better the increased drag coefficient effect at low winds. There, it is attributed to the so-called swell-induced stress, which is assumed to depend on swell steepness and on the wave age.…”

Section: Turbulent Drag In Light Windsmentioning

confidence: 74%

Air‐Sea Turbulent Fluxes From a Wave‐Following Platform During Six Experiments at Sea

Bourras

Cambra

Marié³

et al. 2019

JGR Oceans

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Turbulent fluxes at the air‐sea interface are estimated with data collected in 2011 to 2017 with a low‐profile platform during six experiments in four regions. The observations were carried out with moderate winds (2–10 m/s) and averaged wave heights of 1.5 m. Most of the time, there was a swell, with an averaged wave age (the ratio between wave phase speed and wind speed) being equal to 2.8 ± 1.6. Three flux calculation methods are used, namely, the eddy covariance (EC), the inertial dissipation (ID), and the bulk methods. For the EC method, a spectral technique is proposed to correct wind data from platform motion. A mean bias affecting the friction velocity (u*) is then evaluated. The comparison between EC u* and ID u* estimates suggests that a constant imbalance term (ϕimb) equal to 0.4 is required in the ID method, possibly due to wave influence on our data. Overall, the confidence in the calculated u* estimates is found to be on the order of 10%. The values of the drag coefficient (CD) are in good agreement with the parameterizations of Smith (1988, https://doi.org/10.1029/JC093iC12p15467) in medium‐range winds and of Edson et al. (2013, https://doi.org/10.1175/JPO-D-12-0173.1) in light winds. According to our data, the inverse wave age varies linearly with wind speed, as in Edson et al. (2013, https://doi.org/10.1175/JPO-D-12-0173.1), but our estimates of the Charnock coefficient do not increase with wind speed, which is possibly related to sampling swell‐dominated seas. We find that the Stanton number is independent from wind speed.

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“…While the model captures the basic roughness dynamics that would take place on the inner shelf under purely wind-driven conditions, the surface drag in a more realistic situation would be affected by swell (e.g., mixed sea conditions). In that case, swell would complicate the purely wind-sea drag by accepting or releasing momentum at the air-sea boundary as a function of wind speed (Garcia-Nava et al, 2012). However, the transition from mixed conditions to swellsheltered estuarine wind seas is beyond the scope of the present work.…”

Section: 1029/2018jc014585mentioning

confidence: 98%

Effects of Locally Generated Wind Waves on the Momentum Budget and Subtidal Exchange in a Coastal Plain Estuary

2019

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A numerical model with a vortex force formalism is used to study the role of wind waves in the momentum budget and subtidal exchange of a shallow coastal plain estuary, Delaware Bay. Wave height and age in the bay have a spatial distribution that is controlled by bathymetry and fetch, with implications for the surface drag coefficient in young, underdeveloped seas. Inclusion of waves in the model leads to increases in the surface drag coefficient by up to 30% with respect to parameterizations in which surface drag is only a function of wind speed, in agreement with recent observations of air‐sea fluxes in estuaries. The model was modified to prevent whitecapping wave dissipation from generating breaking forces since that contribution is integrally equivalent to the wind stress. The proposed adjustment is consistent with previous studies of wave‐induced nearshore currents and with additional parameterizations for breaking forces in the model. The mean momentum balance during a simulated wind event was mainly between the pressure gradient force and surface stress, with negligible contributions by vortex, wave breaking (i.e., depth‐induced), and Stokes‐Coriolis forces. Modeled scenarios with realistic Delaware bathymetry suggest that the subtidal bay‐ocean exchange at storm time scales is sensitive to wave‐induced surface drag coefficient, wind direction, and mass transport due to the Stokes drift. Results herein are applicable to shallow coastal systems where the typical wave field is young (i.e., wind seas) and modulated by bathymetry.

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On the parameterization of the drag coefficient in mixed seas

Cited by 10 publications

References 26 publications

On the early stages of wind-wave generation under accelerated wind conditions

On the early stages of wind-wave generation under accelerated wind conditions

Air‐Sea Turbulent Fluxes From a Wave‐Following Platform During Six Experiments at Sea

Effects of Locally Generated Wind Waves on the Momentum Budget and Subtidal Exchange in a Coastal Plain Estuary

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