Wind-Wave Effects on Estuarine Turbulence: A Comparison of Observations and Second-Moment Closure Predictions

Abstract

Observations of turbulent kinetic energy, dissipation, and turbulent stress were collected in the middle reaches of Chesapeake Bay and were used to assess second-moment closure predictions of turbulence generated beneath breaking waves. Dissipation scaling indicates that the turbulent flow structure observed during a 10-day wind event was dominated by a three-layer response that consisted of 1) a wave transport layer, 2) a surface log layer, and 3) a tidal, bottom boundary layer limited by stable stratification. Below the wave transport layer, turbulent mixing was limited by stable stratification. Within the wave transport layer, where dissipation was balanced by a divergence in the vertical turbulent kinetic energy flux, the eddy viscosity was significantly underestimated by second-moment turbulence closure models, suggesting that breaking waves homogenized the mixed surface layer to a greater extent than the simple model of TKE diffusing away from a source at the surface. While the turbulent transport of TKE occurred largely downgradient, the intermittent downward sweeps of momentum generated by breaking waves occurred largely independent of the mean shear. The underprediction of stress in the wave transport layer by second-moment closures was likely due to the inability of the eddy viscosity model to capture the nonlocal turbulent transport of the momentum flux beneath breaking waves. Finally, the authors hypothesize that large-scale coherent turbulent eddies played a significant role in transporting momentum generated near the surface to depth.

Long‐Term Statistics of Observed Bubble Depth Versus Modeled Wave Dissipation

“…Some of this work is based on observations of bubble clouds (Thorpe, ; Wang et al, ). It is nevertheless clear that the law‐of‐the‐wall scaling does not hold in the breaking zone (Fisher et al, ; Gemmrich, ; Sutherland & Melville, ; Thomson et al, ; Zippel et al, ). Vertical size‐specific bubble distributions for radii between 8 and 130 m were measured by Vagle and Farmer () using a multifrequency acoustic backscatter technique.…”

Accuracy of high resolution coastal flow speed simulations during and outside of wind, wave and stratification events (Gulf of Lion, NW Mediterranean)

“…This splitting of the flow energy is essential to describe transport and mixing processes in the numerical simulations. Turbulence closure is expected to play a more prominent role when energetic transfer happens at scales smaller than the spatio-temporal grid, such as during wind-wave (Fisher et al, 2018) or river flooding events (Reffray et al, 2004). Evaluating model accuracy during selected events when the classical assumptions of ocean models aren't met has been frequent practice in the coastal modelling community over the last two decades (Marsaleix et al, 1998;Estournel et al, 2001;Reffray et al, 2004;Petrenko et al, 2005;Ulses et al, 2008;Estournel et al, 2016, in the Gulf of Lion).…”