Wave-induced Forcing Research Articles

A study of the wave effects on the current circulation in Galway Bay, using the numerical model COAWST

A model coupling waves and currents is set up for Galway Bay, Ireland, using COAWST. The impact of the coupling on the accuracy of the model is shown to be marginal on the overall statistics. The model is in good agreement with the available in-situ observations that are used for the validation. However a closer look at the data during storms shows a deterioration of the agreement, which is barely improved by the coupling. A special analysis of the different processes ruling the ocean circulation and wave propagation is conducted during Storm Hector (2018/06/14). It shows a strong wave-induced surge by 7cm in the back of Galway Bay, and a strong response in terms of currents along the coast of Clare of about 30cm/s. Most of the surge is attributed to the impact of the conservative wave-induced forcing terms, but the main process involved is not identified. The response in currents is partly attributed to the conservative wave-induced forcing terms, but mostly to the wave-enhanced surface roughness.

Relationship between Three-Dimensional Radiation Stress and Vortex-Force Representations

In numerical ocean models, the effect of waves on currents is usually expressed by either vortex-force or radiation stress representations. In this paper, the differences and similarities between those two representations are investigated in detail in conditions of both conservative and nonconservative waves. In addition, comparisons between different sets of equations of mean motion that apply different representations of wave-induced forcing terms are included. The comparisons are useful for selecting a suitable numerical ocean model to simulate the mean current in conditions of waves combined with currents.

The impact of wave energy converter arrays on wave-induced forcing in the surf zone

An alternative metric for assessing nearshore hydrodynamic impact due to Wave Energy Converter (WEC) arrays is presented that is based on the modeled changes in alongshore radiation stress gradients in the lee of the array. The metric is developed using a previously observed relationship between measured radiation stresses and alongshore current magnitudes. Next, a parametric study is conducted using the spectral model SWAN to analyze the nearshore impact of different WEC array designs. A realistic range of array configurations, locations, and incident wave conditions are examined and conditions that generate alongshore radiation stress gradients exceeding a chosen impact threshold on a uniform beach are identified. Finally, the methodology is applied to two permitted WEC test sites to assess the applicability of the results to sites with more realistic bathymetries. For these sites, the overall trends seen in the changes in wave height, direction, and radiation stress gradients in the lee of the array are similar to those seen in the parametric study. However, interactions between the wave field and real bathymetry induce additional alongshore variability in wave-induced forcing. Results indicate that array-induced changes can exceed the natural variability up to 15% of the time with certain array designs and locations.

Effects of wave-induced forcing on a circulation model of the North Sea

The effect of wind waves on water level and currents during two storms in the North Sea is investigated using a high-resolution Nucleus for European Modelling of the Ocean (NEMO) model forced with fluxes and fields from a high-resolution wave model. The additional terms accounting for wave-current interaction that are considered in this study are the Stokes-Coriolis force, the sea-state-dependent energy and momentum fluxes. The individual and collective role of these processes is quantified and the results are compared with a control run without wave effects as well as against current and water-level measurements from coastal stations. We find a better agreement with observations when the circulation model is forced by sea-state-dependent fluxes, especially in extreme events. The two extreme events, the storm Christian (25–27 October 2013), and about a month later, the storm Xaver (5–7 December 2013), induce different wave and surge conditions over the North Sea. Including the wave effects in the circulation model for the storm Xaver raises the modelled surge by more than 40 cm compared with the control run in the German Bight area. For the storm Christian, a difference of 20–30 cm in the surge level between the wave-forced and the stand-alone ocean model is found over the whole southern part of the North Sea. Moreover, the modelled vertical velocity profile fits the observations very well when the wave forcing is accounted for. The contribution of wave-induced forcing has been quantified indicating that this represents an important mechanism for improving water-level and current predictions.

Numerical Study of the Effects of Wave-Induced Forcing on Dynamics in Ocean Mixed Layer

Numerical experiments using hybrid coordinate ocean model (HYCOM) are designed to quantify the effects of wind wave-induced Coriolis-Stokes forcing (CSF) on mixed layer (ML) dynamics in a global context. CSF calculated by the wave parameters simulated by using the WaveWatch III (WW3) model is introduced as a new driving force for HYCOM. The results show that noticeable influence on ocean circulation in ML can be caused by CSF. Over most of the global oceans the direction of Stokes transport is different from that of the change in current transport caused by CSF. This is not unusual because CSF is normal to Stokes drift. However, the CSF-caused change in current transport and the wave-induced Stokes transport have the same magnitude. The seasonal variabilities of mixed layer temperature (MLT) and mixed layer depth (MLD) caused by CSF are analyzed, and the possible relationship between them is also given.

On the coupling of wave and three-dimensional circulation models: Choice of theoretical framework, practical implementation and adiabatic tests

Many theoretical approaches and implementations have been proposed for the coupling of the three-dimensional ocean circulation with waves. The theoretical models are reviewed and it is shown that the formulation in terms of the quasi-Eulerian velocity circumvents the essential difficulty of alternative formulations for the Lagrangian mean velocity. Namely, models based on this Lagrangian velocity require an estimation of wave-induced motions to first order in the horizontal gradients of the wave field in order to estimate the vertical flux of wave pseudo-momentum. So far, only three-dimensional wave models have been able to provide these estimates, and all published theories based on the simpler Airy theory are not consistent at the leading order, because they ignore or incorrectly estimate the vertical momentum flux. With an adiabatic example on a sloping bottom it is shown that this inconsistency produces very large spurious velocities. These errors are independent of the slope for the inviscid case, and are still significant when a realistic vertical mixing is applied. A quick diagnostic of the potential accuracy of a theoretical model is the vertical profile of the wave-induced forcing terms: if it is not uniform over depth in adiabatic conditions then it will produce spurious artificial flow patterns in conditions with shoaling waves. Although conceptually more challenging, the quasi-Eulerian velocity theories only introduce minor modifications of the solution procedure for the standard primitive equations: a modification of the surface boundary condition for the mass conservation, the addition of the Stokes drift in the tracer advection equations, and sources of momentum and turbulent kinetic energy with associated surface and bottom fluxes. All the necessary modifications of primitive equation models are given in detail. This implementation is illustrated with the MARS3D model, which passes the test of the adiabatic shoaling waves.

Drift and deformation of oil slicks due to surface waves

We present a theoretical model for the wave-induced drift and horizontal deformation of an oil slick. The waves and the mean flow are coupled through the influence of the mean flow on the concentration of slick material, which in turn determines the damping rate of the waves and hence the transfer of momentum from the waves to the mean flow. We also briefly discuss a simplified version of the model that can be used when remote sensing data are available. With this simpler model the wave-induced forcing of the mean flow is obtained directly from observations of the wave field, hence knowledge of any specific slick properties is not required.

A Quasi-Eulerian, Quasi-Lagrangian View of Surface-Wave-Induced Flow in the Ocean

Abstract In this study the influence of surface waves on the mean flow in an ocean of arbitrary depth is examined. The wave-induced forcing on the mean flow is obtained by integrating the Eulerian equations for mass and momentum balance from the bottom to an undulating material surface within the water column. By using the mean position of the material surface as the vertical coordinate, the authors obtain the depth dependence of the mean flow and the wave-induced forcing. Substitution of the vertical coordinate makes the model Lagrangian in the vertical direction. The model is Eulerian in the horizontal direction, allowing one to model the effects of a spatially nonuniform wave field or varying depth in a straightforward way.

Scaling Properties and Wave Interactions in Confined Supersonic Turbulent Bluff-Body Wakes

Results are presented from an experimental study that compares the large-scale structure, mean velocities, and scaling properties of confined, supersonic, planar, turbulent, bluff-body wakes at M∞ ≈ 2 and 3 with corresponding results from incompressible wakes. The large-scale structure of incompressible planar turbulent wakes is recovered at downstream locations x where the local relative Mach number Mr(x) has decreased to about 0.4. Interactions between these large-scale structures and reflected expansion waves from the near-field recompression region lead to substantial changes in the scaling properties of the flow. Wave interactions with the flow create local self-induced forcing as the large-scale structures pass through the reflected expansion waves. In the M∞ ≈ 2 wake, a subsonic upstream path exists from the first wave interaction point that allows upstream propagation of the wave-induced forcing, and consequently the measured wake scaling constants match values reported from forced incompressible wakes. In the M∞ ≈ 3 wake, no subsonic upstream path exists from the first interaction point, and as a consequence the measured scaling constants match values reported from unforced incompressible wakes. Downstream of each wave interaction point, the self-induced forcing leads to an increase in the wake growth rate and in the entrainment of free-stream fluid. The wake subsequently detrains this fluid and returns to its original growth rate. This local forcing mechanism and its effects on the wake scaling properties repeat at each downstream location where the reflected expansion waves intersect the wake.

Understanding fluid mud in a dynamic environment

The generation, transportability and dewatering of fluid mud under current- and wave-induced forcing is a multi-variate problem that requires knowledge not only of mud density and composition, but also of rheology, which ultimately characterizes mud dissipative properties. It is, therefore, self-evident that the predictive accuracy of mathematical modeling, a powerful tool for simulating mud dynamics, will depend strongly on the reliability of rheological description. Defining the state of mud solely in terms of density, as done commonly, only leads to ambiguities in quantifying the rates of transport of fluid mud.