OS23E-01 INVITED 13:40h
An Asymptotic and Stochastic Theory for the Effects of Surface Gravity Waves on Currents and Infragravity Waves
Oceanic surface gravity waves are approximately irrotational, weakly nonlinear, and conservative, and they have a much shorter time scale than oceanic currents and longer waves (e.g., infragravity waves) --- except where the primary surface waves break. This provides a framework for an asymptotic theory, based on separation of time (and space) scales, of wave-averaged effects associated with the conservative primary wave dynamics combined with a stochastic representation of the momentum transfer and induced mixing associated with non-conservative wave breaking. Such a theory requires only modest information about the primary wave field from measurements or operational model forecasts and thus avoids the enormous burden of calculating the waves on their intrinsically small space and time scales. For the conservative effects, the result is a vortex force associated with the primary wave's Stokes drift; a wave-averaged Bernoulli head and sea-level set-up; and an incremental material advection by the Stokes drift. This can be compared to the "radiation stress" formalism of Longuet-Higgins, Stewart, and Hasselmann; it is shown to be a preferable representation since the radiation stress is trivial at its apparent leading order. For the non-conservative breaking effects, a population of stochastic impulses is added to the current and infragravity momentum equations with distribution functions taken from measurements. In offshore wind-wave equilibria, these impulses replace the conventional surface wind stress and cause significant differences in the surface boundary layer currents and entrainment rate, particularly when acting in combination with the conservative vortex force. In the surf zone, where breaking associated with shoaling removes nearly all of the primary wave momentum and energy, the stochastic forcing plays an analogous role as the widely used nearshore radiation stress parameterizations. This talk describes the theoretical framework and presents some preliminary solutions using it. McWilliams, J.C., J.M. Restrepo, & E.M. Lane, 2004: An asymptotic theory for the interaction of waves and currents in coastal waters. J. Fluid Mech. 511, 135-178. Sullivan, P.P., J.C. McWilliams, & W.K. Melville, 2004: The oceanic boundary layer driven by wave breaking with stochastic variability. J. Fluid Mech. 507, 143-174.
OS23E-02 INVITED 14:00h
Tools for forecasting in the coastal ocean: a coherent hydrodynamic formalism for numerical models and some first steps toward a coherent use of radar remote sensing.
Operational oceanography systems are being developed for many coastal areas, often based on global or basin scale concept whereas dynamics and relevant observations may be very different. Waves, in particular are generally poorly represented. We give here practical equations for momentum and mass conservation, coupling waves, mean flow and turbulence, uniformly valid from oceanic scales to the nearshore. These are generalized from Mellor (J. Phys. Oceanogr. 2003) and re-derived from the Generalized Lagrangian Mean equations (Andrews and McIntyre, J. Fluid Mech. 1978). They include a Generalized Lagrangian Mean equation for the turbulent kinetic energy that describes the stretching of turbulence by the Stokes drift shear. Consistent boundary conditions and parameterizations are also described. The equations derived here are readily integrated by coupling a spectral wave model to an ocean circulation model, with practical applications to the forecasting of surface drift, upper ocean mixing, coastal ocean circulation and wave forecasting. With this view in mind we discuss some early model results on surface Eulerian and drift velocity, and the first steps towards a coherent processing of radar remote sensing (HF and InSAR or SAR Doppler centroid) to constrain ocean forecasts.
http://surfouest.free.fr/WOO2003
OS23E-03 14:20h
Forcing a Three-Dimensional, Hydrostatic Primitive-Equation Model for Application in the Surf Zone.
A hydrostatic primitive equation model, the Princeton Ocean Model (POM), has been adapted for studies of three-dimensional, wave-averaged circulation in the nearshore surf zone. As part of the NOPP Nearshore Modeling program, POM has been embedded as a subroutine in the community model (NearCoM) master program. Here we examine the influence of the waves on the circulation with emphasis on the three-dimensional aspects of the forcing. The surf zone wave-averaged circulation is forced primarily by the gradients of the radiation stress tensor that arise from the breaking waves. Traditionally, modeling of surf zone currents has used the shallow water (horizontally two-dimensional) approximation. Forcing a fully three-dimensional model such as POM requires that the depth dependence of these forces be examined. Additional wave-current interaction forcing terms not usually included in radiation stress forcing of shallow-water circulation models and the effect of rotation on the wave induced circulation are also important issues for combined nearshore/coastal circulation models. Here we examine the three-dimensional wave force formulation applicable to the surf zone and some of the implications for three-dimensional surf zone modeling.
OS23E-04 14:35h
Coupling of NearCoM, ROMS and SWAN in a MCEL System
The NOPP Nearshore Community Model (NearCoM) is coupled with the shelf-scale circulation model ROMS and the shelf-scale wave generation and transformation model SWAN. A Model Coupling Environment Library (MCEL) technique is used for model coupling through a centralized server and client applications. The model coupling system includes the real-time interactions between models with different theoretical basis and different-scales and thus provides a comprehensive model of waves, tides, wave-induced nearshore circulation in tidal inlet regions. An application of the system is conducted in the Delaware Bay including the Indian River Inlet as a target prediction region. The model is validated using measured data and the model coupling efficiency is tested in both a 4-processor cluster and distributed linux workstations with a Common Object Request Broker Architecture (CORBA)-based network. Based on the MCEL model coupling framework, the system is going to be enhanced by integrating more functional components such as the meteorological model and coastal flooding model in the system for real-time predictions of waves, currents, storm surges, and coastal inundations.
http://www.coastal.udel.edu/~fyshi/funding/funding.html
OS23E-05 14:50h
Representing Nearshore Nonlinearity within the NOPP NearCoM Modeling System
NearCoM, the NOPP-supported Nearshore Community Model, is a modeling system linking together advanced scientific models for simulating the high spatial and temporal variability of nearshore processes. One model slated for inclusion in the modeling system is REFDIF-SNL, a fully dispersive, weakly nonlinear frequency domain model essentially representing a nonlinear extension of the mild slope equation commonly used in nearshore wave propagation problems. In this study we discuss some potential uses of the model, particularly in support of sediment transport calculations. We further discuss some implemented and planned improvements to the model, designed to increase model utility and performance without sacrificing physical fidelity. In particular, we investigate the efficacy of using recently-developed spectral parameterizations to represent the evolution of the high frequency spectral tail in nearshore wave transformations.
OS23E-06 15:05h
NUMERICAL EXPERIMENTS OF GENERATION OF LOW-FREQUENCY WAVES
Much work has been undertaken in the last two decades to understand the process by which low-frequency surface gravity waves (LFWs) are generated in the nearshore region. In this presentation numerical experiments are undertaken to illustrate a generation mechanism, similar to but nevertheless distinct from that of Symonds et al (1982). The mechanism is that of 'bore reflection' (Peregrine, 1974) and results indicate that it is possible that this process can also be active in LFW generation on real beaches, perhaps particularly for highler modulated wave groups. Discrepancies are noted (e.g. short wave to lFW amplitude) as well as similarities with Symonds et al (1982) breakpoint hypothesis are noted.
OS23E-07 15:20h
Modeling Tidal and Wind-Induced Alongshore Currents in the Nearshore
Under storm conditions, obliquely incident breaking waves drive strong (1--2~m/s) alongshore currents in the nearshore. When wave breaking is either absent or confined to a very narrow region at the beachface, nearshore alongshore currents may still be significant (up to $\approx$ 1~m/s), driven largely by tidally induced alongshore surface slopes, wind, or buoyancy effects. There are, however, few detailed observational and modeling studies of alongshore nearshore currents in the absence of breaking waves. Here, predictions of a single-point model in the vertical driven by tidally induced 10--100 km scale alongshore surface slopes, wind stress, and Earth's rotation [Houwman, 2000] are compared to alongshore currents measured under nonbreaking conditions at two heights (0.3 and 1.2~m above the bed) at four positions (water depths 3--10~m) in the nearshore of Terschelling, Netherlands. The observations span several thousands of hours and include maximum near-bed alongshore currents of 0.8~m/s. All calculations are performed using a time-dependent eddy viscosity derived from a two-equation $k-\epsilon$ model and a quadratic partial slip bottom boundary condition. Because the overall performance of the model is satisfactory (e.g., skill $r^2$ exceeds 0.9 at all positions and heights), the model output for a single tide-wind situation is examined in more detail. In the selected situation, a shore-parallel wind (wind stress up to 0.5 N/m$^2$) in the direction of the (positive) flood current is predicted to yield positive near-bed currents even during the ebbing phase of the tide in water depths less than 6~m, whereas modeled alongshore currents in deeper water do change sign, in good quantitative agreement with the observations. Also, model simulations suggest that the wind strongly alters the vertical velocity profile of the alongshore current during a tidal cycle, related to a marked change in the temporal evolution of the eddy viscosity compared to a tide-only situation. Houwman, K.T., 2000. Tide, wind- and wave-driven flow processes in the nearshore zone. Ph.D. thesis, Utrecht University, 235 pp.