Ocean Modeling and Prediction

Development of a new Large Eddy Simulations (LES) model

for estuarine and coastal applications

LES techniques have been used successfully to investigate oceanic boundary layers; however, they are generally limited to horizontally-homogeneous flows in simple geometries. In a collaborative research project funded by National Science Foundation (with Dr. Rocky Geyer at Woods Hole Oceanographic Institution and Prof. Ugo Piomelli at University of Maryland at College Park), we are developing a new Large Eddy Simulation (LES) model to investigate turbulent mixing processes in estuaries and coastal oceans. We add temperature and salinity equations to an advanced LES code that has been validated in engineering flows. The model will include LES equations generalized to incorporate large-scale density and pressure gradients, non-uniform meshes in two directions, allowing for fine resolution of the stratified pycnocline and other regions of interest, and an immersed-boundary method to represent variable-bottom bathymetry and curvy coastlines. The new LES model can be used to tackle a wide range of turbulent mixing problems in coastal oceanography and generate turbulence data for calibrating and improving turbulence parameterization schemes. This model can also be used to investigate horizontal mixing processes such as mixed-layer restratification driven by horizontal density gradients.

Figure 1. A list of ocean models as identified in an ocean modeling document by National Science Foundation. The black numbers indicate current capabilities and the blue numbers show the expected future capabilities. In this NSF-funded project, we are developing a new LES model for coastal and estuarine systems.

In spite of the increasing sophistication of numerical models for simulating oceanographic phenomena, turbulence closure remains a significant impediment to accurate simulations, particularly in the presence of stable stratification. This is particularly evident in estuaries, in which stratified mixing processes are fundamentally important. Several modeling studies of estuaries using the Mellor-Yamada closure scheme have noted “runaway” stratification, indicating that the turbulence within the stratified interior was underestimated during relatively stable conditions (Simpson and Sharples, 1991; Sharples and Simpson, 1993; Monismith et al 1996; Stacey 1996). Other studies (Warner et al., 2005a, b; Li et al., 2005a) have compared different closure schemes, including Mellor-Yamada, k-epsilon, k-omega and KPP, and they found that the differences between the schemes were much smaller than the differences between any of the simulations and the data. In the simulations of Chesapeake Bay, the selection of the background diffusivity was found to be far more important than the selection of a particular closure method. Li et al. (2005c) compared the salinity distribution in an along-channel section between the ROMS model and high-resolution hydrographical data acquired from a ship-towed undulating vehicle (Scanfish). The observations show a much sharper pycnocline than the model predictions during high runoff periods. When the pycnocline is more diffused, on the other hand, the model has difficulty in simulating the landward salt transport in the bottom layer. In simulations of the Hudson River none of the closure schemes adequately resolved the transition in mixing intensity between the boundary layer and the stratified interior (Warner et al, 2005b). The consistency of the different closure models with each other, and their failure to simulate estuarine observations, indicate that more attention needs to be focused on the physical processes controlling turbulence within the stratified interiors of estuaries.

Large Eddy Simulation (LES) has potential to shed new light on the estuarine mixing problem and lead to better turbulence parameterizations. LES is a numerical technique that directly resolves flux-carrying turbulent eddies. It was originally developed for studying atmospheric boundary layers, but has been extended to engineering and oceanographic flows. In engineering, LES has mostly been used to solve the Navier-Stokes equations in complex geometries at moderate Reynolds numbers. In geophysical fluid dynamics, on the other hand, the stress has been on the application of LES in simpler geometries, but including the effects of the Coriolis force and stratification. In oceanography, LES has been used to investigate turbulent mixing processes in the ocean-surface mixed layer (e.g. McWilliams et al., 1997). Recently, Li et al. (2005b) applied LES to oscillating tidal flows in an unstratified bottom boundary layer. However, the LES model developed for boundary-layer studies usually assumes horizontal homogeneity and is not well suited for coastal and estuarine flows constrained by irregular bottom and lateral boundaries.

This is a project in progress. Please come back for updates.

Publications:

Li, M., J. Trowbridge and W.R. Geyer. 2005a. Asymmetric tidal mixing due to the horizontal density gradient. J. Phys. Oceanogr., In revision.

Li, M., L. Sanford and S-Y Chao. 2005b. Time-dependent effects in unstratified tidal flows: results from Large Eddy Simulations. Estuarine, Coastal and Shelf Sci., 62, 193-204.

Li, M., L. Zhong, and W. C. Boicourt. 2005c. Simulations of Chesapeake Bay estuary: Sensitivity to turbulence mixing parameterizations and comparison with observations, J. Geophys. Res., 110, C12004, doi:10.1029/2004JC002585.