Summary and Conclusions

1. A Study of the Effect of Surface Ocean Waves on Ocean Circulation Abstract: A coupled current-wave model used to study the net effect of surface ocean waves on ocean circulation during extreme weather events.By J. Wallmark (NAVO), V.Kamenkovich (USM), D.Nechaev (USM)), J. Veeramony (NRL) PIs: V. Kamenkovich and D. Nechaev (USM) SWAN Model Hsig(m) Flat Basin Mellor Model Hsig(m) Flat Basin o Wave Model Validation Modified Model Equations Wave Model Comparisons: Mellor vs. SWAN Significant Wave Heights for Identical Conditions Validation of the Mellor-Donelan-Oey Wave Model In order to assure that the wave model introduced by Mellor et al (Mellor, 20081), it is checked against two observational datasets and against the widely-used SWAN (Simulating Waves Nearshore) wave model. The test case is constructed to examine the fetch-limited case, since two datasets of fetch-limited wave height observations are available in the literature (Hwang and Wang, 20042, Donelan 19923). The test grid is constructed so that a 10 m/s wind is blowing eastward from a coastline. Cyclic boundary conditions are applied so that the test case is as one-dimensional as possible, and the MDO model is run until fully established seas are obtained. The resulting wave energy curve compares favorably with both SWAN and the observational datasets. 1. Mellor, G. L., M. A. Donelan, and L-Y. Oey, 2008: A surface wave model for coupling with numerical ocean circulation models. J. Atmos. Oceanic Tech., 25, 1785-1807. 2. Hwang, P. A. and D. W. Wang, 2004: Field measurements of duration-limited growth of wind-generated ocean surface waves at young stage of development. J. Phys. Oceanogr., 34, 2316-2326. 3. Donelan, M., M. Skafel, H. Graber, P.Liu, D.Schwab, and S. Venkates, 1992: On the growth of wind-generated waves. Atmosphere-Ocean, 30, 457-478. Uniform Wind Case Flat, enclosed basin, 20 meter depth. Constant uniform wind of 20 m/s Cyclonic Wind Case Flat, enclosed basin, 20 meter depth. Cyclonic wind pattern of peak 20 m/s In this case four separate models were compared. The original POM2K from 2002, the most recent POM2K (with a revised turbulence transport scheme), The Mellor-Donelan-Oey (MDO) model with waves disabled, and the MDO model with waves enabled. In this case we see significantly smaller water velocities with waves enabled. This is an interesting results since it was expected that the wave-induced modification of the surface roughness in the MDO model would mean more energy transferred from the wind to the water column. It is suspected that a significant amount of this energy is being lost due to the wave-breaking mechanism in this model. Vertical profiles of water velocities differ significantly, with the Mellor-Donelan-Oey (MDO) model yielding a distinct 2-layer flow scheme with strong shear at the transition depth. In contrast POM2K, while it has a return flow at the bottom of the basin, has a much more smoother vertical distribution of velocities, with no strong shear zone. The interaction between ocean waves and currents has long been recognized as an important aspect of ocean dynamics, particularly in near shore environments. Until recently, these interactions have been modeled using depth-integrated wave models coupled one way with circulation models. In 2006, Mellor et al introduced a three-dimensional wave model with two-way coupling to the Princeton Ocean Model (POM), to more realistically simulate both three-dimensional circulation and surface waves. Such a model promises to more accurately simulate the vertical transport of energy from the surface into the water column, with corresponding improvements in predictions of current structure and net transport. The purpose of this investigation is to determine whether this coupled model yields results sufficiently different from the standard POM model to justify its use. The coupled wave model was developed by George Mellor, Mark Donelan, and Leo Oey. (Mellor, Donelan, Oey, 2008: A Surface Wave Model for Coupling with Numerical Ocean Circulation Models. J. Atmos. Oceanic Tech, 25, 1785-1807.) Funds for this project have been provided through The National Science Foundation. Examination of the coefficient of vertical mixing (Km) and of turbulent energy (Q2) of both models shows the effect of the wave model on the downward transport of turbulent energy supplied by the wind stress at the surface. The MDO model has much larger values of Km in the upper water column than POM2K. Consequently, we see much larger values of Q2 in the upper water column as well. Cyclonic Wind Field Summary and Conclusions The Mellor-Donelan-Oey (MDO) wave model compared well to the SWAN model and empirical wave datasets. When the MDO model was compared to POM2K, it was seen that the wave-enabled model yielded significantly different results from the non-wave-enabled models. This differences are attributed to; the modified vertical turbulence transport formulation; the modified (wave-dependent) surface stress coefficient; and the presence of a new energy-loss mechanism through wave breaking. It is felt that this new set of wave-current coupling factors makes the MDO coupled model a promising new tool with which to study ocean dynamics. The next phase of this study will be to compare the MDO model against empirical data collected during Hurricane Katrina. Jay Wallmark Jay.wallmark@navy.mil University of Southern Mississippi Coastal Modeling