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A new coupled ocean-wave model has been developed and tested as a new component of the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS®). The modeling system is comprised of the Simulating WAves Nearshore (SWAN) wave model and the Navy Coastal Ocean Model (NCOM). The models are two-way coupled using the Earth System Modeling Framework (ESMF). The ocean model has been modified to incorporate the effect of the Stokes drift current, wave radiation stresses due to horizontal gradients of the momentum flux of surface waves, enhancement of bottom drag in shallow water, and enhanced vertical mixing due to Langmuir turbulence. The wave model ingests surface currents (wave-current interaction) and water levels. The system is designed to support the Navy's ocean forecast requirements for regional and coastal domains. Validation studies for the Florida Straits and Virginia coastal area are presented. The system will run at the Naval Oceanographic Office and at the Fleet Numerical Meteorology and Oceanography Center.

In this study, we present a new parameterization for the enhancement of vertical mixing brought by the inclusion of the Stokes drift for the turbulent mixing schemes in ocean circulation models. The new scheme (KPP-LT) uses the K-Profile Parameterization (Large et al., 1994) as a template, and attempts to include the effect of the penetration decay scale of the Stokes drift (δ) and the misalignment between the wind stress and Stokes drift (θ w w). The effect of the wind–wave angle of misalignment is guided by a set of idealized Large Eddy Simulations (LES) and the Langmuir Turbulence (LT) parameterization is developed based on LES of the ocean surface boundary layer at Ocean Weather Station Papa, for a period of 20 days under observed atmospheric and oceanic conditions. The KPP-LT model is implemented in the Navy Coastal Ocean Model (NCOM) and compared to in situ oceanographic measurements, LES and other Second Moment Closure (SMC) schemes available in NCOM, namely the model of Kantha and Clayson (2004) and Harcourt (2013, 2015). Comparisons with temperature observations suggest better performance of the KPP-LT model over SMC models within the boundary layer, which are supported by comparisons of inertially averaged eddy viscosity profiles estimated from LES. • A new Langmuir turbulence model was developed and implemented in the Navy Coastal Ocean Model. • Model development is based on Large Eddy Simulation experiments under realistic forcing conditions. • The K-Profile Parameterization based model accounts for the wind–wave angle of misalignment. • Comparison with observations shows improvement over other second moment closure models. [ABSTRACT FROM AUTHOR]

In this paper, we discuss the validation of water level and current predictions from three coastal hydrodynamic models and document the resource and operational requirements for each modeling system. The ADvanced CIRCulation Model (ADCIRC), the Navy Coastal Ocean Model (NCOM), and Delft3D have been configured and validated for the Chesapeake Bay region during a Navy exercise. Water level predictions are compared with a NOAA/NOS water level gauge at the Chesapeake Bay Bridge Tunnel location while current predictions are validated with Acoustic Doppler Profiler (ADP) measurement records at three locations in the lower Chesapeake Bay. Statistical metrics such as correlation coefficient and root mean square error (RMSE) are computed. Both the vertically-integrated currents and currents at varying water depths are compared as well. The model-data comparisons for surface elevation indicate all three models agreed well with water level gauge data. The two-dimensional version of ADCIRC, ADCIRC2D, and NCOM yield better statistics, in terms of correlation and RMSE, than Delft3D. For vertically-integrated currents, ADCIRC2D has the smallest RMSE at Thimble Shoal and Naval Station locations while NCOM has the smallest RMSE at Cape Henry. For the horizontal currents over the water column, the fully three-dimensional, baroclinic ADCIRC model, ADCIRC3D, and NCOM both showed better agreement with the ADP measurements. [ABSTRACT FROM AUTHOR]

In Part 2 of our application of the Navy coastal ocean model (NCOM) to the Atchafalaya Bay system, we examine the wind- and tide-forced three-dimensional baroclinic circulation of the Lower Atchafalaya and Wax Lake Outlet river plumes. The salinity and the current velocity are examined during a time period when three cold fronts passed over the region. The baroclinic circulation of NCOM was validated for the same time period in Part 1 of this study (COBB, KEEN, and WALKER, 2008. Modeling the circulation of the Atchafalaya Bay region, 1: Model description and validation. Journal of Coastal Research, this issue). We find that the westward transport of plume water and the offshore cold-front- induced circulation are determined to a large extent by the alongshore and cross-shore bathymetric structure. Wind-driven plume water moves parallel to the alongshore bathymetric contours unless forced to mix with higher salinity water by strong cross-shore directed winds. The mixing of plume water with offshore water occurs over bathymetric shoals during periods of strong post-frontal winds. This mixing process involves the offshore transport of plume water over the entire water column in addition to the strong surface transport. The model results for offshore circulation are in qualitative agreement with past observations. In addition, the hydrodynamic processes that control the salinity fronts in Vermilion and West Cote Blanche Bays, areas where the model salinity was validated in Part 1, are examined as well. [ABSTRACT FROM AUTHOR]

The Atchafalaya Bay system consists of a series of five shallow bays in southern Louisiana (U.S.A.) that are dominated by the circulation of the Atchafalaya River plume. Winter cold fronts have a significant impact on the resuspension and transport of sediments in this region, and a better understanding of the circulation during these events is absolutely necessary for determining the sediment transport patterns of the Atchafalaya Bay system and the adjacent shelf area. Understanding the circulation of this region is also crucial for environmental studies as well. This work describes the implementation of the Navy Coastal Ocean Model (NCOM), a three-dimensional numerical circulation model for tide, river, and wind-forced circulation in the Atchafalaya Bay system. The model has a cell size (Δx) of ∼800 m and is nested to a northern Gulf of Mexico model (Δx ∼ 5000 m), which is itself nested to the global NCOM (Δx ∼ 1/8°). Atmospheric forcing is supplied by the Navy Operational Global Atmospheric Prediction System (NOGAPS) (Δx = 1°). These models are used to simulate the hydrodynamics of the Atchafalaya Bay system and Atchafalaya river plume between December 1997 and January 1998 during the passage of three winter cold fronts. The water levels, salinity, and currents predicted by NCOM are in reasonable agreement with available measurements and tide-gauge elevation data. Errors in ebb tides and wind-driven circulation are attributable to uncertainties in the bathymetry and the low spatial and temporal resolution of the NOGAPS wind fields. [ABSTRACT FROM AUTHOR]

The unsteady-state process of deep-water convection in the Gulf of Lion has been observed and investigated in recent decades. However, the mechanisms of the uncertainty and irregularity of the deep-water convection in this region have not yet been fully understood. In this study, the effects of time variation of the surface buoyancy flux on the formation of the deep-water convection are examined. Numerical simulations using the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS®) with the NRL Coastal Ocean Model (NCOM) as an oceanic component were conducted for the period from October 1998 to September 2000 to cover two winters, 1998–1999 and 1999–2000, over the Gulf of Lion region. The results show large differences in the deep-water convection between the two winters, even though the total surface heat fluxes during the two winter seasons are similar. The differences are related to the time variation of the surface buoyancy flux that causes large differences in the preconditioning and mixing stages of the convection. [ABSTRACT FROM AUTHOR]

Abstract: The sensitivity of vertical grid choice in the hybrid Navy Coastal Ocean Model (NCOM) is discussed for the Yellow Sea, East Asian Sea, Japan/East Sea domain. In particular, the logarithmically stretched hybrid vertical profile used operationally at the Naval Research Laboratory is compared to six variations. The variations include a full z-level run, a full sigma coordinate run, and other hybrid constructs that modify the hybrid’s transitions depth or the structure of the operational grid. The results are compared to each other and some limited observations in the framework of setting up a rapidly relocatable ocean model. The comparisons show that the operational vertical grid structure is a good first guess design for a rapidly relocatable ocean model. [Copyright &y& Elsevier]

Abstract: A 1/8° global version of the Navy Coastal Ocean Model (NCOM) is described with details of its formulation, implementation, and configuration of the vertical coordinate. NCOM is a baroclinic, hydrostatic, Boussinesq, free-surface ocean model that allows its vertical coordinate to consist of σ coordinates for the upper layers and z-levels below a user-specified depth. This flexibility allows implementation of a hybrid σ–z coordinate system that is expected to mitigate some of the weaknesses that can be associated with either pure coordinate option. For the global NCOM application, the σ–z coordinate is used to allow terrain-following σ coordinates in the upper ocean, providing better resolution and topographic fidelity in shelf regions where flow is most sensitive to its representation. Including z coordinates for deeper regions efficiently maintains high near-surface vertical resolution in the open ocean. Investigation into the impact of the selected coordinate system focuses on differences between atmospherically-forced free-running (no assimilation) global solutions using σ–z and pure z coordinates. Comparisons with independent temperature observations indicate that global NCOM using the σ–z coordinate has improved skill relative to its z coordinate implementation. Among other metrics, we show that in comparison with time series of surface temperature from National Oceanic Data Center (NODC) buoys, mostly located in coastal regions, root mean squared differences (RMSD) improved for 63% and correlation improved for 71% of the stations when σ–z coordinates were used instead of pure z. For the exclusively open-ocean Tropical Atmosphere-Ocean (TAO) buoys, differences between the simulations were small, with the σ–z showing smaller RMSD for 45% of the stations and higher correlation for 65% of the stations. Additional comparisons using temperature profile observations further confirm a tendency for improved performance using the hybrid σ–z coordinates. [Copyright &y& Elsevier]