Most Downloaded Ocean Modelling Articles
1. Physical processes that impact the evolution of global mean sea level in ocean climate models
July 2012
Stephen M. Griffies | Richard J. Greatbatch
Abstract: This paper develops an analysis framework to identify how physical processes, as represented in ocean climate models, impact the evolution of global mean sea level. The formulation utilizes the coarse grained equations appropriate for an ocean model, and starts from the vertically integrated mass conservation equation in its Lagrangian form. Global integration of this kinematic equation results in an evolution equation for global mean sea level that depends on two physical processes: boundary fluxes of mass and the non-Boussinesq steric effect. The non-Boussinesq steric effect itself contains contributions from boundary fluxes of buoyancy; interior buoyancy changes associated with parameterized subgrid scale processes; and motion across pressure surfaces. The non-Boussinesq steric effect can be diagnosed in either volume conserving Boussinesq or mass conserving non-Boussinesq ocean circulation models, with differences found to be negligible.We find that surface heating is the dominant term affecting sea level arising from buoyancy fluxes, contributing to a net positive tendency to global mean sea level, largely due to low latitude heating and because the thermal expansion coefficient is much larger in the tropics than high latitudes. Subgrid scale effects from parameterized quasi-Stokes transport, vertical diffusion, cabbeling, and thermobaricity are also found to be significant, each resulting in a reduction of global mean sea level. Sea level rise through low latitude heating is largely compensated by a sea level drop from poleward eddy heat transport and ocean mixing. Spatial variations in the thermal expansion coefficient provide an essential modulation of how physical effects from mixing and eddy induced advective transport impact global mean sea level.
2. A modeling study on the response of Chesapeake Bay to hurricane events of Floyd and Isabel
June 2012
Kyoung-Ho Cho | Harry V. Wang | Jian Shen | Arnoldo
Valle-Levinson | Yi-cheng Teng
Abstract: The response of Chesapeake Bay to forcing from two hurricanes is investigated using an unstructured-grid three-dimensional hydrodynamic model SELFE. The model domain includes Chesapeake Bay, its tributaries, and the extended continental shelf in the mid-Atlantic Bight. The hurricanes chosen for the study are Hurricane Floyd (1999) and Hurricane Isabel (2003), both of which made landfall within 100km of the mouth of the Bay. The model results agree reasonably well with field observations of water level, velocity, and salinity. From the Bay’s water level response to the hurricanes, it was found that the storm surge in the Bay has two distinct stages: an initial stage set up by the remote winds and the second stage – a primary surge induced by the local winds. For the initial stage, the rising of the coastal sea level was setup by the remote wind of both hurricanes similarly, but for the second stage, the responses to the two hurricanes’ local winds are significantly different. Hurricane Floyd was followed by down-Bay winds that canceled the initial setup and caused a set-down from the upper Bay. Hurricane Isabel, on the other hand, was followed by up-Bay winds, which reinforced the initial setup and continued to rise up against the head of the Bay. From the perspective of volume and salt fluxes, it is evident that an oceanic saltwater influx is pushed into the Bay from the continental shelf by the remote wind fields in the initial stages of the storm surge for both Floyd and Isabel. In the second stage after the hurricane made landfall, the Bay’s local wind plays a key role in modulating the salinity and velocity fields through vertical mixing and longitudinal salt transport. Controlled numerical experiments are conducted in order to identify and differentiate the roles played by the local wind in stratified and destratified conditions. Down-estuary local wind stress (of Hurricane Floyd-type) tends to enhance stratification under moderate winds, but exhibits an increasing-then-decreasing stage when the wind stress increases. The up-estuary local wind stress (of Hurricane Isabel-type) tends to penetrate deeper into the water column, which reduces stratification by reversing gravitational circulation. To characterize mixing conditions in the estuary, a modified horizontal Richardson number that incorporates wind stress, wind direction, horizontal salinity gradient, and vertical eddy viscosity is used for both hurricanes. Finally, the direct precipitation of rainfall into the Bay during Hurricane Floyd appears to create not only a thin surface layer of low salinity but also a seaward barotropic pressure gradient that affects the subsequent redistribution of salinity after the storm.
3. Two-way nesting in split-explicit ocean models: Algorithms, implementation and validation
June 2012
Laurent Debreu | Patrick Marchesiello | Pierrick Penven | Gildas
Cambon
Abstract: A full two-way nesting approach for split-explicit, free surface ocean models is presented. It is novel in three main respects: the treatment of grid refinement at the fast mode (barotropic) level; the use of scale selective update schemes; the conservation of both volume and tracer contents via refluxing. An idealized application to vortex propagation on a β plane shows agreement between nested and high resolution solutions. A realistic application to the California Current System then confirm these results in a complex configuration. The selected algorithm is now part of ROMS_AGRIF. It is fully consistent with ROMS parallel capabilities on both shared and distributed memory architectures. The nesting implementation authorizes several nesting levels and several grids at any particular level. This operational capability, combined with the inner qualities of our two-way nesting algorithm and generally high-order accuracy of ROMS numerics, allow for realistic simulation of coastal and ocean dynamics at multiple, interacting scales.
4. Oceanic three-dimensional Lagrangian coherent structures: A study of a mesoscale eddy in the Benguela upwelling region
July 2012
João H. Bettencourt | Cristóbal López | Emilio
Hernández-García
Abstract: We study three dimensional (3D) oceanic Lagrangian coherent structures (LCSs) in the Benguela upwelling region obtained from an output of the regional ocean modeling system (ROMS). To do that, we first computed finite-size Lyapunov exponent (FSLE) fields in the region volume that characterize mesoscale stirring and mixing. Average FSLE values generally decreased with depth, but there was a local maximum at a depth of approximately 100m. LCSs are extracted as the ridges of the calculated FSLE fields. They present a “curtain-like” geometry in which the strongest attracting and repelling structures appear as quasivertical surfaces. LCSs around a particular cyclonic eddy, pinched off from the upwelling front, are also calculated. The LCSs are confirmed to provide pathways and barriers to transport into and out of the eddy.
5. The regional oceanic modeling system (ROMS): a split-explicit, free-surface, topography-following-coordinate oceanic model
2005
Alexander F. Shchepetkin | James C. McWilliams
Abstract: The purpose of this study is to find a combination of optimal numerical algorithms for time-stepping and mode-splitting suitable for a high-resolution, free-surface, terrain-following coordinate oceanic model. Due to mathematical feedback between the baroclinic momentum and tracer equations and, similarly, between the barotropic momentum and continuity equations, it is advantageous to treat both modes so that, after a time step for the momentum equation, the computed velocities participate immediately in the computation of tracers and continuity, and vice versa, rather than advancing all equations for one time step simultaneously. This leads to a new family of time-stepping algorithms that combine forward–backward feedback with the best known synchronous algorithms, allowing an increased time step due to the enhanced internal stability without sacrificing its accuracy. Based on these algorithms we design a split-explicit hydrodynamic kernel for a realistic oceanic model, which addresses multiple numerical issues associated with mode splitting. This kernel utilizes consistent temporal averaging of the barotropic mode via a specially designed filter function to guarantee both exact conservation and constancy preservation properties for tracers and yields more accurate (up to second-order), resolved barotropic processes, while preventing aliasing of unresolved barotropic signals into the slow baroclinic motions. It has a more accurate mode-splitting due to redefined barotropic pressure-gradient terms to account for the local variations in density field, while maintaining the computational efficiency of a split model. It is naturally compatible with a variety of centered and upstream-biased high-order advection algorithms, and helps to mitigate computational cost of expensive physical parameterization of mixing processes and submodels.
6. On the stability and accuracy of the harmonic and biharmonic isoneutral mixing operators in ocean models
August 2012
F. Lemarié | L. Debreu | A.F. Shchepetkin | J.C.
McWilliams
Abstract: Ocean models usually rely on a tracer mixing operator which diffuses along isoneutral directions. This requirement is imposed by the highly adiabatic nature of the oceanic interior, and a numerical simulation needs to respect these small levels of dianeutral mixing to maintain physically realistic results. For non-isopycnic models this is however non-trivial due to the non-alignment of the vertical coordinate isosurfaces with local isoneutral directions, rotated mixing operators must therefore be used. This paper considers the numerical solution of initial boundary value problems for the harmonic (Laplacian) and biharmonic rotated diffusion operators. We provide stability criteria associated with the conventional space–time discretizations of the isoneutral Laplacian operator currently in use in general circulation models. Furthermore, we propose and study possible alternatives to those schemes. A new way to handle the temporal discretization of the rotated biharmonic operator is also introduced. This scheme requires only the resolution of a simple one-dimensional tridiagonal system in the vertical direction to provide the same stability limit of the non-rotated operator. The performance of the various schemes in terms of stability and accuracy is illustrated by idealized numerical experiments of the diffusion of a passive tracer along isoneutral directions.
7. Modeling the west Florida coastal ocean by downscaling from the deep ocean, across the continental shelf and into the estuaries
May 2012
Lianyuan Zheng | Robert H. Weisberg
Abstract: We arrive at a coastal ocean circulation model, suitable for downscaling from the deep ocean, across the continental shelf and into the estuaries, by nesting the unstructured grid, Finite Volume Coastal Ocean Model (FVCOM, inner model) into the structured grid, Global Hybrid Coordinate Model (HYCOM, outer model). The coastal ocean circulation model is three-dimensional, density dependent and inclusive of tides (eight constituents). A calendar year 2007 simulation for the west Florida continental shelf is quantitatively tested against in situ observations of sea level from coastal tide gauges and water column currents and temperature from moored acoustic Doppler current profilers. Agreements between model simulations and observations for both tides and low frequency variability over the calendar year demonstrate the usefulness of our approach. Model horizontal resolution varies from around 12km at the open boundary to 150m in the estuaries. Sensitivity experiments for vertical resolution led to the adoption of 21 σ-layers. Several model limitations are discussed, including seasonal steric effects and deep ocean (outer) model errors that may propagate through the inner model. With adequate observations spanning the inner model domain, we may determine when the outer model is in error at the nesting zone. This finding further highlights the need for coordinating coastal ocean observing and modeling programs. The nesting of unstructured and structured grid models is a new approach to coastal ocean circulation modeling. It provides a means for circulation hindcasts and nowcasts/forecasts, and, after combining with biological process models, may provide a framework for multi-disciplinary modeling of coastal ocean ecology from the deep ocean to the head of tides.
8. Importance of wave age and resonance in storm surges: The case Xynthia, Bay of Biscay
2012
Xavier Bertin | Nicolas Bruneau | Jean-François Breilh | André B.
Fortunato | Mikhail Karpytchev
Abstract: This study aims to hindcast and analyze the storm surge associated with Xynthia, a mid-latitude depression that severely hit the French central part of the Bay of Biscay on the 27–28th of February 2010. The main losses in human lives and damages were caused by the associated storm surge, which locally exceeded 1.5m and peaked at the same time as a high spring tide, causing the flooding of low-lying coasts. A new storm surge modeling system was developed, based on the unstructured-grid circulation model SELFE and the spectral wave model WaveWatchIII. The modeling system was implemented over the North-East Atlantic Ocean and resulted in tidal and wave predictions with errors of the order of 3% and 15%, respectively. The storm surge associated with Xynthia was also well predicted along the Bay of Biscay, with only a slight underestimation of the surge peak by 3–8%. Numerical experiments were then performed to analyze the physical processes controlling the development of the storm surge and revealed firstly that the wind caused most of the water level anomaly through an Ekman setup process. The comparison between a wave-dependant and a quadratic parameterization to compute wind stress showed that the storm surge was strongly amplified by the presence of steep and young wind-waves, related to their rapid development in the restricted fetch of the Bay of Biscay. In the central part of the Bay of Biscay, both observed and predicted water level anomalies at landfall displayed ∼6h oscillations, with amplitudes of up to 0.2m (10–20% of the surge peak). An analytical shelf resonance model and numerical experiments demonstrated that the period of the observed oscillations corresponds to the resonant mode of the continental shelf in the central part of the Bay of Biscay. It is concluded that these oscillations originate from the interactions between the water level perturbation and the continental shelf and this phenomenon is expected to be relevant at other places along the world’s coastlines.
9. Implementation of the vortex force formalism in the coupled ocean-atmosphere-wave-sediment transport (COAWST) modeling system for inner shelf and surf zone applications
2012
Nirnimesh Kumar | George Voulgaris | John C. Warner | Maitane
Olabarrieta
Abstract: The coupled ocean-atmosphere-wave-sediment transport modeling system (COAWST) enables simulations that integrate oceanic, atmospheric, wave and morphological processes in the coastal ocean. Within the modeling system, the three-dimensional ocean circulation module (ROMS) is coupled with the wave generation and propagation model (SWAN) to allow full integration of the effect of waves on circulation and vice versa. The existing wave-current coupling component utilizes a depth dependent radiation stress approach. In here we present a new approach that uses the vortex force formalism. The formulation adopted and the various parameterizations used in the model as well as their numerical implementation are presented in detail. The performance of the new system is examined through the presentation of four test cases. These include obliquely incident waves on a synthetic planar beach and a natural barred beach (DUCK’ 94); normal incident waves on a nearshore barred morphology with rip channels; and wave-induced mean flows outside the surf zone at the Martha’s Vineyard Coastal Observatory (MVCO).Model results from the planar beach case show good agreement with depth-averaged analytical solutions and with theoretical flow structures. Simulation results for the DUCK’ 94 experiment agree closely with measured profiles of cross-shore and longshore velocity data from Garcez Faria et al. (1998, 2000). Diagnostic simulations showed that the nonlinear processes of wave roller generation and wave-induced mixing are important for the accurate simulation of surf zone flows. It is further recommended that a more realistic approach for determining the contribution of wave rollers and breaking induced turbulent mixing can be formulated using non-dimensional parameters which are functions of local wave parameters and the beach slope. Dominant terms in the cross-shore momentum balance are found to be the quasi-static pressure gradient and breaking acceleration. In the alongshore direction, bottom stress, breaking acceleration, horizontal advection and horizontal vortex forces dominate the momentum balance. The simulation results for the bar/rip channel morphology case clearly show the ability of the modeling system to reproduce horizontal and vertical circulation patterns similar to those found in laboratory studies and to numerical simulations using the radiation stress representation. The vortex force term is found to be more important at locations where strong flow vorticity interacts with the wave-induced Stokes flow field. Outside the surf zone, the three-dimensional model simulations of wave-induced flows for non-breaking waves closely agree with flow observations from MVCO, with the vertical structure of the simulated flow varying as a function of the vertical viscosity as demonstrated by Lentz et al. (2008).
10. A sea-ice sensitivity study with a global ocean-ice model
July 2012
P. Uotila | S. O’Farrell | S.J. Marsland | D. Bi
Abstract: The sensitivity of the global sea-ice distribution in the Australian Climate Ocean Model (AusCOM) to a range of parameter values related to sea-ice physics was explored. The sea-ice component of AusCOM is the U.S. Los Alamos National Laboratory Sea Ice Model (CICE4.1) and the ocean component is the Modular Ocean Model developed at NOAA’s Geophysical Fluid Dynamics Laboratory (GFDL MOM4p1). We aimed to determine optimal sets of parameter values to produce as realistic a global sea-ice distribution as possible. A small number of sets of optimal parameter values was selected based on the closest match between the model and observationally constrained model climatologies. New detailed information is provided on the sensitivity of the global sea-ice distribution to the parameters not studied this extensively before. The sea-ice distribution shows a similar degree of sensitivity to parameters determining ice-ocean stress, mechanical redistribution, oceanic heat and shortwave radiation. Accordingly, AusCOM can be effectively tuned to produce realistic sea ice by parameters internal to the sea-ice model. The sensitivity of ice volume is stronger than that of ice area indicating that the internal ice model parameters mostly influence the ice thickness. The sea-ice area has significantly weaker sensitivity to the sea-ice model parameters considered, particularly in winter. Then, the evolution of sea ice is dominated by external factors, such as location of land, and atmospheric and oceanic forcing. The performance of the ocean model is crucial in producing a realistic sea-ice cover, in both the Arctic and the Antarctic.
11. Variational assimilation of HF radar surface currents in a coastal ocean model off Oregon
June 2012
Peng Yu | Alexander L. Kurapov | Gary D. Egbert | John S. Allen
| P. Michael Kosro
Abstract: The impact of assimilation of sea surface velocity fields observed by a set of high-frequency (HF) radars is studied using a three-dimensional ocean circulation model configured along the Oregon coast. The study period is June–July 2008 featuring upwelling and separation of the coastal currents into the adjacent interior ocean. The nonlinear model is based on the Regional Ocean Modeling System (ROMS) and the data assimilation (DA) component on the AVRORA system utilizing the representer-based variational algorithm. Assimilation proceeds in a series of 3-day windows, providing an analysis solution in each window and a 3-day forecast into the next window. Experiments with two different initial condition error covariances are compared (one is dynamically balanced, based on the linearized equation of state, temperature-salinity relation, and geostrophic and thermal wind balance relations and the other is multivariate unbalanced). While the assimilation impact is statistically better in the case of the balanced covariance, the case with the unbalanced covariance also provides sensible improvement in terms of surface velocity and sea surface temperature (SST) model-data forecast statistics. The analysis of representer functions shows that even if the initial condition error covariance is unbalanced, the correction fields at the model initial time are partially balanced after each dynamical field is smoothed independently, due to inherent dynamical properties of the adjoint model. Assimilation of the HF radar surface currents improves not only surface velocity forecasts, but also geometry of the upwelling SST front and the sea surface height (SSH) slope near the coast, as verified against unassimilated satellite SSH and SST data. The assimilation also alters the latitudinal distribution of the time-averaged offshore transport. Combined HF radar velocity and other observations, e.g., altimetry, is needed to better constrain surface geostrophic currents in the entire model domain, including the area not covered by the HF radar data.
12. Large-scale impacts of submesoscale dynamics on phytoplankton: Local and remote effects
2012
M. Lévy | D. Iovino | L. Resplandy | P. Klein | G. Madec | A.-M.
Tréguier | S. Masson | K. Takahashi
Abstract: The sensitivity of an idealized bio-physical model of seasonally varying subtropical and subpolar gyres to increased horizontal resolution is presented. Switching from mesoscale-resolving (1/9°) to submesoscale-resolving (1/54°) allows the emergence of a denser and more energetic vortex population sustained by submesoscale physics. The experiments display a global decrease in phytoplankton abundance of ∼10–20% as the resolution is refined. This result contrasts with previous studies, which suggested that eddy-driven vertical fluxes of nutrients locally boost phytoplankton growth at mid-latitudes in the open ocean. The explanation is that the long-term (50years) modification of the large-scale, or basin-scale, circulation and distribution of nutrients by submesoscale processes (remote effects), not taken into account by previous studies because of their much shorter time integration, balance the local, small-scale effects. More precisely, dynamical remote effects (involving only the physics) affect the location of the boundary between the two gyres as well as the large-scale mixed-layer depth (MLD) and thermocline depth. Biological remote effects result from the dynamical remote effects that strongly modify the physical–biological interactions at all scales, including at large scales. These biological remote effects involve changes of the nutricline depth. In the mid-latitude subpolar gyre, phytoplankton decrease in abundance at higher resolution is due to the shallower MLD and nutricline, which promote zooplankton grazing; in the subtropical gyre, it is due to deeper MLD and nutricline, which diminishes regenerated production. In addition, remote effects modulate the dynamical supplies of nutrients to the euphotic layer through a combination of changes in mean advection, eddy advection and vertical mixing.
13. Wave climate variability in the North-East Atlantic Ocean over the last six decades
2010
Guillaume Dodet | Xavier Bertin | Rui Taborda
Abstract: Ocean surface gravity waves play a major role in many engineering and environmental problems, both in the open ocean and in coastal zones. Therefore, it is essential to improve our knowledge on spatial and temporal variability of wave climate. This study aims at investigating this variability in the North-East Atlantic Ocean (25°W–0°W and 30°N–60° N), using a 57-year hindcast (1953–2009) obtained with a spectral wave model forced with reanalysis wind fields. The hindcast analysis reveals firstly strong seasonal fluctuations of wave climate, with winters characterized by large and long-period waves of mean direction spreading from south-west to north-west, and summers characterized by smaller and shorter-period waves originating from norther directions. From northern (55°N) to southern (35°N) latitudes, the significant wave height (Hs) decreases by roughly 40%, the mean wave direction (Mwd) rotates clockwise by about 25% while the peak period (Tp) only grows by 5%. These three parameters also exhibit a strong inter-annual variability, particularly when winter-means (from 1st of December to 1st of April) are considered. Linear trend analysis over the studied period shows spatially variable long-term trends, with a significant increase of Hs (up to 0.02m yr−1) and a counterclockwise shift of Mwd (up to −0.1°yr−1) at northern latitude, contrasting with a fairly constant trend for Hs and a clockwise shift of Mwd (up to +0.15°yr−1) at southern latitudes. Long-term trends of Tp are less significant, with still a slight increase in the north-eastern part of the study area (up to +0.01 syr−1). Eventually, a comparison between the inter-annual variability of the winter-means of the three selected wave parameters and the North Atlantic Oscillation (NAO) reveals: (1) a strong positive correlation between Hs and the NAO index at northern latitudes (correlation coefficient up to R=0.91) and a significant negative correlation at southern latitudes (up to R=−0.6); (2) no significant correlation for Mwd north of 40°N and a clear positive correlation southward of 40°N (up to R=0.8) and (3) a northward increasing correlation for Tp (up to R=0.8). Long-term trends for Hs, Mwd and Tp are finally explained by a significant increase in the NAO index over the studied period.
14. A dike–groyne algorithm in a terrain-following coordinate ocean model (FVCOM): Development, validation and application
2012
Jianzhong Ge | Changsheng Chen | Jianhua Qi | Pingxing Ding | Robert
C. Beardsley
Abstract: A dike–groyne module is developed and implemented into the unstructured-grid, three-dimensional primitive equation finite-volume coastal ocean model (FVCOM) for the study of the hydrodynamics around human-made construction in the coastal area. The unstructured-grid finite-volume flux discrete algorithm makes this module capable of realistically including narrow-width dikes and groynes with free exchange in the upper column and solid blocking in the lower column in a terrain-following coordinate system. This algorithm used in the module is validated for idealized cases with emerged and/or submerged dikes and a coastal seawall where either analytical solutions or laboratory experiments are available for comparison. As an example, this module is applied to the Changjiang Estuary where a dike–groyne structure was constructed in the Deep Waterway channel in the inner shelf of the East China Sea (ECS). Driven by the same forcing under given initial and boundary conditions, a comparison was made for model-predicted flow and salinity via observations between dike–groyne and bed-conforming slope algorithms. The results show that with realistic resolution of water transport above and below the dike–groyne structures, the new method provides more accurate results. FVCOM with this MPI-architecture parallelized dike–groyne module provides a new tool for ocean engineering and inundation applications in coastal regions with dike, seawall and/or dam structures.
15. Sensitivity of a marine coupled physical biogeochemical model to time resolution, integration scheme and time splitting method
August 2012
Momme Butenschön | Marco Zavatarelli | Marcello Vichi
Abstract: Coupled marine biogeochemical models are composed of a hydrodynamic component with a transport model for the ecological state variables and a model for the biogeochemical dynamics. The combination of these components involves the implementation of a numerical coupling method, that performs the spatial–temporal integration of the combined system, introducing an additional source of error to the system (splitting error). In this article we demonstrate the sensitivity of a comparatively complex 1D hydrodynamical biogeochemical model to the coupling method, showing that for an inadequate choice of the coupling method, the splitting error may dominate the numerical error of the system. It is demonstrated that for this type of system the tracer transport time scale clearly dominates over the scale of the biogeochemical processes, that maybe computed on significantly coarser time scales. In between the implemented coupling schemes Operator Splitting and Source Splitting, the Source Splitting method inserting the biogeochemical rates into the transport tracer integration is to be preferred for these type of models.
16. Open boundary conditions for long-term integration of regional oceanic models
2001
Patrick Marchesiello | James C. McWilliams | Alexander
Shchepetkin
Abstract: Regional oceanic models can be developed and used efficiently for the investigation of regional and coastal domains, provided a satisfactory prescription for the open boundary conditions (OBCs) is found. We propose in this paper an adaptive algorithm where inward and outward information fluxes are treated separately. Because of the essentially hyperbolic nature of the incompressible, hydrostatic Primitive Equations, external data are required only for inward boundary fluxes. The outward fluxes are treated with a new algorithm for two-dimensional radiation. Special attention is given to the estimation of the radiation phase speed, essential for detecting the direction of boundary fluxes. The boundary conditions are applied and assessed on a US West Coast (USWC) configuration of the Regional Oceanic Modeling System (ROMS). Our guiding principles are that the numerical solution be stable over multiple years, reach a meaningful statistical equilibrium, and be realistic with respect to the available observational data. A sensitivity analysis suggests that the oblique radiation is robust and sufficiently accurate to detect the direction of information fluxes. The adaptive nudging adequately incorporates the external information minimizing over- and under-specification problems. In addition, a volume constraint based on global correction of normal barotropic velocities improves the overall performances of the open boundary conditions.
17. Ocean–atmosphere dynamics during Hurricane Ida and Nor’Ida: An application of the coupled ocean–atmosphere–wave–sediment transport (COAWST) modeling system
2012
Maitane Olabarrieta | John C. Warner | Brandy Armstrong | Joseph B.
Zambon | Ruoying He
Abstract: The coupled ocean–atmosphere–wave–sediment transport (COAWST) modeling system was used to investigate atmosphere–ocean–wave interactions in November 2009 during Hurricane Ida and its subsequent evolution to Nor’Ida, which was one of the most costly storm systems of the past two decades. One interesting aspect of this event is that it included two unique atmospheric extreme conditions, a hurricane and a nor’easter storm, which developed in regions with different oceanographic characteristics. Our modeled results were compared with several data sources, including GOES satellite infrared data, JASON-1 and JASON-2 altimeter data, CODAR measurements, and wave and tidal information from the National Data Buoy Center (NDBC) and the National Tidal Database. By performing a series of numerical runs, we were able to isolate the effect of the interaction terms between the atmosphere (modeled with Weather Research and Forecasting, the WRF model), the ocean (modeled with Regional Ocean Modeling System (ROMS)), and the wave propagation and generation model (modeled with Simulating Waves Nearshore (SWAN)). Special attention was given to the role of the ocean surface roughness. Three different ocean roughness closure models were analyzed: DGHQ (which is based on wave age), TY2001 (which is based on wave steepness), and OOST (which considers both the effects of wave age and steepness). Including the ocean roughness in the atmospheric module improved the wind intensity estimation and therefore also the wind waves, surface currents, and storm surge amplitude. For example, during the passage of Hurricane Ida through the Gulf of Mexico, the wind speeds were reduced due to wave-induced ocean roughness, resulting in better agreement with the measured winds. During Nor’Ida, including the wave-induced surface roughness changed the form and dimension of the main low pressure cell, affecting the intensity and direction of the winds. The combined wave age- and wave steepness-based parameterization (OOST) provided the best results for wind and wave growth prediction. However, the best agreement between the measured (CODAR) and computed surface currents and storm surge values was obtained with the wave steepness-based roughness parameterization (TY2001), although the differences obtained with respect to DGHQ were not significant. The influence of sea surface temperature (SST) fields on the atmospheric boundary layer dynamics was examined; in particular, we evaluated how the SST affects wind wave generation, surface currents and storm surges. The integrated hydrograph and integrated wave height, parameters that are highly correlated with the storm damage potential, were found to be highly sensitive to the ocean surface roughness parameterization.
18. Performance of four turbulence closure models implemented using a generic length scale method
2005
John C. Warner | Christopher R. Sherwood | Hernan G. Arango | Richard
P. Signell
Abstract: A two-equation turbulence model (one equation for turbulence kinetic energy and a second for a generic turbulence length-scale quantity) proposed by Umlauf and Burchard [J. Marine Research 61 (2003) 235] is implemented in a three-dimensional oceanographic model (Regional Oceanographic Modeling System; ROMS v2.0). These two equations, along with several stability functions, can represent many popular turbulence closures, including the k–kl (Mellor–Yamada Level 2.5), k–ε, and k–ω schemes. The implementation adds flexibility to the model by providing an unprecedented range of turbulence closure selections in a single 3D oceanographic model and allows comparison and evaluation of turbulence models in an otherwise identical numerical environment. This also allows evaluation of the effect of turbulence models on other processes such as suspended–sediment distribution or ecological processes. Performance of the turbulence models and sediment–transport schemes is investigated with three test cases for (1) steady barotropic flow in a rectangular channel, (2) wind-induced surface mixed-layer deepening in a stratified fluid, and (3) oscillatory stratified pressure-gradient driven flow (estuarine circulation) in a rectangular channel. Results from k–ε, k–ω, and gen (a new closure proposed by Umlauf and Burchard [J. Marine Research 61 (2003) 235]) are very similar for these cases, but the k–kl closure results depend on a wall-proximity function that must be chosen to suit the flow. Greater variations appear in simulations of suspended–sediment concentrations than in salinity simulations because the transport of suspended–sediment amplifies minor variations in the methods. The amplification is caused by the added physics of a vertical settling rate, bottom stress dependent resuspension, and diffusive transport of sediment in regions of well mixed salt and temperature. Despite the amplified sensitivity of sediment to turbulence models in the estuary test case, the four closures investigated here all generated estuarine turbidity maxima that were similar in their shape, location, and concentrations.
19. Saltwater intrusion and ETM dynamics in a tidally-energetic stratified estuary
June 2012
Michel A.J. de Nijs | Julie D. Pietrzak
Abstract: We explore the dynamics of the salt wedge and estuarine turbidity maxima (ETMs) in the Rotterdam Waterway using three-dimensional model simulations. These are compared to 13-h time series of profiles of velocity, salinity and suspended particulate matter (SPM) at a number of boat stations along the estuary, and long-term water level and salinity records. Evaluation of the numerical results shows that while good agreement is found between predicted and measured water levels and tidal discharges, the model under-predicts saltwater intrusion and stratification, and it over-predicts the height of the pycnocline above the bed. This leads to deficiencies in predictions of (1) the magnitude and vertical distribution of the baroclinic pressure gradient and subsequently of local shear and (2) vertical SPM gradients and concentrations near the bed because salinity stratification determines this distribution. However, the stability of the salt wedge during tidal excursions, the dominant role played by currents caused by the baroclinic pressure gradient and the damping of turbulence at the pycnocline with subsequent trapping of fluvial SPM at the head of the salt wedge are all well reproduced. A single stable ETM is formed when the salt wedge remains in the Rotterdam Waterway at low water slack. When saltwater intrudes farther up-estuary, multiple stable along-channel ETMs are maintained by localized trapping of fluvial SPM at the respective heads of saltwater by salinity gradients. Our results demonstrate that (1) the saltwater intrusion length is one of the main parameters controlling the SPM trapping probability and (2) the ETM is an advective phenomenon determining the timing of the availability of SPM for exchange with harbours. The model results indicate that all sediments deposited in the harbours along the Rotterdam Waterway and New Meuse are of fluvial origin.
20. Shock-capturing non-hydrostatic model for fully dispersive surface wave processes
2012
Gangfeng Ma | Fengyan Shi | James T. Kirby
Abstract: This paper describes NHWAVE, a shock-capturing non-hydrostatic model for simulating wave refraction, diffraction, shoaling, breaking and landslide-generated tsunami in finite water depth. The governing equations are the incompressible Navier–Stokes equations in conservative form, written in surface and terrain-following form using a σ coordinate. In order to apply a Godunov-type scheme, the velocities are defined at cell centers. The dynamic pressure is defined at vertically facing cell faces so that the pressure boundary condition at free surface can be precisely imposed. The HLL Riemann approximation is employed to estimate fluxes at horizontal cell faces. The nonlinear Strong Stability-Preserving (SSP) Runge–Kutta scheme is used to obtain second-order accuracy in time. The model is validated using seven test cases based on analytical solutions and experimental data. The computed results show that the model can well predict wave propagation over submerged bar, wave refraction and diffraction over an elliptical shoal as well as wave breaking, run-up and longshore current on a plane beach using relatively few (three to five) vertical layers. The model is also shown to provide accurate reproduction of a generated tsunami wave resulting from a solid translating bottom feature.
21. Impact of different tidal renewable energy projects on the hydrodynamic processes in the Severn Estuary, UK
2010
Junqiang Xia | Roger A. Falconer | Binliang Lin
Abstract: The Severn Estuary, located in the UK between south east Wales and south west England, is an ideal site for tidal renewable energy projects, since this estuary has the third highest tidal range in the world, with a spring tidal range approaching 14m. The UK Government recently invited proposals for tidal renewable energy projects from the estuary and many proposals were submitted for consideration. Among the proposals submitted and subsequently shortlisted were: the Cardiff–Weston Barrage, the Fleming Lagoon and the Shoots Barrage, all three of which are nationally public interest. Therefore a two-dimensional finite volume numerical model, based on an unstructured triangular mesh, has been refined to study the hydrodynamic impact and flood inundation extent, post construction, of all three of these proposed tidal power projects. The model-predicted hydrodynamic processes have been analysed in detail, both without and with the structures, including the discharge processes at key sections, the contours of maximum and minimum water levels, the envelope curves of high and low water levels, the maximum tidal currents, the local velocity fields around the structures and the mean power output curves. Simulated results indicate that: (i) although the construction of the Cardiff–Weston Barrage would have an adverse impact on a range of environmental aspects, due to there being approximately a 50% decrease in the peak discharge entering the upstream region, it would reduce the maximum water levels upstream of the barrage by typically 0.3–1.2m, which could be positive in respect of coastal flooding; (ii) the construction of the Fleming Lagoon would have little influence on the hydrodynamic processes in the Severn Estuary; and (iii) the construction of the Shoots Barrage would decrease the maximum water levels upstream of the M4 bridge by between 0.3 and 1.0m, but it could lead to an increase in the maximum water levels downstream of the barrage by typically 20–30cm.
22. Grid degradation of submesoscale resolving ocean models: Benefits for offline passive tracer transport
May 2012
M. Lévy | L. Resplandy | P. Klein | X. Capet | D. Iovino | C.
Ethé
Abstract: A numerical solution for an idealized double-gyre is used to investigate the sensitivity of ocean dynamics and passive tracer advection to horizontal resolution (Δx) in a mesoscale eddy rich regime. In agreement with previous studies, we find that ocean dynamical solutions are strongly sensitive to grid resolution. With mesoscale resolution (Δx∼O(10)km), eddies are marginally resolved and their impact on tracer transport is not well represented. At submesoscale resolution (Δx∼O(1)km), the number of mesoscale eddies and their energy is increased, due to the resolved submesoscales. The changes are mostly seen in the vorticity and vertical velocity fields, and are less obvious in the temperature field. In contrast, we demonstrate that the offline transport of passive tracer is not altered when the finest scales (O(1)km) present in the dynamical solutions are discarded. We do so by showing that dynamical solutions obtained with Δx∼O(1)km can be degraded (following a flux preserving procedure) down to resolutions ΔX∼O(10)km without significantly impacting passive tracer solutions. The reason for this stems from the level of dissipation/diffusion required during the integration of the dynamical model which smoothes variance at wavelength smaller than at least 5–10 Δx. This result is used to derive a method which alleviates data storage needs and accelerates tracer advection simulations, with a gain of the order of 103 in computing time. The method involves three steps: (1) on-line resolution of the dynamics with Δx∼O(1)km, (2) degradation of the 3D velocity field on a ΔX∼O(10)km grid and (3) off-line tracer transport with the degraded velocity on the ΔX grid. It opens promising perspectives for submesoscale bio-physical modelling at reduced numerical cost.
23. Coordinated Ocean-ice Reference Experiments (COREs)
2009
Stephen M. Griffies | Arne Biastoch | Claus Böning | Frank Bryan |
Gokhan Danabasoglu | Eric P. Chassignet | Matthew H. England | Rüdiger Gerdes |
Helmuth Haak | Robert W. Hallberg | Wilco Hazeleger | Johann Jungclaus | William
G. Large | Gurvan Madec | Anna Pirani | Bonita L. Samuels | Markus Scheinert |
Alex Sen Gupta | Camiel A. Severijns | Harper L. Simmons | Anne Marie Treguier |
Mike Winton | Stephen Yeager | Jianjun Yin
Abstract: Coordinated Ocean-ice Reference Experiments (COREs) are presented as a tool to explore the behaviour of global ocean-ice models under forcing from a common atmospheric dataset. We highlight issues arising when designing coupled global ocean and sea ice experiments, such as difficulties formulating a consistent forcing methodology and experimental protocol. Particular focus is given to the hydrological forcing, the details of which are key to realizing simulations with stable meridional overturning circulations.The atmospheric forcing from [Large, W., Yeager, S., 2004. Diurnal to decadal global forcing for ocean and sea-ice models: the data sets and flux climatologies. NCAR Technical Note: NCAR/TN-460+STR. CGD Division of the National Center for Atmospheric Research] was developed for coupled-ocean and sea ice models. We found it to be suitable for our purposes, even though its evaluation originally focussed more on the ocean than on the sea-ice. Simulations with this atmospheric forcing are presented from seven global ocean-ice models using the CORE-I design (repeating annual cycle of atmospheric forcing for 500 years). These simulations test the hypothesis that global ocean-ice models run under the same atmospheric state produce qualitatively similar simulations. The validity of this hypothesis is shown to depend on the chosen diagnostic. The CORE simulations provide feedback to the fidelity of the atmospheric forcing and model configuration, with identification of biases promoting avenues for forcing dataset and/or model development.
24. Enhanced vertical mixing within mesoscale eddies due to high frequency winds in the South China Sea
2012
Yuley Cardona | Annalisa Bracco
Abstract: The South China Sea is a marginal basin with a complex circulation influenced by the East Asian Monsoon, river discharge and intricate bathymetry. As a result, both the mesoscale eddy field and the near-inertial energy distribution display large spatial variability and they strongly influence the oceanic transport and mixing.With an ensemble of numerical integrations using a regional ocean model, this work investigates how the temporal resolution of the atmospheric forcing fields modifies the horizontal and vertical velocity patterns and impacts the transport properties in the basin. The response of the mesoscale circulation in the South China Sea is investigated under three different forcing conditions: monthly, daily and 6-hourly momentum and heat fluxes.While the horizontal circulation does not display significant differences, the representation of the vertical velocity field displays high sensitivity to the frequency of the wind forcing. If the wind field contains energy at the inertial frequency or higher (daily and 6-hourly cases), then submesoscale fronts, vortex Rossby waves and near inertial waves are excited as ageostrophic expression of the vigorous eddy field. Those quasi- and near-inertial waves dominate the vertical velocity field in the mixed layer (vortex Rossby waves) and below the first hundred meters (near inertial waves) and they are responsible for the differences in the vertical transport properties under the various forcing fields as quantified by frequency spectra, vertical velocity profiles and vertical dispersion of Lagrangian tracers.
25. The Max-Planck-Institute global ocean/sea ice model with orthogonal curvilinear coordinates
2003
S.J. Marsland | H. Haak | J.H. Jungclaus | M. Latif | F. Röske
Abstract: The Hamburg Ocean Primitive Equation model has undergone significant development in recent years. Most notable is the treatment of horizontal discretisation which has undergone transition from a staggered E-grid to an orthogonal curvilinear C-grid. The treatment of subgridscale mixing has been improved by the inclusion of a new formulation of bottom boundary layer (BBL) slope convection, an isopycnal diffusion scheme, and a Gent and McWilliams style eddy-induced mixing parameterisation. The model setup described here has a north pole over Greenland and a south pole on the coast of the Weddell Sea. This gives relatively high resolution in the sinking regions associated with the thermohaline circulation. Results are presented from a 450 year climatologically forced integration. The forcing is a product of the German Ocean Model Intercomparison Project and is derived from the European Centre for Medium Range Weather Forecasting reanalysis. The main emphasis is on the model’s representation of key quantities that are easily associated with the ocean’s role in the global climate system. The global and Atlantic northward poleward heat transports have peaks of 1.43 and 0.84 PW, at 18° and 21° N respectively. The Atlantic meridional overturning streamfunction has a peak of 15.7 Sv in the North Atlantic and an outflow of 11.9 Sv at 30° S. Comparison with a simulation excluding BBL shows that the scheme is responsible for up to a 25% increase in North Atlantic heat transport, with significant improvement of the depths of convection in the Greenland, Labrador and Irminger Seas. Despite the improvements, comparison with observations shows the heat transport still to be too weak. Other outstanding problems include an incorrect Gulf Stream pathway, a too strong Antarctic Circumpolar Current, and a too weak renewal of Antarctic Intermediate Water. Nevertheless, the model has been coupled to the atmospheric GCM ECHAM5 and run successfully for over 250 years without any surface flux corrections.
