Unstructured-grid Model Research Articles

A Two-Way Nesting Unstructured Quadrilateral Grid, Finite-Differencing, Estuarine and Coastal Ocean Model with High-Order Interpolation Schemes

The balance between the need of improving horizontal resolution in simulating local small-scale ocean processes and computational costs makes it desirable to refine model mesh locally. A three-dimensional, two-way nesting unstructured quadrilateral grid, primitive equations, finite-differencing, estuarine and coastal ocean model is developed for multi-scale modeling. Because the model grid is capable of multi-area nesting and multi-level refinement at each subdomain, the model is highly compatible with simulations involved in complex topography and studies of local small-scale ocean processes. The two-way information exchange is achieved by a virtual grid method, and its basic idea is to implement numerical integrations of variables at nesting interfaces with the support of virtual grid variables, which are interpolated or updated from actual grid variables. The model is novel for two interpolation schemes: the high-order spatial interpolation at the middle temporal level (HSIMT) parabolic interpolation scheme and HSIMT advection-equivalent interpolation scheme, and they have high-order accuracy and good consistency with the advection scheme applied to solving the tracer equations. The conservation of both volume and tracer contents is ensured via a flux correction algorithm. The two original interpolation schemes are examined in an ideal salinity advection experiment in the peak preservation skill, stability, and conservation properties. A realistic application to the Deep Waterway Project area in the Changjiang Estuary showed that the nested grid model can reproduce the hydrodynamic processes at the observed sites successfully while it failed to maintain the performance with the structured grid model in simulating the variance of salinity, for which the enforced conservation had primary responsibility. The HSIMT parabolic interpolation scheme was distinguished from other schemes for its outstanding performances in simulating salinity.

Effects of coastal reclamation on the storm surge in the Bohai Bay

In recent years, the Bohai Bay has experienced frequent reclamation activities and significant changes in the coastline with the rapid development of economy. Coastline changes will influence the propagation of tide and ocean waves, and the storm surges may get worse as well. Based on the ADCIRC storm surge model, an unstructured triangular grid model driven by tide and wind was established to simulate the storm surge in Bohai Sea. The numerical simulation of hydrodynamic environment during typhoon NO.9711 under four coastline scenarios which are 1980, 2004, 2008 and 2015 had been carried out to investigate how coastal reclamation affects storm surges. The results reveal that the storm surge level and hazard risk in the reclamation areas has increased along with the changes of the coastline.

Primery mezomasshtabnogo i submezomasshtabnogo chislennogo vikhrerazreshayushchego modelirovaniya okeana

Purpose. The study addresses rotational motion of geophysical fluids in the horizontal and vertical planes. It is aimed mainly at tracing the development of high-resolution numerical modeling of the ocean, as well as at demonstrating new physical processes due to more correct consideration both of the tides in the eddy-resolving numerical models and sub-mesoscale dynamics in the models of the sea straits. Methods and Results. The ocean eddies and their interaction with tides are studied using numerical simulations by four NEMO models for the European North-West shelf with the resolutions ranging from 7 to 1.5 km. The vertical characteristics of motion in the Bosporus Strait were studied using numerical simulations with SCHISM , the unstructured grid model with the ultra-fine model resolution (less than 100 m). The barotropic tidal forcing resulted in substantial flattening of the slopes of the spectral curves. The most important difference between the spectral featurws of four models occurs in the motion rotational component. In the model with the 1.5 km resolution, the magnitude of the vorticity power spectral density at the scales ~70 km is by an order of magnitude higher than in the other three models. Although most of the tidal flattening is associated with the internal tides, beyond a certain horizontal resolution, the eddy dynamics become affected by the barotropic tides. The shelf of the Biscay Bay and the shallows around the Faroe Islands are the most sensitive areas to adding of the barotropic tides to the model forcing. Due to the grid ultra-fine resolution, new elements of physical motion emerged in the Bosporus region. The lateral circulation is dominated by the systems of multiple circulation cells with the scales ~ 1 km. In some areas, the lateral flow magnitude exceeds 0.5 m/s, which is comparable with the magnitude of the axial flow. This reveals importance of the helical elements of the strait circulation for overturning of water masses in the Bosporus. Conclusions. Without proper resolution, the models of tidal oceanic dynamics simulate the ocean general circulation, but do not describe correctly the energy cascades at the eddy scales including interaction between the tides and the mesoscale eddies. Absence of this sub-mesoscale dynamics in the models can largely affect their capability to simulate the two-layer inter-basin exchange.

Coupling an unstructured grid three‐dimensional model with a laterally averaged two‐dimensional model for shallow water hydrodynamics and transport processes

AbstractThis article presents a dynamically coupled 3D‐2DV hydrodynamic model, which combines an unstructured Cartesian grid three‐dimensional hydrodynamic model named UnLESS3D with a laterally averaged model for estuaries (LAMFE). Both UnLESS3D and LAMFE are shallow water models, which solve unsteady Reynolds‐averaged Navier–Stokes equations. The coupled hydrodynamic model, named UnLESS, is very suitable and efficient for rivers, lakes, and estuaries, which have not only large waterbodies but also long, meandering, and narrow tributaries. The dynamic coupling of UnLESS3D and LAMFE is facilitated with a semiimplicit scheme called the free‐surface correction (FSC) method. The resulting coupled model UnLESS is unconditionally stable with respect to the gravity waves, bottom shear stress, and vertical eddy viscosity terms. The UnLESS model was tested with an idealized estuary case which has a large basin and two narrow tributaries, providing freshwater flow to the basin. Model results using UnLESS were compared with those using the UnLESS3D model. It was found that the UnLESS model can reproduce the model results of UnLESS3D. The UnLESS model was also applied to Charlotte Harbor, with its major tributaries flowing to its northern part being included in the simulation domain. A good agreement of model results of water level, current, salinity, and temperature simulated by the UnLESS model with real‐time data measured in Charlotte Harbor and its tributaries suggest that the method presented here is robust in dynamically coupling the 3D unstructured Cartesian Grid model UnLESS3D with the laterally averaged model LAMFE.

Modeling of groundwater flow and transport in coastal karst aquifers

Numerical modeling of karst water systems is a challenge due to vastly different flow regimes in the solution channels and the porous medium, and because of the complex interactions between them. Recent developments with unstructured grid models have enabled robust simulation of such systems due to full coupling of the system of equations and the interactions, within a flexible gridding framework that allows these domains to be discretized independently of each other. These capabilities have been incorporated into the MODFLOW-USG code, which has been developed and released in the public domain by the United States Geological Survey. Further developments to MODFLOW-USG have continued and have also been released in the public domain. The significant features that enhance simulations in karst environments include various optional turbulent flow formulations, solute transport capability and a density-dependent flow simulation option that can be used to model transport of nonaqueous phase liquids (NAPLs) or freshwater–seawater interactions, for example. A dual-porosity-flow and dual-porosity-transport formulation is also included to further expand model scale and flexibility. A physically based representation of flow and transport within the karst conduits (channels) and porous medium eliminates the need for various surrogate modeling approaches in karst, thus enabling solutions to some of the common problems that groundwater professionals face when dealing with groundwater resources management in coastal karst areas. A demonstrative example shows the impact of karst on the saltwater system in a coastal aquifer.

A Numerical Study of Hypoxia in Chesapeake Bay Using an Unstructured Grid Model: Validation and Sensitivity to Bathymetry Representation

AbstractA three‐dimensional unstructured‐grid hydrodynamic and water quality model (Semi‐implicit Cross‐scale Hydroscience Intergrated System Model‐Integrated Compartment Model) is applied successfully for Chesapeake Bay. The model is validated with observations of salinity, chlorophyll‐a, dissolved oxygen, nutrients, and phytoplankton productions from the year 1991 to 1995 for the mainstem and some major tributaries, based on multiple model skill scores. Model experiments are conducted to test the importance of having (1) an accurate representation of bathymetry to correctly predict hypoxia and other processes and (2) a high‐resolution model grid for tributaries to correctly simulate water quality variables. Comparison with the model experiment results with bathymetry smoothing indicates that bathymetry smoothing, as commonly used for many systems, changes the stratification and lateral circulation pattern, resulting in more salt intrusion into shallow water regions, and an increase in the freshwater age. Consequently, a model with bathymetry smoothing can lead to an unrealistic prediction of the distribution of hypoxia and phytoplankton production. Local grid refinement shows significant improvement of model simulations on local stratification and water quality variables. Overall, the use of high‐resolution unstructured grid model leads to a faithful representation of the complex geometry, and thus a seamless cross‐scale capability for simulating water quality processes in the Bay including tributaries and tidal creeks.

Hydrodynamic Simulations of Charlotte Harbor and its Major Tributaries in Florida Using a Dynamically Coupled 3D – 2DV model

Hydrodynamics, salinity transport processes, and thermodynamics in Charlotte Harbor and its major tributaries in Florida, USA were simulated using a coupled three-dimensional (3D) and laterally averaged two-dimensional (2DV) hydrodynamic model named UnLESS, in support of the re-evaluation of minimum freshwater inflows for two water bodies in the system, namely the lower Peace River (LPR) and lower Shell Creek (LSC). The UnLESS model involves a dynamic and two-way coupling of the 3D unstructured Cartesian grid model UnLESS3D (Chen, 2011) with the laterally averaged model LAMFE (Chen, 2003a) using a semi-implicit scheme named the free-surface correction (FSC) method (Chen, 2003b.)The simulation period was January 2013 through August 2014, during which best real-time data of water elevation, current, salinity, and temperature at five measurement stations within the simulation domain were available. Comparisons of model results with these real-time field data show that UnLESS is capable of simulating hydrodynamics, salinity transport processes, and thermodynamics well in the Charlotte Harbor estuary. Good matches of model results with measured data suggest that both estimated ungauged flows and downstream boundary conditions were reasonable. Major physical characteristics in the Charlotte Harbor system are correctly simulated by the UnLESS model. It is demonstrated that the dynamically coupled 3D-2DV model UnLESS is robust and very efficient for complex systems such as Charlotte Harbor, which has a large waterbody with islands and several narrow and meandering tributaries.

Changes in sea ice and future accessibility along the Arctic Northeast Passage

Abstract Retreating Arctic sea ice under rapid warming is projected to continue. A new transarctic route, the Northeast Passage (NEP), may open in the near future, with considerable impacts on global shipping transportation. Comprehensive research on the past changes in sea ice in September and future accessibility along the NEP is essential. In this investigation, an unstructured-grid model was used for accurate fitting to the irregular coastal boundary, and accessibility was assessed under two different shared socioeconomic pathways (SSPs) and two vessel classes with the Arctic transportation accessibility model from 2021−2050. Significant warming was presented in the deep layer in the Arctic seas along the NEP in recent decades (1988−2016), with a distinct band on the outer edge of the abyssal zone. The positive anomaly of seawater temperature moved westward and increased notably in coastal areas, which made sea ice disappear, and the area was dominated by the negative anomaly in the last decade. The NEP is projected to be navigable for open water ships in September from 2021 to 2025, which would extend to August−October during 2025−2050 under both SSP2-4.5 and SSP5-8.5. In addition, Polar Class 6 ships would be capable of crossing the NEP from August to December during 2021−2025 and from July to December during 2026−2050. The Vilkitsky Strait and Dmitrii Laptev Strait, which are close to the coast, have higher accessibility than the Shokalskiy Strait and Sannikov Strait, especially in the next five years.

A 2D well-balanced, coupled model of water flow, sediment transport, and bed evolution based on unstructured grids with efficient variable storage strategy

In recent decades, a few Godunov-type, finite volume two-dimensional (2D) unstructured grid, coupled flow, and sediment models (GF2DUCM) have been developed for flows over erodible beds. These kinds of models are generally analyzed as a Vertex Model (VM) that define topography at the cell vertex, which can lead to the non-conservation of mass regarding flow, sediment, and bed evolution. Here, a full cell-cantered variable storage method (Central Model or CM) is applied as the solution of the GF2DUCM. In this method, terrain elevation is defined at the cell centroids; this accurately describes the physical relations between the water depth and topography deformation. This approach can fully eliminate calculation errors in topography deformation at local cells caused by the interpolation of topography deformation at the cell vertex, and reduced uncertainty in the computation of the GF2DUCM. The model performance is systematically tested using a series of laboratory experiments, which demonstrate the mass conservation feature and high accuracy in reproducing hydrodynamic and morphological processes.

Evaluation of Structured and Unstructured Models for Application in Operational Ocean Forecasting in Nearshore Waters

The oceanography sub-initiative of Canada’s Oceans Protection Plan was tasked to develop high-resolution nearshore ocean models for enhanced marine safety and emergency response, fitting into the multi-scale, multi-level nested operational ocean forecasting systems. For decision making on eventual 24/7 operational support, two ocean models (a structured grid model, NEMO (Nucleus for European Modelling of the Ocean); and an unstructured grid model, FVCOM (Finite Volume Coastal Ocean Model), were evaluated. The evaluation process includes the selection of the study area, the requirements for model setup, and the evaluation metrics. The chosen study area, Saint John Harbour in the Bay of Fundy, features strong tides, significant river runoff and a narrow tidal-river channel. Both models were configured with the same sources of bathymetry and forcing data. FVCOM achieved 50-100 m horizontal resolution in the inner harbour and included wetting/drying. NEMO achieved 100 m resolution in the harbour with a three-level one-way nesting configuration. Statistical metrics showed that one-year simulations with both models achieved comparable accuracies against the observed tidal and non-tidal water levels and currents, temperature and salinity, and the trajectories of surface drifters, but the computational cost of FVCOM was significantly less than that of NEMO.

Simulation of High Water Levels in Waters off the Mackenzie Delta during an Arctic Storm

Xu, F.; Li, C., and Perrie, W., 2020. Simulation of high water levels in waters off the Mackenzie Delta during an Arctic storm. In: Malvárez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 1361–1365. Coconut Creek (Florida), ISSN 0749-0208.Arctic storms are among the most frequent and devastating phenomena that occur in waters off the Mackenzie Delta and Southern Beaufort Sea, and changes in water level are a serious concern in the open water summer season. This study focuses on finding a reasonable method to simulate the storm surge induced by an Arctic storm, in waters off the Mackenzie Delta, by applying an unstructured grid model, FVCOM, and by developing an evaluation method to resolve the complex multiple channels and the associated water discharge distributions. This study shows that wind-induced storm surge is a dominant factor among the factors affecting increases in total water level. During the storm, water levels rises and surface current activity intensify in shallow coastal areas.

The effect of nasal and oral breathing on airway collapsibility in patients with obstructive sleep apnea: Computational fluid dynamics analyses.

The purpose of this study was to investigate the effect of breathing route on the collapsibility of the pharyngeal airway in patients with obstructive sleep apnea by using computational fluid dynamics technology. This study examined Japanese men with obstructive sleep apnea. Computed tomography scans of the nose and pharynx were taken during nasal breathing with closed mouth, nasal breathing with open mouth, and oral breathing while they were awake. Three-dimensional reconstructed stereolithography models and digital unstructured grid models were created and airflow simulations were performed using computational fluid dynamics software. Airflow velocity was significantly higher during oral breathing than during nasal breathing with open or closed mouth. No significant difference in maximum velocity was noted between nasal breathing with closed and open mouth. However, airflow during nasal breathing with open mouth was slow but rapidly sped up at the lower level of the velopharynx, and then spread and became a disturbed, unsteady stream. In contrast, airflow during nasal breathing with closed mouth gradually sped up at the oropharyngeal level without spreading or disturbance. Negative static pressure during oral breathing was significantly decreased; however, there were no significant differences between nasal breathing with closed or open mouth. Computational fluid dynamics results during nasal and oral breathing revealed that oral breathing is the primary condition leading to pharyngeal airway collapse based on the concept of the Starling Resistor model. Airflow throughout the entirety of the breathing route was smoother during nasal breathing with closed mouth than that with open mouth.

Sensitivity study on the main tidal constituents of the Gulf of Tonkin by using the frequency-domain tidal solver in T-UGOm

Abstract. Consequences of tidal dynamics on hydro-sedimentary processes are a recurrent issue in estuarine and coastal processes studies, and accurate tidal solutions are a prerequisite for modeling sediment transport, especially in macro-tidal regions. The motivation for the study presented in this publication is to implement and optimize a model configuration that will satisfy this prerequisite in the frame of a larger objective in order to study the sediment dynamics and fate from the Red River Delta to the Gulf of Tonkin from a numerical hydrodynamical–sediment coupled model. Therefore, we focus on the main tidal constituents to conduct sensitivity experiments on the bathymetry and bottom friction parameterization. The frequency-domain solver available in the hydrodynamic unstructured grid model T-UGOm has been used to reduce the computational cost and allow for wider parameter explorations. Tidal solutions obtained from the optimal configuration were evaluated from tide measurements derived from satellite altimetry and tide gauges; the use of an improved bathymetry dataset and fine friction parameter adjustment significantly improved our tidal solutions. However, our experiments seem to indicate that the solution error budget is still dominated by bathymetry errors, which is the most common limitation for accurate tidal modeling.

Simulating vegetation effects on flows in 3D using an unstructured grid model: model development and validation

Prevalence of vegetation (either submerged or emergent) in shallow water significantly affects the flow and turbulence structure in this environment. In this paper, we develop a new 3D unstructured grid hydrostatic model that accounts for the 3D effects of vegetation on flows. The model uses a semi-implicit time stepping method and treats the new vegetation-related terms implicitly to enhance numerical stability, so the time step does not need to be reduced as compared with the no-vegetation cases. The stability is also independent of the vegetation parameters, so as to efficiently account for large shear that can occur around the canopy. We validate the model using lab data before applying it to a field study in San Francisco Bay-Delta to illustrate the influence of the vegetation on the flow structure as well as tidal energetics. The efficiency of the model enabled by the implicit method allows, for the first time, the simulation of the vegetation effects during multi-year evolution of vegetation in full three dimensions at large spatial scales.

Cross-scale operational oceanography in the Adriatic Sea

ABSTRACTThe oceanographic forecast capability in coastal seas is often limited by the capacity of the numerical models in correctly reproducing the complex morphology of the coastline and the exchange processes between the shelf and the open seas. In the marginal Adriatic Sea this task is of uppermost importance due to the presence of several coastal water bodies and rivers. We present here a new operational oceanographic system, called Tiresias, based on the unstructured grid model SHYFEM and representing the whole Adriatic Sea together with the lagoons of Marano-Grado, Venice and Po Delta. The novelty of this oceanographic system resides in the very high-resolution, up to 10 m, of the numerical mesh, and in the high spatial and temporal resolution of the forcing and boundary conditions that drive the forecasts. The forecast results are evaluated against sea temperature and salinity profiles, mean circulation fields derived from a regional ocean model, tide gauges and drifter trajectory. The presented results highlighted the capacity of Tiresias in forecasting the general circulation in the Adriatic Sea, as well as several relevant coastal dynamics, such as saltwater intrusion, storm surge and riverine waters dispersion.

Integration of fully 3D fluid dynamics and geophysical fluid dynamics models for multiphysics coastal ocean flows: Simulation of local complex free-surface phenomena

A modeling system is presented for simulation of multiphysics coastal ocean flows at scales from O (1) m to O (1,000) km, especially for high-fidelity simulation of local, complicated, free-surface phenomena. The system integrates the solver for incompressible flow on unstructured mesh (SIFUM) and the finite volume coastal ocean model (FVCOM) on the basis of domain decomposition. In this system, the former is built on the Navier–Stokes equations and simulates small-scale, fully three-dimensional phenomena in local flows, whereas the latter is based on the geophysical fluid dynamics equations and predicts large-scale, background ocean currents. The integrated SIFUM–FVCOM system is developed from a previous system consisting of a structured-grid model and an unstructured-grid model (Tang et al., J. Comput. Phys. 273, 2014), and it combines two unstructured-grid models and has the capability of dealing with free-surface phenomena and complex geometries in local flows. In the new system, SIFUM and FVCOM are two-way coupled via Schwarz iteration, and they march in time together as a single system. The SIFUM–FVCOM system performs as intended with regard to capturing physical phenomena (e.g., generation of dam-break wave and slashing of water at a structure), converging with grid spacing and time step, and permitting seamless transition of solutions for far and near fields. In addition, its prediction of benchmark flow problems matches well with analytical, computational, and experimental data. The system is able to simultaneously and directly simulate many multiscale, multiphysics, real-world phenomena that could not be handled before. Such capability is illustrated by its application to coastal flooding and the resulting impact on a coastal bridge and a beachfront house.

German Bight estuaries: An inter-comparison on the basis of numerical modeling

The Semi-implicit Cross-scale Hydroscience Integrated System Model (SCHISM), which is an unstructured-grid model, was coupled with a 3D sediment model and established for the German Bight and its estuaries. The horizontal resolution of the model ranges from ~10 m in the estuaries to ~400 m in most of the open sea. Validation against tidal gauge data, fixed station data, and FerryBox data demonstrated that the model adequately simulated tidal dynamics in the entire area as well as the salinity fronts and estuarine turbidity maxima in the Ems, Weser and Elbe estuaries. Comparisons of model output with that of experiments with constant density allowed to estimate the role of density control, which appeared to be strongest in the landward reaches of salinity front. The increase of tidal range caused by density effects was most prominent in the Weser and Elbe estuaries, and relatively small in the Ems, where the runoff was also small. The magnitude of the density effect on the sea level oscillations was comparable to that of the M4 tide amplitude, demonstrating its importance in shaping the tidal asymmetry in the estuaries. Density effects not only reduced the dissipation of tidal energy; density stratification suppressed also the resuspension of sediment and contributed to a displacement of the position of the estuarine turbidity maximum upstream compared to the case of an homogeneous estuary. In the case of hyper-turbid Ems Estuary, density effects on stratification caused by high sediment concentrations resulted in a suppression of turbulence and further increase of concentration of suspended matter at the bottom. Although the three estuaries, which are only ~100 km apart, were driven by similar tidal and atmospheric forcings, they exhibited different extensions of both fronts and vertical stratification, mainly due to different river runoff conditions. At intra-tidal time scales, the dependencies between sea surface height and sea surface salinity varied considerably from estuary to estuary, and the largest flood asymmetry appeared in the Elbe Estuary. Wind acted as the dominant factor driving the longer-term estuarine variability; the correlation between zonal wind magnitude and sea surface height appeared to be very strong. The simulated suspended particulate matter dynamics and position of the estuarine turbidity maxima (ETM) were in agreement with observations. Secondary ETM appeared at different locations depending on the grain size, providing an illustration of sediment sorting.

Thetis coastal ocean model: discontinuous Galerkin discretization for the three-dimensional hydrostatic equations

Abstract. Unstructured grid ocean models are advantageous for simulating the coastal ocean and river–estuary–plume systems. However, unstructured grid models tend to be diffusive and/or computationally expensive, which limits their applicability to real-life problems. In this paper, we describe a novel discontinuous Galerkin (DG) finite element discretization for the hydrostatic equations. The formulation is fully conservative and second-order accurate in space and time. Monotonicity of the advection scheme is ensured by using a strong stability-preserving time integration method and slope limiters. Compared to previous DG models, advantages include a more accurate mode splitting method, revised viscosity formulation, and new second-order time integration scheme. We demonstrate that the model is capable of simulating baroclinic flows in the eddying regime with a suite of test cases. Numerical dissipation is well-controlled, being comparable or lower than in existing state-of-the-art structured grid models.

Hydrodynamics of a Tidally Forced Coral Reef Atoll

AbstractThe hydrodynamics of a tidally forced semi‐enclosed coral reef atoll (North Scott) at the edge of the continental shelf of northwestern Australia were investigated by combining field observations and numerical modeling. The observations revealed that the spring tidal range outside the atoll reaches 4 m, and as the water level drops below mean sea level, the reef rim surrounding the shallow (~10–15 m) lagoon becomes exposed. During this time, the lagoon can only exchange with the open ocean through two narrow channels, resulting in highly asymmetric water levels and velocities that were most pronounced during spring tide. On average, the ebb tide duration was ~2 hr longer than the flood, with rapid flood velocities in the channel reaching 2 m/s. We applied an unstructured grid model Delft3D‐Flexible Mesh to simulate the atoll hydrodynamics and were able to replicate the asymmetric water levels and complex velocities in the lagoon. The results revealed that at higher tidal stages, a dominant momentum balance exists between the pressure gradient (established by the propagation of the tide on the shelf) and the local flow acceleration of water throughout the interior of the atoll. At lower tidal stages, which coincided with a reversal of the offshore tidal pressure gradient, the lagoon became isolated from offshore dynamics and all momentum terms were negligible. This resulted in a tidally averaged residual westward flow within the lagoon that drove an asymmetric flushing pattern within the atoll, which we propose would be a common flushing mechanism within other tide‐dominated atolls worldwide.

A 3D unstructured-grid model for Chesapeake Bay: Importance of bathymetry

We extend the 3D unstructured-grid model previously developed for the Upper Chesapeake Bay to cover the entire Bay and its adjacent shelf, and assess its skill in simulating saltwater intrusion and the coastal plume. Recently developed techniques, including a flexible vertical grid system and a 2nd-order, monotone and implicit transport solver are critical in successfully capturing the baroclinic responses. Most importantly, good accuracy is achieved through an accurate representation of the underlying bathymetry, without any smoothing. The model in general exhibits a good skill for all hydrodynamic variables: the averaged root-mean-square errors (RMSE's) in the Bay are 9 cm for sub-tidal frequency elevation, 17 cm/s for 3D velocity time series, 1.5 PSU and 1.9 PSU for surface and bottom salinity respectively, 1.1 °C and 1.6 °C for surface and bottom temperature respectively. On the shelf, the average RMSE for the surface temperature is 1.4 °C. We highlight, through results from sensitivity tests, the central role played by bathymetry in this estuarine system and the detrimental effects, from a common class of bathymetry smoothers, on volumetric and tracer fluxes as well as key processes such as the channel-shoal contrast in the estuary and plume propagation in the coast.