Cowles Geoffrey W.

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Cowles

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Geoffrey W.

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FVCOM validation experiments : comparisons with ROMS for three idealized barotropic test problems

(American Geophysical Union, 2008-07-26) Huang, Haosheng ; Chen, Changsheng ; Cowles, Geoffrey W. ; Winant, Clinton D. ; Beardsley, Robert C. ; Hedstrom, Kate S. ; Haidvogel, Dale B.

The unstructured-grid Finite-Volume Coastal Ocean Model (FVCOM) is evaluated using three idealized benchmark test problems: the Rossby equatorial soliton, the hydraulic jump, and the three-dimensional barotropic wind-driven basin. These test cases examine the properties of numerical dispersion and damping, the performance of the nonlinear advection scheme for supercritical flow conditions, and the accuracy of the implicit vertical viscosity scheme in barotropic settings, respectively. It is demonstrated that FVCOM provides overall a second-order spatial accuracy for the vertically averaged equations (i.e., external mode), and with increasing grid resolution the model-computed solutions show a fast convergence toward the analytic solutions regardless of the particular triangulation method. Examples are provided to illustrate the ability of FVCOM to facilitate local grid refinement and speed up computation. Comparisons are also made between FVCOM and the structured-grid Regional Ocean Modeling System (ROMS) for these test cases. For the linear problem in a simple rectangular domain, i.e., the wind-driven basin case, the performance of the two models is quite similar. For the nonlinear case, such as the Rossby equatorial soliton, the second-order advection scheme used in FVCOM is almost as accurate as the fourth-order advection scheme implemented in ROMS if the horizontal resolution is relatively high. FVCOM has taken advantage of the new development in computational fluid dynamics in resolving flow problems containing discontinuities. One salient feature illustrated by the three-dimensional barotropic wind-driven basin case is that FVCOM and ROMS simulations show different responses to the refinement of grid size in the horizontal and in the vertical.
Tidal dynamics in the Gulf of Maine and New England Shelf : an application of FVCOM

(American Geophysical Union, 2011-12-10) Chen, Changsheng ; Huang, Haosheng ; Beardsley, Robert C. ; Xu, Qichun ; Limeburner, Richard ; Cowles, Geoffrey W. ; Sun, Yunfang ; Qi, Jianhua ; Lin, Huichan

The unstructured-grid, Finite-Volume Community Ocean Model (FVCOM) was used to simulate the tides in the Gulf of Maine (GoM) and New England Shelf (NES) for homogeneous and summer stratified conditions. FVCOM captures the near-resonant nature of the semidiurnal tide and energy flux in the GoM and the complex dynamics governing the tide in the NES. Stratification has limited impact on tidal elevation, but can significantly modify the tidal current profile. Internal tides are energetic in the stratified regions over steep bottom topography, but their contribution to the total tidal energy flux is only significant over the northeast flank of Georges Bank. The model suggests that the tidal flushing-induced eddy east of Monomoy Island is the dynamic basis for the locally observed phase lead of the M2 tide. The southward propagating tidal wave east of Cape Cod encounters the northeastward propagating tidal wave from the NES south of Nantucket Island, forming a zone of minimum sea level along a southeast-oriented line from Nantucket Island. These two waves are characterized by linear dynamics in which bottom friction and advection are negligible in the momentum balance, but their superposition leads to a strong nonlinear current interaction and large bottom stress in the zone of lowest sea elevation.
Comparison of observed and model-computed low frequency circulation and hydrography on the New England Shelf

(American Geophysical Union, 2008-09-09) Cowles, Geoffrey W. ; Lentz, Steven J. ; Chen, Changsheng ; Xu, Qichun ; Beardsley, Robert C.

The finite volume coastal ocean model (FVCOM) is configured to study the interannual variability of circulation in the Gulf of Maine (GoM) and Georges Bank. The FVCOM-GoM system incorporates realistic time-dependent surface forcing derived from a high-resolution mesoscale meteorological model (MM5) and assimilation of observed quantities including sea surface temperature and salinity and temperature fields on the open boundary. An evaluation of FVCOM-GoM model skill on the New England shelf is made by comparison of computed fields and data collected during the Coastal Mixing and Optics (CMO) Program (August 1996–June 1997). Model mean currents for the full CMO period compare well in both magnitude and direction in fall and winter but overpredict the westward flow in spring. The direction and ellipticity of the subtidal variability correspond but computed magnitudes are around 20% below observed, partially due to underprediction of the variability by MM5. Response of subtidal currents to wind-forcing shows the model captures the directional dependence, as well as seasonal variability of the lag. Hydrographic results show that FVCOM-GoM resolves the spatial and temporal evolution of the temperature and salinity fields. The model-computed surface salinity field compares well, except in May when there is no indication of the fresh surface layer from the Connecticut River discharge noted in the observations. Analysis of model-computed results indicates that the plume was unable to extend to the mooring location due to the presence of a westward mean model-computed flow during that time that was stronger than observed. Overall FVCOM-GoM captures well the dynamics of the mean and subtidal flow on the New England shelf.
A wet/dry point treatment method of FVCOM, part I: stability experiments

(MDPI, 2022-06-28) Chen, Changsheng ; Qi, Jianhua ; Liu, Hedong ; Beardsley, Robert C. ; Lin, Huichan ; Cowles, Geoffrey W.

A 3-dimensional wet/dry point treatment method was developed for the unstructured-grid Finite-Volume Community Ocean Model (FVCOM). Analytical equations were derived to examine discretized errors that occurred during the flooding/drying process by the wet/dry point treatment for the flooding/drying process. Numerical experiments were carried out for an idealized estuary, including the inter-tidal zone. The model results show that if the ratio of internal to external mode time steps (Isplit) is appropriately selected, FVCOM was capable of simulating the flooding/drying process with sufficient accuracy to ensure the mass conservation. The up-bound limit of Isplit was restricted by the bathymetric slope of the inter-tidal zone, external mode time step, horizontal/vertical resolution, and amplitude of tidal forcing at the open boundary, as well as the thickness of the viscous layer specified in the model. Criteria for time steps via these parameters were derived from these experiments, which provide a helpful guide in selectingIsplit for applying FVCOM to realistic geometric estuaries.
Physical mechanisms for the offshore detachment of the Changjiang Diluted Water in the East China Sea

(American Geophysical Union, 2008-02-02) Chen, Changsheng ; Xue, Pengfei ; Ding, Pingxing ; Beardsley, Robert C. ; Xu, Qichun ; Mao, Xianmou ; Gao, Guoping ; Qi, Jianhua ; Li, Chunyan ; Lin, Huichan ; Cowles, Geoffrey W. ; Shi, Maochong

Physical mechanisms for the summertime offshore detachment of the Changjiang Diluted Water (CDW) into the East China Sea are examined using the high-resolution, unstructured-grid, Finite-Volume Coastal Ocean Model (FVCOM). The model results suggest that isolated low salinity water lens detected west of Cheju Island can be formed by (1) a large-scale adjustment of the flow field to the Changjiang discharge and (2) the detachment of anticyclonic eddies as a result of baroclinic instability of the CDW front. Adding the Changjiang discharge intensifies the clockwise vorticity of the subsurface current (originating from the Taiwan Warm Current) flowing along the 50-m isobath and thus drives the low-salinity water in the northern coastal area of the Changjiang mouth offshore over a submerged plateau that extends toward Cheju Island. Given a model horizontal resolution of less than 1.0 km, the CDW front becomes baroclinically unstable and forms a chain of anticyclonic and cyclonic eddies. The offshore detachment of anticyclonic eddies can carry the CDW offshore. This process is enhanced under northward winds as a result of the spatially nonuniform interaction of wind-induced Ekman flow and eddy-generated frontal density currents. Characteristics of the model-predicted eddy field are consistent with previous theoretical studies of baroclinic instability of buoyancy-driven coastal density currents and existing satellite imagery. The plume stability is controlled by the horizontal Ekman number. In the Changjiang, this number is much smaller than the criterion suggested by a theoretical analysis.
Application and comparison of Kalman filters for coastal ocean problems : an experiment with FVCOM

(American Geophysical Union, 2009-05-13) Chen, Changsheng ; Malanotte-Rizzoli, Paola ; Wei, Jun ; Beardsley, Robert C. ; Lai, Zhigang ; Xue, Pengfei ; Lyu, Sangjun ; Xu, Qichun ; Qi, Jianhua ; Cowles, Geoffrey W.

Twin experiments were made to compare the reduced rank Kalman filter (RRKF), ensemble Kalman filter (EnKF), and ensemble square-root Kalman filter (EnSKF) for coastal ocean problems in three idealized regimes: a flat bottom circular shelf driven by tidal forcing at the open boundary; an linear slope continental shelf with river discharge; and a rectangular estuary with tidal flushing intertidal zones and freshwater discharge. The hydrodynamics model used in this study is the unstructured grid Finite-Volume Coastal Ocean Model (FVCOM). Comparison results show that the success of the data assimilation method depends on sampling location, assimilation methods (univariate or multivariate covariance approaches), and the nature of the dynamical system. In general, for these applications, EnKF and EnSKF work better than RRKF, especially for time-dependent cases with large perturbations. In EnKF and EnSKF, multivariate covariance approaches should be used in assimilation to avoid the appearance of unrealistic numerical oscillations. Because the coastal ocean features multiscale dynamics in time and space, a case-by-case approach should be used to determine the most effective and most reliable data assimilation method for different dynamical systems.
Turbulent and numerical mixing in a salt wedge estuary : dependence on grid resolution, bottom roughness, and turbulence closure

(John Wiley & Sons, 2017-01-28) Ralston, David K. ; Cowles, Geoffrey W. ; Geyer, W. Rockwell ; Holleman, Rusty C.

The Connecticut River is a tidal salt wedge estuary, where advection of sharp salinity gradients through channel constrictions and over steeply sloping bathymetry leads to spatially heterogeneous stratification and mixing. A 3-D unstructured grid finite-volume hydrodynamic model (FVCOM) was evaluated against shipboard and moored observations, and mixing by both the turbulent closure and numerical diffusion were calculated. Excessive numerical mixing in regions with strong velocities, sharp salinity gradients, and steep bathymetry reduced model skill for salinity. Model calibration was improved by optimizing both the bottom roughness (z0), based on comparison with the barotropic tidal propagation, and the mixing threshold in the turbulence closure (steady state Richardson number, Rist), based on comparison with salinity. Whereas a large body of evidence supports a value of Rist ∼ 0.25, model skill for salinity improved with Rist ∼ 0.1. With Rist = 0.25, numerical mixing contributed about 1/2 the total mixing, while with Rist = 0.10 it accounted for ∼2/3, but salinity structure was more accurately reproduced. The combined contributions of numerical and turbulent mixing were quantitatively consistent with high-resolution measurements of turbulent mixing. A coarser grid had increased numerical mixing, requiring further reductions in turbulent mixing and greater bed friction to optimize skill. The optimal Rist for the fine grid case was closer to 0.25 than for the coarse grid, suggesting that additional grid refinement might correspond with Rist approaching the theoretical limit. Numerical mixing is rarely assessed in realistic models, but comparisons with high-resolution observations in this study suggest it is an important factor.
A finite volume numerical approach for coastal ocean circulation studies : comparisons with finite difference models

(American Geophysical Union, 2007-03-28) Chen, Changsheng ; Huang, Haosheng ; Beardsley, Robert C. ; Liu, Hedong ; Xu, Qichun ; Cowles, Geoffrey W.

An unstructured grid, finite volume, three-dimensional (3-D) primitive equation coastal ocean model (FVCOM) has been developed for the study of coastal ocean and estuarine circulation by Chen et al. (2003a). The finite volume method used in this model combines the advantage of finite element methods for geometric flexibility and finite difference methods for simple discrete computation. Currents, temperature, and salinity are computed using an integral form of the equations, which provides a better representation of the conservative laws for mass, momentum, and heat. Detailed comparisons are presented here of FVCOM simulations with analytical solutions and numerical simulations made with two popular finite difference models (the Princeton Ocean Model and Estuarine and Coastal Ocean Model (ECOM-si)) for the following idealized cases: wind-induced long-surface gravity waves in a circular lake, tidal resonance in rectangular and sector channels, freshwater discharge onto the continental shelf with curved and straight coastlines, and the thermal bottom boundary layer over the slope with steep bottom topography. With a better fit to the curvature of the coastline using unstructured nonoverlapping triangle grid cells, FVCOM provides improved numerical accuracy and correctly captures the physics of tide-, wind-, and buoyancy-induced waves and flows in the coastal ocean. This model is suitable for applications to estuaries, continental shelves, and regional basins that feature complex coastlines and bathymetry.
Connectivity of the bay scallop (Argopecten irradians) in Buzzards Bay, Massachusetts, U.S.A.

( 2015-06) Liu, Chang ; Cowles, Geoffrey W. ; Churchill, James H. ; Stokesbury, Kevin D. E.

The harvest of bay scallops (Argopecten irradians) from Buzzards Bay, Massachusetts, USA undergoes large interannual fluctuations, varying by more than an order of magnitude in successive years. To investigate the extent to which these fluctuations may be due to yearly variations in the transport of scallop larvae from spawning areas to suitable juvenile habitat (settlement zones), a high-resolution hydrodynamic model was used to drive an individual-based model of scallop larval transport. Model results revealed that scallop spawning in Buzzards Bay occurs during a time when nearshore bay currents were principally directed up-bay in response to a persistent southwesterly sea breeze. This nearshore flow results in substantial transport of larvae from lower-bay spawning areas to settlement zones further up-bay. Averaged over the entire bay, the spawning-to-settlement zone connectivity exhibits little interannual variation. However, connectivities between individual spawning and settlement zones vary by up to an order of magnitude. The model results identified spawning areas that have the greatest probability of transporting larvae to juvenile habitat. Because managers may aim to increase scallop populations either locally or broadly, the high-connectivity spawning areas were divided into: 1) high larval retention and relatively little larval transport to adjoining settlement areas, 2) both significant larval retention and transport to more distant settlement areas, and 3) little larval retention but significant transport to distant settlement areas.
A nonhydrostatic version of FVCOM : 1. Validation experiments

(American Geophysical Union, 2010-11-13) Lai, Zhigang ; Chen, Changsheng ; Cowles, Geoffrey W. ; Beardsley, Robert C.

The unstructured grid finite volume coastal ocean model (FVCOM) system has been expanded to include nonhydrostatic dynamics. This addition uses the factional step method with both split mode explicit and semi-implicit schemes. The unstructured grid finite volume method, combined with a correction of the final free surface from its intermediate value with inclusion of nonhydrostatic effects, efficiently reduces numerical damping and thus ensures second-order accuracy of the solutions with local/global volume conservation. Numerical experiments have been made to fully validate the nonhydrostatic FVCOM, including surface standing and solitary waves in idealized flat- and sloping-bottomed channels in homogeneous conditions, the density adjustment problem for lock exchange flow in a flat-bottomed channel, and two-layer internal solitary wave breaking on a sloping shelf. The model results agree well with the relevant analytical solutions and laboratory data. These validation experiments demonstrate that the nonhydrostatic FVCOM is capable of resolving complex nonhydrostatic dynamics in coastal and estuarine regions.
A nonhydrostatic version of FVCOM : 2. Mechanistic study of tidally generated nonlinear internal waves in Massachusetts Bay

(American Geophysical Union, 2010-12-21) Lai, Zhigang ; Chen, Changsheng ; Cowles, Geoffrey W. ; Beardsley, Robert C.

The generation, propagation, and dissipation processes of large-amplitude nonlinear internal waves in Massachusetts Bay during the stratified season were examined using the nonhydrostatic Finite-Volume Coastal Ocean Model (FVCOM-NH). The model reproduced well the characteristics of the high-frequency internal waves observed in Massachusetts Bay in August 1998. The model experiments suggested that internal waves over Stellwagen Bank are generated by the interaction of tidal currents with steep bottom topography through a process of forming a large-density front on the western slope of the bank by the release of an initial density perturbation near ebb-flood transition, nonlinear steepening of the density front into a deep density depression, and disintegrating of the density depression into a wave train. Earth's rotation tends to transfer the cross-bank tidal kinetic energy into the along-bank direction and thus reduces the intensity of the density perturbation at ebb-flood transition and density depression in the flood period. The internal wave packet propagates as a leading edge feature of the internal tidal wave, and the faster propagation speed of the high-frequency internal waves in Massachusetts Bay is caused by Earth's rotation. The model experiments suggested that bottom friction can significantly influence the cross-bank scale of the density perturbation and thus the density depression during wave generation and the dissipation during the wave's shoaling. Inclusion of vertical mixing using the Mellor-Yamada level 2.5 turbulence closure model had only a marginal effect on wave evolution. The model results support the internal wave theory proposed by Lee and Beardsley (1974) but are in disagreement with the lee-wave mechanism proposed by Maxworthy (1979).
A new high-resolution unstructured grid finite volume Arctic Ocean model (AO-FVCOM) : an application for tidal studies

(American Geophysical Union, 2009-08-27) Chen, Changsheng ; Gao, Guoping ; Qi, Jianhua ; Proshutinsky, Andrey ; Beardsley, Robert C. ; Kowalik, Zygmunt ; Lin, Huichan ; Cowles, Geoffrey W.

A spherical coordinate version of the unstructured grid 3-D FVCOM (finite volume coastal ocean model) has been applied to the Arctic Ocean to simulate tides with a horizontal resolution ranging from 1 km in the near-coastal areas to 15 km in the deep ocean. By accurately resolving the irregular coastlines and bathymetry in the Arctic Ocean coastal regions, this model reproduces the diurnal (K1 and O1) and semidiurnal (M2 and S2) tidal wave dynamics and captures the complex tidal structure along the coast, particularly in the narrow straits of the Canadian Archipelago. The simulated tidal parameters (harmonic constituents of sea surface elevation and currents) agree well with the available observational data. High-resolution meshes over the continental shelf and slope capture the detailed spatial structure of topographic trapped shelf waves, which are quite energetic along the Greenland, Siberia, and Spitsbergen continental slope and shelf break areas. Water stratification influences the vertical distribution of tidal currents but not the water transport and thus tidal elevation. The comparison with previous finite difference models suggests that horizontal resolution and geometric fitting are two prerequisites to simulate realistically the tidal energy flux in the Arctic Ocean, particularly in the Canadian Archipelago.

Cowles Geoffrey W.

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