The Numerical Methods Laboratory (NML) is a research unit within the Thayer School of Engineering concerned with numerical analysis and advanced scientific computation. The NML Environmental Group pursues research at the intersection of advanced computation and large-scale environmental simulation, under the direction of Professor Daniel R. Lynch.
In the area of advanced computation, the Environmental Group is continuing to develop three-dimensional coastal ocean circulation models. These models vary in sophistication from a linear diagnostic model (which determines the circulation field based entirely on user-specified conditions) to a nonlinear prognostic model (which allows the circulation field to evolve over time).
The primary coastal ocean region under investigation by the group is the Northwest Atlantic; including the Gulf of Maine along with surrounding banks and continental shelf regions (see Figure 1 and Figure 2).
James L. Waugh, Laboratory Manager
Daniel R. Lynch, MacLean Professor
Justin T. C. Ip, Assistant Professor
Christopher E. Naimie, Postdoctoral Fellow
Monica J. Holboke, PhD Candidate
Wendy C. Gentleman, PhD Candidate
A. Governing Equations: The circulation models solve the 3-D shallow water equations with conventional hydrostatic and Boussinesq assumptions:
Continuity Equation:
Conservation of Horizontal Momentum:
Conservation of Heat and Salt:
Constitutive Relationship for Perturbation Density:
Bottom Boundary Conditions (z=-h):
Surface Boundary Conditions (
):
B. 3-D Finite Element Mesh Generation: The models utilize a 3-D (x,y,z,t) finite element mesh that is created by projecting a 2-D (x,y) mesh (see Figure 3 and Figure 15 for example) vertically, bounded by the free surface and the sea floor. The vertical meshing procedure allows variable spacing to increase the resolution of boundary and internal layers. However, the number of nodes under each (x,y) location must be uniform. Projection of the horizontal mesh in this manner creates 3-D prism elements.
C. Finite Element Method Models: Forcing is included from sea-surface elevation boundary conditions, atmospheric conditions, and baroclinic pressure gradients. For the computations presented in this poster, the most advanced diagnostic and prognostic models available were used:
D. Symbol definitions:
Current Research Grants - Sponsoring Agencies:
Circulation Models for the Gulf of Maine - US National Science Foundation
Importance of Physical and Biological Processes to Population Regulation of Cod and Haddock on Georges Bank: A Model-Based Study - Jointly Sponsored by NSF and NOAA through the GLOBEC Program
Tidal Nonlinearities and Evolution of Shallow Estuaries and Inlets - US National Science Foundation
An Observational/Modeling Study of the Western Gulf of Maine Circulation - New Hampshire Sea Grant Program and the NOAA Regional Marine Research Program.
A Data and Information Management System for the Gulf of Maine Regional Marine Research Program Regional Marine Research Program for the Gulf of Maine
Other Acknowledgements:
The work conducted by our research group derives significant benefit from interaction with a variety of individuals associated with the aforementioned research grants. In particular, we acknowledge and appreciate the contributions of Brian Blanton, Wendell Brown, Mary Jo Graça, Dave Greenberg, Charles Hannah, John Loder, Greg Lough, Jim Manning, Fred Page, Ian Perry, Peter Smith, Wally Smith, Mike Sinclair, John Tremblay, and Francisco Werner.
A. Motivation/Overview: The sustainability of valuable fish populations and their associated ecosystems is currently receiving significant attention; in light of concerns regarding increased fishing pressure, pollution, catastrophic events, other human intervention, and possible global change scenarios. The situation is critical on Georges Bank, where both the United States and Canadian governments recently invoked severe fishing restrictions. Understanding the physical circulation is vital to resource management on Georges Bank, as early life stages of key marine species rely on it for survival at various times throughout the year.
In this study, 3-D circulation fields on realistic Georges Bank topography are computed to gain insight regarding contributions from various physical processes to seasonal variations in circulation. The dynamics considered result from the dominant principal lunar tide, density variations, wind stress, up-stream conditions, and turbulence closure. The annual cycle is modeled as the progression through bimonthly, quasi-static circulation fields.
B. Barotropic Tidal Rectification: The persistent semi-diurnal (twice a day, with a period of 12.42 hours) principle lunar tide creates oscillating currents at its frequency and a low frequency clockwise gyre around Georges Bank throughout the year (see Figure 4, Figure 5, and Figure 6).
C. Seasonal Variations in Diagnostic Forcing: The six bimonthly diagnostic solutions indicate important contributions to seasonal intensification of the Georges Bank gyre from tidal rectification, prescribed density gradients, and mean wind stress; with recirculating transport increasing by a factor of seven from winter to late summer. Overall, these solutions are in approximate agreement with observations (see Figure 7, Figure 8, and Figure 9).
D. Prognostic Simulations: Comparison of model results with observational data improves further when the more physically realistic prognostic model is used to compute the circulation for weak (March-April) and strong (July-August) stratification periods. This illustrates the importance of incorporating advanced turbulence closure, tidal-time evolution of density, and improved treatment of nonlinearities (see Figure 10, Figure 11, and Figure 12).
A. Motivation/Overview: The coastal zone is very biologically productive and therefore very important ecologically and economically. This aspect has brought significant interest to understanding and predicting the circulation in this zone. The Maine Coastal Current is thought to be an important factor in the production and distribution of toxic dinoflagellates (``red tide'') and the transport of river borne pollutants alongcoast. It is also the interface between the coast and the gulf across which important nutrient fluxes are believed to occur. The ability to predict pollutant flow and red tides are of extreme importance for environmental managers.
The goal of this study is to resolve the important physical processes that effect the coastal current. These processes are exchange with the Gulf, river discharge, lunar tides and local atmospheric forcing. The first step towards this resolution is to model the coastal circulation prognostically with bi-monthly climatological averaged density and atmospheric forcing, and boundary conditions from a similar Gulf-wide simulation.
B. Gulf-Wide Boundary Conditions: A Gulf-wide simulation does not resolve the coastal region while it does produce Gulf-wide circulation consistent with observations. The results also suggest some of the Gulf-wide mechanisms that may affect the coastal current, thereby confirming the belief that Gulf-forcing must be included to model the coastal current correctly. The Gulf-forcing on the coastal regime is then implied by boundary conditions found from these results (see Figure 13 and Figure 14).
C. Prognostic Simulations: Results on the cstb finite element mesh for March-April and May-June prognostic simulations yield good resolution of the coastal current. These results also reveal a pronounced effect caused by river discharge, seasonal forcing, and coastal upwelling near the coastline (see Figure 15, Figure 16, Figure 17, Figure 18, and Figure 19).
Christopher.E.Naimie@dartmouth.edu and Monica.J.Holboke@dartmouth.edu
