Marine Ecosystem Dynamics Modeling Laboratory

Lake Superior FVCOM

With a joint effort of UMASSD and WHOI, we have successfully developed an unstructured grid, finite-volume three-dimensional primitive equation coastal ocean model (so called FVCOM) (Chen et al., 2003). In common with POM (Princeton Ocean Model) and ECOM-si (semi-implicit code of POM), FVCOM uses the modified Mellor and Yamada level 2.5 (MY-2.5) and Smagorinsky turbulent closures for vertical and horizontal mixing, respectively (Mellor and Yamada, 1982; Galperin et al., 1988; Smagorinsky, 1963) and a sigma coordinate (vertical stretching to follow bottom topography). Distinct difference from POM and other finite-difference or finite-element models is that FVCOM is solved numerically by flux calculation in the integral form of the governing equations over non-overlapping, unstructured triangular grids.  This numerical approach combines the best features of the finite-element method (grid flexibility) and finite-difference method (numerical efficiency and code simplicity). Further, the flux calculation method with an integral form of the equations provides a better representation of momentum, mass, salt, and heat conservation. This method avoids the occurrence of wave energy reflected from open boundaries due to improper radiation boundary condition and also provides exact boundary conditions of no flux of heat and salt at the bottom of the slope. The conservative nature of FVCOM plus its grid flexibility and the simplicity of its code also make FVCOM ideally suited for interdisciplinary application.

FVCOM has been developed with careful validation through comparison with analytical solutions and finite-difference models (POM/ECOM-si and ROMs). The model validations were conducted for idealized cases of wind-induced surface gravity waves in a circular lake, tidal resonance in a channel, freshwater discharges over the continental shelf and in a sloping bottom circular lake, thermal bottom boundary layer over the slope with steep bottom topography, equatorial Rossby Soliton, and hydraulic jump, etc. With better fit of the curvature of the coastlines using unstructured non-overlapped triangular cells, FVCOM demonstrates significant improvement in numerical accuracy and capability to capture the right physics of tide-, wind- and buoyancy-induced waves and flow as well as the bottom boundary dynamics in the coastal ocean.

FVCOM has been successfully applied to Lake Superior to replace our old hydrodynamic model (a modified version of ECOM-si) used in that lake (for the old model study, please refers to Chen et al., 2001; Zhu et al., 2001; Chen et al., 2002a, Chen et al. 2004c, 2004d). Comparisons between FVCOM- and ECOM-si-based Lake Superior hydrodynamic models show that FVCOM is better in 1) resolving the high resolution thermal and currents near the coast, 2) enabling to build a linkage with small lakes in the adjacent coastal region, 3) capturing the right physics of the bottom boundary layer at the steep bottom topography off the Keweenaw coast, and 4) ensuring the mass conservation of passive tracers. Updated FVCOM has migrated to Fortran 95/2K. This was done primarily to facilate the use of allocatable arrays that beneficial when one does not know a priori the dimension of the sub-domains resulting from the an arbitrary partitioning. Four-dimensional nudging and Kalman Filter data assimilation methods are incorporated into the updated code to merge the model-predicted water temperature and currents to observations taken by mooring current meters, AVHRR-derived SST, and regional hydrographic surveys.

Current Vector Fields

Animation of the near-surface current vectors for May and July 1999. The current vectors were selected to provide a viewable picture. The wind stress over the lake was calculated based on the interpolation field of NOAA buoys and coastal meteorological stations with land-lake correction.

Thermal Front (or Thermal Bar)

Animation of the near-surface water temperature for May and July 1999. The heat flux was estimated using empirical functions that were used for Lake Michigan model.

 

Lagrangian Particle Tracking

The Largrangian particle tracking experiment results along the Keweenaw coast. Exmperiments were also made to examine the water exchange between Lake Superior and small lakes Portage and Torch

 

 

«PreviousNext» Posted on January 15, 2014