QUODDY Barotropic Coastal Ocean Modeling

(Sample animations of the real-time animations are available.)

Model Description: QUODDY is a 3-D, nonlinear, prognostic, f-plane, finite-element coastal ocean circulation model with advanced turbulence closure (Lynch et al., 1996, 1997). In this application, bottom flow is subject to quadratic bottom boundary stress, according to , where the time/space constant bottom drag coefficient is 0.005. There was no surface forcing for this study.

The QUODDY model domain (see Figure 1) is defined by the Holboke (1998) GHSD mesh. The mesh resolution varies from about 10 km in the gulf to about 5 km near the coastlines with even finer resolution in the regions of steep bathymetric slopes like the north flank Georges Bank. A 10-m minimum depth was adopted for the coastal boundary elements. Vertically here are 21 sigma layers.

Figure1. The Holboke (1998) GHSD mesh for the QUODDY model domain, with the deep ocean – western cross-shelf, Halifax cross-shelf and Bay of Fundy open boundaries highlighted by the thick red, blue and black lines respectively. The water depths (in meters) are color-coded according to the scale on the right.

The conditions imposed on the different QUODDY open ocean boundaries (see Figure 1) are:
Deep Ocean and Watch Hill, RI Cross-Shelf Sections: The predicted semidiurnal M2 tidal elevation forcing, zero steady residual or non-tidal elevation, and the Holboke (1998) inhomogeneous and barotropic radiation conditions;
Bay of Fundy Section: The predicted M2, M4, M6 normal flow, constrained by a condition of zero non-tidal transport normal to the section; and
Halifax Cross-Shelf Section: The predicted M2 tidal elevations, a zero steady residual elevation, and the Holboke (1998) inhomogeneous and barotropic radiation boundary conditions.

The model, with homogeneous water density, was initialized with zero velocity and elevation fields for this barotropic calculation. It was then run with a 21.83203125 second (= the 12.42-hour tidal period/2048) time-step. So that the model nonlinearities and advection could dynamically adjust to the initial fields (Holboke, 1998), the prescribed M2 tidal sea level forcing-only (due to Lynch et al., 1997) was linearly increased to full forcing (i.e. ramped-up) during the first six tidal cycles. Holboke (1998) has shown with QUODDY runs with a similar model configuration reach dynamical equilibrium after 6 tidal cycles.

Model/Observation Sea level Comparisons: For the model/observation sea level comparisons, the model was run (after ramp-up) for 2 full M2 tidal cycles. These model sea level time series were extracted at the 49 nodes which were nearest to the selected set of Moody et al. (1984) observation stations for which tidal harmonic constants were available. Two-month time series were produced at each of the 49 sites by repeatedly joining the two M2 tidal cycle series end-to-end. The model and observed M2 tidal sea level harmonic constants are compared in Table 1. For stations in the Gulf of Maine and on Georges Bank, the observed and model M2 tidal sea level amplitude differences are typically within 10% of each other; with corresponding phase differences typically within 10 degrees of each other.

REFERENCES
Holboke, Monica J., 1998. “Variability of the Maine Coastal Current under Spring Conditions”, Ph.D. Dissertation -Thayer School of Engineering -Dartmouth College, pp. 193.

Lynch, D. R., M. J. Holboke, and C.E. Naimie, 1997. The Maine coastal current: spring climatological circulation. Continental Shelf Research, 17, 605-634.

Lynch, D.R., J.T.C. Ip, C.E. Naimie, and F.E. Werner, 1996. Comprehensive coastal circulation model with application to the Gulf of Maine. Continental Shelf Research, 16, 875-906.

Moody, J., B. Butman, R.C. Beardsley, W.S. Brown, W. Boicourt, P. Daifuku, J.D. Irish, D.A. Mayer, H.E. Mofjeld, B. Petrie, S. Ramp, D. Smith and W.R. Wright, 1984."Atlas of Tidal Elevation and Current Observations on the Northeast American Continental Shelf and Slope," U.S. Geological Survey Bulletin No. 1611, U.S. Government Printing Office, pp. 122.