Hydrodynamics

Model Grids

It is desirable that all the dominant physical processes are captured in a local model domain of Moreton Bay, including motion due to processes originating on the shelf. For this to be accomplished a two grid nesting strategy was employed (Figure 2.1). The large scale (regional) grid is designed to capture processes occurring on the shelf, including intrusions of the EAC, and extends from Fraser Island to northern NSW. This grid has a minimum depth of 1 m imposed at the coast, and maximum depth is 4000 m. The grid is polar in nature, where the centre contains high resolution sufficient to adequately resolve Moreton bay (~500 m), with resolution increasing in a radial direction to ~3 km at the offshore boundary. The model output from this grid is used to force the open boundaries of the local Moreton Bay grid.

The local grid is required to contain high resolution in order to resolve the mouths of the numerous estuaries feeding into the Bay, and also resolve the narrow channels connecting the Bay to the shelf. A portion of the shelf was also required to be included in the domain, so that any tracer delivered outside the bay was allowed to be subsequently transported back into the bay. High resolution was also required in the geographically complex southern portion of the Bay. High resolution of ~300 m is maintained in the western and southern Bay, with cell width increasing to 500 m in the eastern and northern Bay, and a maximum of 1400 m on the offshore open boundary. The maximum depth of this grid is ~200 m on the offshore open boundary and a minimum depth of 0.5 m is imposed. The model includes wetting and drying, hence these shallow regions become dry when the tide drops below 0.5 m.

The maximum depth of the models affects the gravity wave speed, hence the time step used for the 2-D mode and consequently the run time of the model. The regional model has 3D/2D time-steps of 60/3 s respectively. The grid size is 181 x 147 with 45 layers in the vertical, with ~21% of the grid containing wet cells. This results in a run-time ratio of ~80:1 using parallel processing over eight partitions. The local grid has a size of 135 x 360 with 25 vertical layers (~18% wet cells) and 3D/2D time-steps of 20/2.5 s. Parallel processing this grid over eight partitions yields a run-time ratio of ~70:1.

All grids use ‘z’ vertical discretization with exponentially increasing grid spacing near the surface and constant spacing at depth. Surface layer thickness is 1 and 1.5 m for the regional and local grids respectively. The bathymetry for all grids is smoothed with a 9 point convolution filter, and a maximum gradient of 0.05 is imposed. All models use the QUICKEST advection scheme with ULTIMATE limiter (Binliang and Falconer, 1997) for tracer advection and Mellor-Yamada 2.0 turbulence closure.

Model nesting strategy. The regional grid is on the left, and local grid on the right.
Figure 1: Model nesting strategy. The regional grid is on the left, and local grid on the right.

Bathymetry

Coastline with resolution greater than 20 m was supplied by the Department of Environment and Resource Management (DERM), which adequately resolved the complex channels of southern Moreton Bay but does not include the rivers. The latter were manually digitised to the heads of the estuaries from GoogleEarth images and from naval charts AUS237 and AUS238.

Bathymetry data was mainly comprised of a Digital Elevation Model provided by WBM. This gridded data set has a resolution of approximately 20m, but does not span the entire region and was consequently supplemented with gridded bathymetry products produced by GeoScience Australia’s (GA) 2005 product (e.g. Petkovic and Buchanan, 2002) at 250 m resolution for the offshore region, data digitised from chart AUS235 for a region north of Moreton Bay and data digitised from chart AUS814 for the Gold Coast Seaway region. The bathymetry for the regional and local grids is displayed in Figure 3.1 and 3.2 respectively.

Bathymetry of the regional grid.
Figure 2: Bathymetry of the regional grid.

 

Bathymetry of the local grid.
Figure 3: Bathymetry of the local grid.

Forcing

The regional model was forced with global ocean and atmospheric model products. The regional model was nested in BRAN2.3 (Schiller et. al, 2008); these outputs considered the best global product to date suitable for forcing the nested suite. BRAN2.3 is a data assimilating global model with 10 km resolution in the Australasian region, using MOM4p1 as the code base. The BRAN2.3 hindcast terminates in May 2008, therefore at this stage model simulation can only proceed to this date. These data were used as initial conditions for temperature, salinity and sea level, and boundary forcing on the regional grid for temperature, salinity and velocity. An upstream advection open boundary condition (OBC) was used for temperature and salinity. The velocity open boundary condition follows the methodology of Herzfeld (2009), where a local flux adjustment is used to prevent long term basin-wide divergence, which may lead to filling or emptying of the domain. A tidal signal was superimposed on the low frequency sea level oscillation provided by BRAN2.3 on the regional grid open boundary. This tidal signal was introduced via the local flux adjustment. The OTIS tidal model was used to generate the tidal signal from amplitude and phase information for 8 constituents (M2, S2, K1, O1, Q1, P1, N2 and K2) at every regional boundary node. The local grid open boundary was forced with temperature, salinity (again using an upstream advection OBC) and velocity (with local flux adjustment) derived from the regional grid. Note that the velocity forcing on the local grid contains the tidal component.

Atmospheric forcing products (wind, pressure, heatflux) are supplied by the Bureau of Meteorology‘s (BOM) MesoLAPS atmospheric model at 1/8 degree resolution. This modelled product is preferable to spatially interpolated meteorological data supplied by BOM weather stations for surface forcing, since its spatial detail is superior. MesoLAPS provides wind, mean sea level pressure, cloud amount, air temperature and dew point temperature. From these variables, bulk schemes (Kondo, 1975) are used to compute sensible and latent heat fluxes, and black body radiation may be used to compute long wave radiation (Zillman, 1972). The sum of these and computed short wave input provides a net heat budget.

Fresh Water Inputs

Freshwater inputs are classified as the head-of-estuary flows, and lateral inputs along the river. Belonging to the former are (from north to south) the Caboolture, Pine, Brisbane, Logan, Pimpana, Coomera and Nerang Rivers. The Brisbane and Bremer rivers were treated as one combined head-of-estuary flow, as was the Albert and Logan Rivers. These are input as a velocity OBC where the velocity is input as a parabolic profile with maximum flow at the surface and zero at a nominated pycnocline depth (usually the sea bed). The lateral inflows are input as point sources. Locations of all freshwater inputs are displayed in Figure 4. Major flows occurred in the summer of 2008, with combined flow in the Logan and Albert rivers reaching ~1400 m3s-1 and in the Brisbane and Bremer rivers ~350 m3s-1.

Locations of freshwater flow inputs.
Figure 4: Locations of freshwater flow inputs.

References

Binliang, L. and A. Falconer (1997) Tidal flow and transport modelling using ULTIMATE QUICKEST scheme. Journal of Hydraulic Engineering, 123, 303 – 314.

Blumberg, A.F. and J. Herring (1987) Circulation modelling using orthogonal curvilinear coordinates, in Three-Dimensional Models of marine and Estuarine Dynamics, Ed. J.C.J. Nihoul and B.M. Jamart, Elsevier.

Herzfeld, M. (2006) An alternative coordinate system for solving finite difference ocean models. Ocean Modelling, 14, 174 – 196.

Herzfeld, M. (2009) Improving stability of regional numerical ocean models. Ocean Dynamics, 59, 21- 46. http://dx.doi.org/10.1007/s10236-008-0158-1

Kondo, J. (1975) Air-sea bulk transfer coefficients in diabatic conditions.Boundary-Layer Meteorology, 9, 91-112.

Petkovic, P., Buchanan, C., 2002. January 2002 edition of the AGSO bathymetry. Geographic projection WGS84 Datum. Australian bathymetry and topography grid. Canberra: Geoscience Australia.

Schiller, A., Oke, P. R., Brassington, G. B., Entel, M., Fiedler, R., Griffin, D. A., and Mansbridge, J. (2008) Eddy-resolving ocean circulation in the Asian-Australian region inferred from an ocean reanalysis effort. Progress in Oceanography,76(3), 334-365.doi:10.1016/j.pocean.2008.01.003.

Van Leer, B. (1979) Towards the ultimate conservative difference scheme. V: a second order sequel to Godanov’s method. J. Comput. Phys., 32, 101-136.

Zillman, J.W. (1972) A study of some aspects of the radiation and heat budgets of the southern hemisphere oceans. Bureua of meteorology, Meteorological study no. 26, Australian Govt. Pub. Service, Canberra.