'Next Generation' unstructured mesh ocean global circulation modelling.

Lead Research Organisation: University of Reading
Department Name: Meteorology


We will build a next generation ocean global circulation model that is more accurate and has more detailed resolution than existing models. This model will be capable of resolving flows simultaneously on global, basin, regional, and process scales. It will be able to change numerical detail in response to both the structure of the modelled flows and user-defined preferences of regions and structures of specific importance. We shall use the model to carry out ocean research which is not possible with existing models. Despite significant advances over the past decade, numerical ocean global circulation models (OGCMs) are based on essentially the same finite-difference methods employed in the earliest ocean models developed in the 1 960s. Meanwhile, unstructured finite element/volume methods have been deployed to great effect in engineering applications and offer several major advantages for ocean modelling. These include the abilities to: conform accurately to complex basin geometries; focus resolution where it is most needed in response to the evolving flow or regional importance; move the mesh in response to error norms and maintain vertical density structures; incorporate various natural boundary conditions in a straightforward manner; and to make rigorous statements about model errors and numerical convergence. The ability of an unstructured mesh ocean model to change resolution smoothly and dynamically in response to changing ocean dynamics can be viewed as the 'ultimate in grid nesting', but avoiding the various difficulties inherent in matching different dynamical regimes at nest boundaries. Although unstructured mesh modelling has long been a goal of many oceanographers, attempts to apply such methods to model the global circulation have failed due to challenges in treating the Coriolis and buoyancy terms accurately and stably on unstructured meshes. Over the past few years important solutions to this problem have been developed (many by us) and incorporated into the Imperial College Ocean Model (ICOM). This model has the best available parallel mesh adaptivity methods, a suite of options for spatial derivatives (such as high-resolution methods for density/tracer advection), novel and robust treatments of balance, optimised bathymetry and coastline geometries and new large eddy mesh adaptive turbulence models. ICOM will form the foundation of the OGCM built by the proposed consortium. The drivers for it are twofold: (a) a new research tool is a necessity as 'standard' finite-difference codes become harder to tweak to improve their accuracy; and (b) there are significant science problems where the disparity between length scales requires the application of new modelling techniques such as ours (e.g. flow through sills and down slopes, eddies, convection within gyres, etc). These methods have the potential to revolutionise the way in which ocean modelling is done, and thus to secure the long-term future of UK Ocean Modelling at the forefront of the field. The next generation ocean model, with its ability to efficiently resolve a wide range of scales, will have numerous applications to the NERC and wider communities. For ocean modellers, it offers the opportunity to efficiently resolve both basin scale circulation and small-scale processes such as boundary currents, through- and overflows and geostrophic eddies. For Earth system and climate modellers, it offers the opportunity to focus resolution in regions of particular importance, such as boundary currents and overflows, without increasing the computational cost above that of a conventional coarse-resolution model. With a wide range of applications in oceanography, climate change, flood defence, pollution and contaminant dispersal, sustainability of water quality and fisheries, the development of such a model is extremely desirable.


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