Global Ocean Modelling with Adaptive Unstructured Grid Methods

Ocean circulation is clearly important on a water-dominated planet in the throes of climate change. Ocean modelling technologies are therefore crucially important to our abilities to forecast change. However, despite significant advances over the past decade, ocean general circulation models are based on essentially the same finite difference methods and fixed structured grids employed in the earliest models developed in the 1960s. Although unstructured mesh modelling has long been a goal of many oceanographers, attempts to apply such methods the global circulation have failed due to challenges with high aspect ratio domains and in treating the Coriolis and buoyancy terms accurately and stably on unstructured meshes. Over the past few years important solutions to these problems have been developed 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 robust treatments of geostrophic and hydrostatic balance, optimised bathymetry and coastline geometries, and large eddy adaptive turbulence models. In this project we will take advantage of more than a decade of development work on our new ocean model ICOM and produce, in managed stages, a global circulation model that fully incorporates 3D adaptive and unstructured mesh technology. As a result we will be able to simultaneously resolve flow at scales ranging from 10s km (boundary currents) to 100s m (downwelling currents) with minimal parametisation and maximal capture of the physics. In addition, by taking advantage of our state-of-the-art meshing tools, initial surface resolution will be focussed on areas of bathymetric change. This will impart computational efficiency since it will reduce the number of nodes and elements required to capture geometric complexity. Our approach will allow the mesh to move in response to eddies or density layers in a manner akin to hybrid coordinate approaches. The topology of the meshes and node density will also be optimised to gain maximum flexibility, power and robustness from our novel modelling approach. The quality of the adaptive model's simulation of the global ocean will be assessed by examining how, in multi-decadal integrations, it reproduces the essential dynamics of the oceans through detailed comparisons with recent in-situ observational datasets and available remote sensing datasets and also against simulations performed at the National Oceanography Centre Southampton (NOC) by current ocean models, such as NEMO and OCCAM, at a variety of grid resolutions. Simulations will be conducted on parallel computing clusters at Imperial College, NOC, and the UK's HECToR supercomputing facility where scaling properties on large numbers of processors will be determined. In addition to developing new adaptive mesh techniques on the sphere we will show how various forms of mesh movement vertically and horizontally, and mesh structural changes can each enhance solution quality. We will study the performance of a range of error measures which are based on representations of the flow dynamics. Our mesh generation package Terreno will be generalised so that static meshes are able to provide high quality simulations in isolation from mesh adaptivity. We will also demonstrate that model spin-up can be rapidly achieved with progressively increased resolution. The resulting advances in ocean modelling capabilities will cement the UK's position firmly at the forefront of unstructured and adaptive mesh ocean modelling, and is expected to be a major ocean modelling milestone for the UK. Overall this project will contribute considerably towards the production of an open source ocean model for the UK and worldwide ocean modelling communities. It will also provide an important contribution to NERC's Oceans2025 programme by delivering Work Package 9.8.