NGWCP - Atmospheric model dynamical core

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At the heart of a weather prediction or climate simulation model is a `dynamical core' that solves the equations describing the fluid dynamics and thermodynamics of the atmosphere on the scales that can be resolved. The dynamical core of the current Met Office Unified Model for weather and climate prediction (the `New Dynamics') and its planned successor (`ENDGame') use highly efficient and accurate methods that exploit the structure of a longitude-latitude grid. However, several aspects of these schemes involve non-local data communication, particularly near the poles of the grid. Such non-local communication is expected to become a serious performance and scalability bottleneck on future massively parallel computer architectures, for which communication between processors is disproportionately slow compared to the calculation itself. It is therefore imperative to examine alternative, quasi-uniform, grids, with much reduced communication demands, as a potential basis for a future atmospheric model dynamical core. The proposed project will examine some of the leading candidate alternative grids. The properties of the candidate grids will be examined through a combination of theoretical analysis and numerical calculations using simplified equation sets. One requirement is for accurate wave propagation. The ideal scheme should capture the frequencies of all resolved waves as well as possible and should not support any unphysical `computational modes'. This will be examined theoretically in idealised cases where the calculation is tractable, and numerically in more complex cases. If a candidate scheme is found to support computational modes, methods for controlling them will be examined. Another set of essential requirements relate to conservation of mass and energy, and that the scheme should respect basic geometrical requirements such as the fact that the gradient of the geopotential should generate no local rotation (vorticity) in the fluid. Also among these requirements is the need for the scheme to be able to support balanced flow, since the atmosphere is observed to be close to balance on large scales. Again, these requirements will be examined through a combination of theoretical analysis and numerical testing. Finally, all of the candidate alternative grids have the potential for the numerical approximations to lead to the grid structure being reflected in persistent errors in the numerical solution, so called `grid imprinting'. Test cases will be developed and applied to quantify the grid imprinting tendencies of the various candidate grids.