Formulation of a balanced Exner pressure in the Lateral ... - Met Office

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Formulation of a balanced Exner pressure in the Lateral Boundary Conditionsof the Unified ModelPeter Lean and Clive WilsonDocument Change HistoryFirst draft, Peter Lean 22/06/2007Comments from Clive Wilson 04/03/2008Approved by Stuart Bell 05/03/20081
AbstractA new formulation of Exner pressure and density in the lateral boundaries conditions of limited areaconfigurations of the Unified Model is presented. This new formulation is designed for use when thevertical grid of the limited area model is different to that in the model used to provide those boundaryconditions. In these situations, the dynamically balanced state in the driving model can becomedisrupted by vertical interpolation onto the new vertical grid. The subsequent vertical accelerations inthe lateral boundary regions resulting from this imbalance can lead to high dynamics solver iterationcounts (increasing the run time of the forecasts) and, in some circumstances, a bias in the mean sealevel pressure.These problems were first noted in the operational NAE model after the introduction of a newvertical grid in the global model. This report summarises the idealised experiments conducted toidentify the cause of these problems, the use of imposed vertical balance as a solution and finally theimplementation of this solution in the operational limited area forecast models.2
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Formulation of a balanced Exner pressure in the Lateral ... - Met Office

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