Implementation of the Semi-Lagrangian Method in a High-Resolution Version of the ECMWF Forecast Model

Abstract In this article the implementation of the semi-Lagrangian method in a high-resolution version of the ECMWF forecast model is examined. Novel aspects include the application of the semi-Lagrangian scheme to a global model using the ECMWF hybrid coordinate in the vertical and its use in a baroclinic spectral model in conjunction with a reduced Gaussian grid in the horizontal. The former Eulerian vorticity-divergence formulation is first converted into a momentum-equation formulation that is considerably more economical, thanks in part to the incorporation of Legendre transform efficiencies that were previously demonstrated for the shallow-water equations. The semi-Lagrangian formulation is presented in detail, together with a discussion of computational aspects that are relevant for executing the high-resolution model efficiently on a modestly parallel supercomputer. The impact of formulation changes is assessed via numerical experiments on a set of 12 independent cases. In particular it is shown t...