A Comparison of the HIRLAM Gridpoint and Spectral Semi-Lagrangian Models

Abstract

A comparison of two semi-Lagrangian limited area models, one spectral, and the other grid point, is described. Forecasts from both models are compared and contrasted, first on a 55-km horizontal mesh and subsequently on a 22-km horizontal mesh. The weaknesses in the respective models exposed by these tests, and the corrections made to overcome them are described. The final models arrived at are shown to be accurate and more efficient than the Eulerian counterpart for the test dataset. It is also found that the spectral model is as accurate as the gridpoint model and is also computationally competitive.. It is concluded that with sufficient thought and effort the gridpoint and spectral models can be made to produce equally good forecasts at comparable computer costs. Finally, a reassessment of the relative merits and drawbacks of the spectral and gridpoint schemes is attempted taking into account the fact that the advection terms are integrated by semi-Lagrangian scheme.