Relationship between spectral and grid resolution

IFS is a model of two resolutions: the spectral resolution determining the number of retained waves and the corresponding grid onto which these waves can be transformed. A greater spectral resolution can improve the forecast quality whilst reduced Gaussian grid resolutions can improve the efficiency of the model. However, aliassing of the quadratic (and cubic) terms in the model equations can result from using linear grids, which needs to be compensated for in the model formulation. A cubic grid removes aliassing and the need for additional filters.

Model timestep

The timestep depends on the horizontal resolution and the choice of the grid. ECMWF typically use a long a timestep as possible for efficiency reasons. However, there are times when this may result in some of the semi-Lagrangian trajectories going underground (messages can be seen in the model logfile), though generally this is not a problem.

Recommended maximum timesteps are given in the table below. Shorter timesteps can be used, though note that the model will not produce the exact same result between two forecasts with different timesteps.

Supported spectral and horizontal grid resolutions

The table below summarizes the supported configurations of the IFS and the relationship between spectral truncation, gridpoint resolution and the recommended timestep for optimum performance.

Note that a linear grid allows a higher spectral truncation for the same number of gridpoints as a quadratic or cubic grid. Linear and quadratic in this sense means enough gridpoints are available to compute the linear and quadratic terms in the equations respectively. Likewise a cubic grid has more gridpoints per spectral wavenumber and provides more gridpoints to describe the highest wavenumbers.