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.

IFS model grids

IFS can make use of different latitude-longitude grids for the same spectral resolution. Each grid type offers different advantages and disadvantages.  A 'reduced' grid is one in which the number of gridpoints around each latitude circle is reduced as the poles are approached. A full grid, or 'regular' grid, has the same number of points around each latitude circle. The terms 'linear', 'quadratic' and 'cubic' refer to the number of gridpoints used to describe the shortest waves retained in spectral space. For example, linear grids use two gridpoints to describe the shortest wave, and so on.

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. 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.

For more information about IFS grids please see: OpenIFS: Gaussian grids and OpenIFS: Octahedral grid in the OpenIFS User Guide.

The following table describes the different grid types together with the filename extension used in the ifsdata filesnames (see: Download OpenIFS).

Linear reduced and Cubic Octahedral are by far the most commonly used, with cubic octahedral grids typically used for the higher resolutions.

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.

Wave model grids

The ECMWF Wave Model, a component of IFS, uses a different grid to the atmospheric grid in IFS.

The wave model uses an irregular latitude-longitude grid. For such grids the distance between latitudes is set to a certain resolution in degrees, where the number of grid points per latitude is adjusted so that the actual distance between points on each latitude is roughly the same as the one between 2 consecutive latitudes.

There is an approximate match between the number of latitudes in the atmospheric grid and the wave model grid, it is still a good approximation to run the wave model on a slightly coarser grid in order to save overall cost during operational forecasts. Future developments of IFS will include a wave model grid that matches the IFS.

The table below summarises the wave model recommended grid (as used by ECMWF) for each atmospheric resolution. This is for information only, the wave initial files provided by ECMWF will already have this taken into account for the requested resolutions.

Supported spectral and horizontal grid resolutions

The table below summarizes the supported configurations of the IFS and the relationship between spectral truncation, atmosphere and wave model grids and the recommended timestep for optimum performance.