WhatIs : The use filtre fft parameter

The longitudinal filter

To minimize stability issues due to grid size diminishing as the pole is approached, some filtering is applied to remove short wavelength (high frequency) structures along longitudes. This operation can be summarized as maintaining an effective grid resolution (along a longitude circle) to that existing at latitudes +/-60° where the longitudinal geometrical resolution is halmf of that at the equator.

the use_filtre_fft parameter

In practice the filtering along a longitude circle of N points requires computations with a cost of the order of N*N. And this is the default behavior in the code. It is however possible to do the same work using efficient FFT (Fast Fourier Transform) operations, which cost less (somethings of the order of N*log(N) operations). In fact the larger N is, the more advantageous it is to use the FFT for filtering.

To activate the FFT filtering, just set

use_filtre_fft=y

in your run.def file. Note that the FFT filtering is only implemented in the parallel dynamical core and not the serial one.

22/11/2021