Bug report: Wrong model aspect ratio in the diagnostic if setting it after specifying the basis

[jodemaey]

What

If during the initialization of the model the aspect ratio n is changed after the setting of the basis of functions, like in :

# Model parameters instantiation with some non-default specs
model_parameters = QgParams({'phi0_npi': np.deg2rad(50.)/np.pi, 'hd':0.3})
# Mode truncation at the wavenumber 2 in both x and y spatial coordinate
model_parameters.set_atmospheric_channel_fourier_modes(2, 2)


model_parameters.set_params({'n': 0.3})

then the model core compute the trajectories with the right aspect ratio, but the diagnostic classes are not. You can thus trust the model output but not the diagnostic ones (see temporary solution below).

Consequences

Due to the fact that the aspect ratio n is incorrect in the diagnostics, it can lead to aberrant spatial field renditions (again, in the spectral domain the model results are correct). For instance, for the example above, the Fourier basis function 2 * sin (y) * cos(n * y) is rendered as

where many maxima and minima can be seen over the domain, instead of only one maxima and one minima (see below). Aliasing due to an incorrect sampling can also be seen.

Again, this impact only your results when you use diagnostic classes to obtain spatial fields.

Temporary solution

Presently, the simplest way to avoid this bug is to instantiate the QgParams object for the model configuration directly with the desired aspect ratio:

# Model parameters instantiation with some non-default specs
model_parameters = QgParams({'phi0_npi': np.deg2rad(50.)/np.pi, 'hd':0.3, 'n':0.3})
# Mode truncation at the wavenumber 2 in both x and y spatial coordinate
model_parameters.set_atmospheric_channel_fourier_modes(2, 2)

This ensures that the aspect ratio is properly communicated to the diagnostics. In this case, the Fourier basis function above is rendered as:

Final solution

This bug will be resolved soon, but we are in the middle of a very thorough modification of the model's core and thus we need to wait that other issues are resolved before moving to this one. The final bugfix should come at the end of 2022.

[jodemaey]

Solved by #15 . Will be merged to master soon with a new release.

[jodemaey]

Solved by merging develop to master.