Revise TEM module to reduce aliasing errors and calculate values on pressure surfaces

[patcal]

What is the feature/what would you like to discuss?

The current module to calculate Transverse Eulerian Mean (TEM) values operates on model level values. While the results are useful for the uppermost hybrid pressure levels, they are not meaningful for pressure levels near the surface and for MPAS levels which are based on height. To address this deficiency, the module should be revised to first interpolate state values onto nominal hybrid pressure levels prior to computations. The resulting TEM output values for the SE and MPAS dycores will be consistent and correspond to values on the nominal hybrid-pressure surfaces.

Secondly, the m=0 spherical harmonic amplitudes computed on unstructured grids are susceptible to unsightly aliasing errors, particularly near the poles. To suppress the effects of these errors, the TEM module needs to be revised to filter the 2D zonal mean residual values prior to the computation of non-linear eddy terms. The 2/3 rule of thumb for spectral modeling will be used as a guide to implement this filtering using the new spherical_harmonic module.

Is there anyone in particular you want to be part of this conversation?

@fvitt

Will this change (regression test) answers?

Yes

Will you be implementing this enhancement yourself?

Yes