[ncl-talk] compute PV on hybrid levels on a regular global grid with function pot_vort_hybrid; weird, fluctuating values toward the poles

[Dennis Shea]

*http://www.ncl.ucar.edu/Applications/pot_vort.shtml*
<http://www.ncl.ucar.edu/Applications/pot_vort.shtml>

[1]
"Previously, I computed PV using data from ERA-Interim on a gaussian
grid *without
any troubles*."

[2]
"Now I try the same, but with ERA-Interim data on a* regular 1x1 grid.*

*---*
*pot_vort_hybrid
<http://www.ncl.ucar.edu/Document/Functions/Contributed/pot_vort_hybrid.shtml>*:
This function works with both gaussian and regular grids. Same code, just a
switch for the grid type

---
I *speculate* that the ECMWFspectral  interpolation from the source
gaussian grid to 1x1 is the issue. *Spectral interpolation can result in
"ringing" due to Gibbs phenomenon.*

*D*





On Wed, Mar 6, 2019 at 10:31 AM De Vries Andries <
andries.devries at env.ethz.ch> wrote:

> Dear NCL team & NCL users,
>
> I have an issue with the computation of potential vorticity (PV) on hybrid
> levels using the function "pot_vort_hybrid". Previously, I computed PV
> using data from ERA-Interim on a gaussian grid without any troubles. Now I
> try the same, but with ERA-Interim data on a regular 1x1 grid. I found that
> values toward the pole are fluctuating with respect to PV that has been
> provided directly by the ECMWF. (The reason for computing PV myself is to
> obtain PV isentropic surfaces with higher intervals than provided by
> ECMWF). In other words, the PV that I compute show increasing, positive &
> negative alternating values as compared to the PV that is provided by the
> ECMWF.
>
> Any idea if I do something wrong, and/or how to fix this issue?
>
> Please, find attached the two scripts for the computation of PV on a
> Gaussian grid and on a regular grid, as well as a short script that prints
> the numerical differences between the two output files. The needed data
> files to compute PV on a guassian and regular grid, as well as files with
> PV from ECMWF are uploaded.
>
> Looking forward for any feedback!
> Many thanks!
>
> Cheers,
> Andries
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