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

[De Vries Andries]

Hi Dennis,

Thank you very much for your helpful response. I looked a bit into this ringing/Gibbs phenomenon and it seems indeed the issue looks very much like that. 

At the moment I see the following options/solutions: 
1) accepting the ringing effect in the PV data on a regular grid. Large differences only occur very close to the Pole and might not be a big problem
2) switching to working with data on a gaussian grid. 

Any comments on this? Or perhaps any other solutions that I miss? By the way, please, find attached the scripts how I compute the PV for the regular and Gaussian grids, and another one to assess the differences in the PV files. I forgot to attach those in the previous email. If there is anything in the scripts that looks suspicious or might explain the the issue, please, let me know.

Cheers,
Andries
________________________________________
From: Dennis Shea [shea at ucar.edu]
Sent: 06 March 2019 19:57
To: De Vries  Andries
Cc: ncl-talk at ucar.edu
Subject: Re: [ncl-talk] compute PV on hybrid levels on a regular global grid with function pot_vort_hybrid; weird, fluctuating values toward the poles

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<mailto: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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