[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]

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