[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]

Hello Andries

I only skimmed the scripts. I did not see anything 'wrong'.
Perhaps some form of filtering of the source 1x1 grid.

The following filters work on the spherical harmonic coefficients.

*tri_trunC   *
<http://www.ncl.ucar.edu/Document/Functions/Built-in/tri_trunC-1.shtml>:
Triangular truncation of spherical harmonic coefficients

*exp_tapershC*
<http://www.ncl.ucar.edu/Document/Functions/Built-in/exp_tapershC.shtml>: :
Performs tapering (filtering) of the spherical harmonic coefficients

I have a 1x1 grid so I used that. See attached script. Substitute your file.

Play with the various parameters.

Of course, the contour intervals will have to be adjusted.

Hope this helps..

D

On Thu, Mar 7, 2019 at 6:03 AM De Vries Andries <andries.devries at env.ethz.ch>
wrote:

> 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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