# [ncl-talk] Converting hybrid sigma-pressure coordinate to pressure level

Xi Chang xi.chang01 at gmail.com
Fri Apr 28 14:26:59 MDT 2017

```Hi Gus,

I am using GFDL dynamical core. Since the output of "pk" from my simulation
is "zero" so I assuming that the coordinate is sigma. Anyway, which
interpolation technique do you think the best for vinth2p: "linear" or
"logarithmic" (?)

Thanks
Xi

On Fri, Apr 28, 2017 at 12:42 PM, Gustavo Correa <gus at ldeo.columbia.edu>
wrote:

> Hi Xi
>
> I am speaking about GFDL AM3/CM3, so if you're using AM2.1/CM2.1
> or older, you need to check if this is correct for those older versions.
>
> I just went through the same problem a couple of weeks ago.
>
> In the history/postprocessed files,
> GFDL renames as  "pk" what is in fact "ak".
>
> Note however, that contrary to NCAR models,
> which uses this conversion formula:
>
> pressure(i,j,k,n)=ak(k)*p0 + bk(k)*ps(i,j,n)
>
> AM3 (maybe older versions as well) use
>
> pressure(i,j,k,n)=ak(k) + bk(k)*ps(i,j,n)
>
>
> where ak is just renamed
> to pk in the netCDF output files.
>
> In the formulas above, i,j,k,n are respectively the indices for longitude,
> latitude, level, and time, and p0 is the reference pressure (at the
> surface).
> Although the mean sea level pressure p0 is 1013hPa, models
> normally use p0=1000hPa.
>
> Note that in the NCAR scheme both ak and bk are
> non-dimensional.
> However, in the GFDL AM3 scheme bk is non-dimensional,
> but ak (renamed pk) has dimensions of pressure.
> Actually ak (or pk) is in Pascal (NOT mb or hPa).
>
> This is probably handled directly by vinth2p, or at most you need
> to rescale ak=pk/p0 before you use vinth2p.
> Please, check the vinth2p documentation before you use it.
>
> A good reference is the Donner et. al. 2013 Journal of Climate
> It has a table clarifying the meaning of ak, bk, etc, although
> it doesn't mention that in the output files ak is renamed pk.
>
> I hope this helps,
> Gus Correa
>
> On Apr 28, 2017, at 12:07 AM, Xi Chang wrote:
>
> > Hi NCL team,
> >
> > I am currently running some simulations with GFDL model + topography.
> > So, I aware that pressure level_interpolation is needed when topography
> is non-zero due to terrain-following sigma coordinates. I know that I can
> use this function from NCL "vinth2p" but I only have "bk", "pk", and "ps".
> from the output file, but not the "ak". Can you suggest me how to solve
> this issue?
> >
> > Thanks
> > Dr. Chang
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