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

[Gus Correa]

Hi Xi

On 04/28/2017 04:26 PM, Xi Chang wrote:
> 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.

It may be sigma only.
However, make sure the bk is NOT zero also!
Otherwise the GFDL dynamical core may be using another scheme
for the vertical coordinate.

>  Anyway,
> which interpolation technique do you think the best for
> vinth2p: "linear" or "logarithmic" (?)
 >
 > Thanks
 > Xi
 >

If it is sigma (a pressure ratio) linear interpolation may be OK.
Note that regardless of the interpolation method,
near the surface you may have areas filled with NaNs or
_FillValue in the output pressure field,
where the isobars cut across the topography.

I hope this helps,
Gus Correa

> On Fri, Apr 28, 2017 at 12:42 PM, Gustavo Correa <gus at ldeo.columbia.edu
> <mailto: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
>     article about AM3.
>     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
>     > _______________________________________________
>     > ncl-talk mailing list
>     > ncl-talk at ucar.edu <mailto:ncl-talk at ucar.edu>
>     > List instructions, subscriber options, unsubscribe:
>     > http://mailman.ucar.edu/mailman/listinfo/ncl-talk
>     <http://mailman.ucar.edu/mailman/listinfo/ncl-talk>
>
>     _______________________________________________
>     ncl-talk mailing list
>     ncl-talk at ucar.edu <mailto:ncl-talk at ucar.edu>
>     List instructions, subscriber options, unsubscribe:
>     http://mailman.ucar.edu/mailman/listinfo/ncl-talk
>     <http://mailman.ucar.edu/mailman/listinfo/ncl-talk>
>
>