[ncl-talk] Question re: Regridding geographic subsets using ESMF_regrid

[Mary Haley]

Hi Jacob,

Welcome back to NCL.

1) You do want to set SrcRegional to True if your source grid is not
global.  If the destination grid is also not global, then set DstRegional
to True as well.

2) The regridding is done based on whatever lat/lon destination grid you
give it.  So, if you tell it to regrid to a "2.5x2.5" deg grid, then
internally it generates a *global* 2.5 x 2.5 grid, regardless of what
SrcRegional or DstRegional are set to. This doesn't hurt anything, but it
can make your regridding go slower if you have a small regional area to
regrid.

If you don't want a global 2.5 x 2.5 grid generated under the hood, then
you can optionally set "

DstLLCorner" and "

DstURCorner" to the lower left and upper right corners of a lat/lon
box that contains the area you want to regrid to. Internally, then, a
2.5 x 2.5 grid will be generated across these lat/lon limits only.


Here's what the ESMF_regrid (
http://www.ncl.ucar.edu/Document/Functions/ESMF/ESMF_regrid.shtml)
documentation
says about the meaning of SrcRegional and DstRegional:

- *SrcRegional* (default = False)

By default, the ESMF regridding application assumes that a logically
rectangular grid is periodic. This means that it connects the ends of the
first dimension to form a sphere and allows extrapolation over the pole. If
this attribute is set to True, then the source grid is assumed to NOT have
a periodic connection and instead is considered just a region of the
sphere.

- *DstRegional* (default = False)

By default, the ESMF regridding application assumes that a logically
rectangular grid is periodic. This means that it connects the ends of the
first dimension to form a sphere and allows extrapolation over the pole. If
this attribute is set to True, then the destination grid is assumed to NOT
have a periodic connection and instead is considered just a region of the
sphere.
3) Dennis Shea has written a nice overview that you can see at:

https://climatedataguide.ucar.edu/climate-data-tools-and-analysis/regridding-overview

Please visit our examples page:

http://www.ncl.ucar.edu/Applications/ESMF.shtml

You can also see some regridding templates at:

http://www.ncl.ucar.edu/Applications/Templates/#ESMFRegriddingTemplates

If you have problems getting your regridding script to work, please post
back to ncl-talk and let us know.  It helps if you can provide us with the
script as you have it so far, and the data file (we can provide an ftp site
for you to upload it to if necessary).

Good luck,

--Mary


On Mon, Oct 26, 2015 at 12:49 PM, Jacob Klee (VirginiaPower - 1) <
jacob.klee at dom.com> wrote:

> Hello,
>
>
>
> I am diving back into NCL after about a decade away, and have a few
> questions about regridding.  I have gathered a few parameters (Z500hPa,
> MSLP, and SST) from numerous reanalysis datasets, having already
> geographically subset all to (as close as possible) 85N, -180E, -10N,
> 180E.  I intend to regrid them all to the NCEP R1 2.5x2.5deg grid.
>
>
>
> Question 1) I assume that I will want to set option “SrcRegional” to True?
>
> Question 2) Should I contract the destination grid to 82.5N, -180E, -7.5N,
> 180E as some of the rectilinear datasets will have 85N & -10N as their
> northern and southernmost data (JRA-55 for ex), or does the script handle
> that for the user with the option “SrcRegional” set to true?
>
> Question 3) Any recommendation on which interpolation method(s) would best
> suite each of the three data types?
>
>
>
> -- Jacob Klee
>
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