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Re: [ferret_users] Regird tripolar grid



This is very good advice (thanks, Russ).

As the link says "In general, regridding is a difficult task to perform accurately and without producing noise or spurious results." This discussion is useful and thoughtful, and applies not just to model output. One cannot assume that regridding done by simple interpolation (as in the first message below) will result in fields that conserve properties, especially products of properties (like fluxes), derivatives or integrals. Computation should be done as much as possible on the model's native grid, then regridding done to make a plot.

Ferret's CURV_TO_RECT function implements the GFDL algorithm, and is better than interpolation, but better still is working on the native grid until it is time to make a plot. And checking is always necessary.

Billy K

On Mar 6, 2013, at 15:57 , Russ Fiedler wrote:


Hi,

The MOM distribution includes tools for the postprocessing of model data
on tripolar grids onto regular lat/lon grids. You need the underlying
grid_spec.nc file and need to create an exchange grid for your 2 degree
resolution grid so that the interpolations are done correctly.

See the file

src/postprocessing/regrid/regrid.F90

in your MOM distribution. See also

http://data1.gfdl.noaa.gov/~arl/pubrel/r/mom4p1/src/mom4p1/doc/mom4_manual.html#Postprocessing%20regrid%20tool



If you are using the newer MOSAIC style of grid file I think you want
the fregrid tool in

src/tools/fregrid

Russ




On Thu, 2013-03-07 at 09:27 +1100, Ansley Manke wrote:
Hi,
Ferret needs special handling for these grids. Grids like the tripolar grid are called curvilinear grids in the Ferret documentation. So check
under "curvilinear" or "regridding, curvilienar" in the documentation
index for the CURV_TO_RECT and CURV_TO_RECT_MAP function.

These functions are correct but slow. We have some newer ones on the
horizon, using methods and code from the Earth System Modeling Framework.

Ansley

On 3/6/2013 1:34 PM, Boyin Huang - NOAA Federal wrote:
Hi Ferreters,
Can we regrid tripolar grid data from GFDL MOM model outputs (NetCDF
format) to regular 2x2 grid using ferret?
I used the following script that failed in the Arctic region
use tos_day_GFDL-ESM2G_historical_r1i1p1_18610101-18651231.nc
SET MEMORY/SIZE=320
define axis/x=0e: 358e:2/unit=degree xlon
define axis/y=88S:88N: 2/unit=degree ylat
define grid/x=xlon/y=ylat mygrid
list/file="tos.dat"/format=stream/L=1:1825/clobber tos[g=mygrid]

Any suggestions are welcomed.
Thanks

--
Boyin Huang
Ingest and Analysis Branch
Global Climate Application Division
NCDC/NOAA, Room-514
Phone 828-271-4851
Fax 828-271-4022, 828-271-4246






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