[ncl-talk] Regrid ECMWF to an equal-area grid (EASE) for SOM

[David Brown]

Hi Mattias,
I'm sorry no one has responded to your question yet. After a bit of
research, I have learned that the EASE grids, as supported by (e.g.)
NSIDC, use two different projections, a lambert equal-area grid for
plots centered on the poles, and a cylindrical equal area grid for
global data. Both of these projections are supported by NCL and
therefore one way to generate a destination grid would be to use NCL's
mapping routines in conjunction with the NCL function ndctodata.
Another way to do this would be to use the proj4 tool. There is
actually an interface to this tool in NCL and so you could do this in
NCL also.

It seems that the output grid you use could be somewhat arbitrary as
long as it is equal area and covers the area of your input data. So I
have created an example using the ndctodata method with a Lambert
EqualArea projection centered at lat 90 lon 280. It writes out a
NetCDF file with the lat and lon points that I think could be used as
a destination grid. This may not be exactly what you are looking for
but hopefully it gives you a starting point to work from.
 -dave


On Mon, Jul 20, 2015 at 6:05 AM, Mattias Cape <mcape at whoi.edu> wrote:
> Hi,
>
> I am working with ECMWF ERA-40 data (2.5 x 2.5º grid, example attached), using self organizing maps to look at patterns of variability in synoptic atmospheric circulation.  These datasets are typically regridded to an equal area grid (250 x 250 km for example) to allow for uniform spatial sampling prior to analysis (e.g. http://onlinelibrary.wiley.com/doi/10.1029/2006JD007460/abstract).
>
> I’ve been having trouble defining the gridding parameters to accomplish this. There are several examples on the NCL website demonstrating how to use ESMF software, but I am unsure how to define the destination (EASE) grid. I am only working with a spatial subset of the ERA-40 dataset covering the region 85°N 80°W 40°N 0°E.
>
> If anyone has any insight into how to accomplish this regridding, or at least how to define the destination grid, I would appreciate it.
>
> Cheers,
>
> Mattias
>
>
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