[ncl-talk] Best interpolation Opt for ESMF_Regrid to regrid a binary mask?

[Greg Deemer]

I am regridding a mask from coarse to fine resolution. The mask is a monthly binary field developed to remove spurious ice from certain satellite products (http://nsidc.org/data/docs/daac/nsidc0622-valid-ice-masks/). Mask = 1 where ice is valid, and Mask = 0 where ocean is valid. There are other flags for land/coastal land that I can ignore as well with the following command:

NIC_mask = where (NIC_Flag.ne.1, default_fillvalue("integer"), NIC_Flag)

As an interim step, I spit out “NIC_mask” to it’s own NC file and opened with NC View. Please see “OriginalMask.png” attached. 

Later on in the code, I chose Opt at InterpMethod = “bilinear” as I figured it was quickest and didn’t matter much since I’m dealing with 1’s and fill values. However, it looks like there is a not so insignificant amount of information being lost near the coastlines in the resulting interpolation. Please see a similar screen grab “BilinearInformationLoss.png” attached for the result generated by ESMF_Regrid. 

Since the regridding takes a while (20+ minutes) on my desktop for bilinear, and an unknown time for other interpolation methods, I have not tested each one. I figured running “neareststod” could produce more accurate results but I’ve let the code run for hours and it seems to be getting hung up…somewhere. 

I was wondering if the folks have some explanation as to why bilinear would smearing the coastlines, or recommendations for avoiding this.

I suppose I can try to regrid a small subset that includes coastal regions  with different methods as a next step to save time….

Thank you for the discussion,
Greg

--

Greg Deemer
User Services Office
National Snow and Ice Data Center
(303) 735-8068
http://nsidc.org
Twitter: @NSIDC

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