Describe the Problem

This issue arose via the dtcenter/METplus#1718 discussion. Further investigation reveals that the MET NetCDF-CF library code needs to be enhanced to instantiate Polar Stereographic grids from NSIDC Sea Ice Edge NetCDF files. Those file are CF-compliant but MET fails to read that data.

Expected Behavior

MET should be able to read data from NSIDC Sea Ice Edge NetCDF file and instantiate the Polar Stereographic grids on which they resides.

Environment

Describe your runtime environment:
1. Machine: (e.g. HPC name, Linux Workstation, Mac Laptop) Not Hardware-specific
2. OS: (e.g. RedHat Linux, MacOS) Not OS-specific
3. Software version number(s) version 10.1.0

To Reproduce

Describe the steps to reproduce the behavior:
1. Retrieve these sample data files:
See NSIDC_sample_data.tar.gz

https://n5eil01u.ecs.nsidc.org/PM/NSIDC-0081.002/2022.07.23/NSIDC0081_SEAICE_PS_N25km_20220723_v2.0.nc
https://n5eil01u.ecs.nsidc.org/PM/NSIDC-0081.002/2022.07.23/NSIDC0081_SEAICE_PS_S25km_20220723_v2.0.nc

2. Run plot_data_plane:

plot_data_plane NSIDC0081_SEAICE_PS_S25km_20220723_v2.0.nc test.ps 'name="F16_ICECON"; level="(0,*,*)";' -v 4

3. See error

ERROR : get_nc_var_att_double() -> Cannot get "longitude_of_projection_origin" from crs variable.

Post relevant sample data following these instructions:
https://dtcenter.org/community-code/model-evaluation-tools-met/met-help-desk#ftp

The CF-Convention documentation defines Polar Stereographic grids here. And this is parsed by the get_grid_mapping_polar_stereographic() function in MET.

This task is to enhance MET's NCCF library to handle these files.

Some initial testing got us closer but still produced an error:

ncatted \
-a longitude_of_projection_origin,crs,a,d,0 NSIDC0081_SEAICE_PS_N25km_20220723_v2.0.nc  \
-a scale_factor_at_projection_origin,crs,a,d,1.0 \
-a units,x,o,c,"m" \
-a units,y,o,c,"m" \
-o NSIDC0081_SEAICE_PS_N25km_20220723_v2.0_mod.nc

But the result still produces this error from MET:

ERROR  : NcCfFile::get_grid_mapping_polar_stereographic() -> MET can only process Polar Stereographic files where the x-axis and y-axis deltas are the same.

The X delta is +25000 m and the Y delta is -25000 m. So the magnitude is the same, but the sign is different.

If we ignore the sign, then MET is able to plot the result but the image is upside-down. So the library code need to detect this situation and flip the data accordingly, as is done in MET's GRIB1/2 libraries.

Please see the images in the attached tarfile to see how the data should actually be oriented.

Relevant Deadlines

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Funding Source

2773542 (was 2793541)

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