We use for that purpose auxiliary data named GeoVals. The GeoVals files contain the model fields interpolated at observation locations. In short, the interpolation has already been performed beforehand and the results stored in a file.
If you’re starting from an obs file and want to save the intermediate GeoVaLs (as suggested by that paragraph in the tutorial), then you can follow the pattern of the fv3-jedi test fv3-jedi/test/testinput/hofx_save_geovals.yaml. There, a GOMsaver obs filter is added to the HofX application yaml to instruct it to write the GeoVaLs.
If instead you’re starting from scratch and you want to generate a random sample of obs locations, you should be able to replace the HofX yaml’s obs space with a “Random Locations” obs space, as is done in fv3-jedi/test/testinput/getvalues.yaml to generate some random locations for the HofX.
@cyemeng — If I correctly understood the error message, I think you would need to modify the MPAS-JEDI Model2GeoVaLs variable change so that it also computes the additional variable needed by your operator.