Fix for running MM5 with NAM grids post-March 2017

by David Ovens, Dept. of Atmospheric Sciences, University of Washington

The Problem:

NCEP modified the NAM GRIB2 files in March 2017 such that the height fields were not read properly by many older codes.
MM5 reads GRIB1 files, but, the NCEP cnvgrib code was not modified to handle the new format.
MM5 programs PREGRID and REGRID seem to work with the files, but INTERPF frequently crashes with an error like this: 
     NHPP vert gradient check   17   9   10.156250E-01-159302.     159302.    
     nhpp_did_not_converge

The Solution:

After some failed attempts to figure out where cnvgrib (NCEP code) or pregrid (part of MM5's REGRID) needed to be modified to fix the problem, I approached the problem in a different way. Since NCAR had already fixed WRF's ungrib to work with the new NAM GRIB2 files, I decided to use ungrid instead of pregrid and modify regridder to read in the slightly different format of those files.
Here are the steps to fix the problem:
  1. Download the WRF's WPS version 3.9 (or later), or patch an earlier version with the ungrib/src/ngl/g2/comunpack.f routine found at this NCAR MMM page.
  2. Replace regridder file REGRID/regridder/src/proc_ingest_first_guess.F with this version.
  3. Optionally, you may also update REGRID/pregrid/util/plotfmt.F with this version
  4. Recompile REGRID. In the top REGRID directory, type "make" or "make intel", or whatever works for your system.
  5. Use WRF's link_grib.csh with NAM GRIB2 files
  6. Use this Vtable file which has been modified for MM5's variable names (TT, UU, and VV renamed to T, U, V)
  7. run ungrib instead of pregrid
  8. run regridder
  9. run interpf and see that it now works!
  10. run the MM5