Prompted by both my own struggles with wgrib2 compilation and a plea on the rNOMADS email listserv, I’m going to describe how to compile and install wgrib2 on Mac OS.

First of all, some background: wgrib2 is an excellent utility written by Wesley Ebisuzaki at NOAA.  It allows for a number of swift and stable operations on GRIB2 files (a common file format for weather and climate data).  It is also a requirement for using grib files in rNOMADS (the function ReadGrib() in particular).

So here’s how to install it on Mac OS.

1.  Get Command Line Tools for Xcode (search for it on Duckduckgo or your engine of choice) and also make sure gcc is installed. If you don’t know what gcc is, stop now and find someone who does (it will save you a lot of time).
2. Download wgrib2 here (note download links are pretty far down the page).
3. Untar the tarball somewhere, and roll up your sleeves.  cd into the resulting wgrib directory.
4.  In the makefile, uncomment the lines
   #export cc=gcc #export FC=gfortran Also search for makefile.darwin in the makefile and uncomment the line containing it.  You’ll see instructions to this effect in the makefile anyway.
5. Now we have to edit the included libpng package, since it is untarred by the makefile and doesn’t inherit our compiler specifications in step 4. Ensuring that we’re in the wgrib directory:
   tar -xvf libpng-1.2.57.tar.gz cd libpng-1.2.57/scripts/
   now edit the makefile.darwin file, changing
   CC=cc
   to
   CC=gcc
   Now, return to the wgrib directory, and re-tar libpng!
   tar -cf libpng-1.2.57.tar libpng-1.2.57 gzip libpng-1.2.57.tar
   If it asks you if you want to replace the original tar.gz file, say “yes”.  What we’ve done here is edited libpng to make sure it uses the right compiler.
6. Finally, type make to build wgrib2.

If you are still having problems (for example, libaec complaining that there is no c compiler), make sure that all the compiler commands (gcc, cc, etc.) all point to something other than clang (the default compiler that comes with OSX).  You may have to edit your bash_profile file to ensure this.

As always, contact me on the form below if you’re having unresolvable issues.