Comparisons to Observations of Simulated Tracers (compost) does what it sounds like. It is a collection of utilities to compare 3D fields of atmospheric constituents, most commonly from a GCM/CTM/DAS, to in situ and satellite data.
Compost is written in Matlab and (please forgive me) C shell. The C shell script will be converted to bash or Python soon. The Matlab code will likely persist for some time. Directions, and any needed updates, for running on GNU Octave, a free Matlab replacement, are forthcoming.