How to Run GEOS Zero Diff Tests

Running regular replay

To run a regular replay from an existing experiment simply uncomment (remove the #M2) these lines in the AGCM.rc file:

# Typical MERRA-2 Regular REPLAY Configuration
# --------------------------------------------
#M2 REPLAY_ANA_EXPID:   MERRA-2
#M2 REPLAY_ANA_LOCATION: /discover/nobackup/projects/gmao/share/gmao_ops/verification/MERRA-2
#M2 REPLAY_MODE: Regular
#M2 REPLAY_FILE: ana/Y%y4/M%m2/MERRA-2.ana.eta.%y4%m2%d2_%h2z.nc4
# -----------------------------------------------------------------

so that it looks like:

# Typical MERRA-2 Regular REPLAY Configuration
# --------------------------------------------
    REPLAY_ANA_EXPID:   MERRA-2
    REPLAY_ANA_LOCATION: /discover/nobackup/projects/gmao/share/gmao_ops/verification/MERRA-2
    REPLAY_MODE: Regular
    REPLAY_FILE: ana/Y%y4/M%m2/MERRA-2.ana.eta.%y4%m2%d2_%h2z.nc4
# -----------------------------------------------------------------

This will "replay" to the MERRA-2 analysis by performing an IAU type procedure. The model will run a predictor loop to produce the background, use the MERRA-2 analysis + background to create an increment, and finally run the corrector loop where the increment is applied. This is repeated for the segment duration

Running zero-increment replay

In addition to the lines uncommented to perform replay, you need to need to set these lines in the AGCM.rc:

   REPLAY_P:  NO   
   REPLAY_U:  NO    
   REPLAY_V:  NO   
   REPLAY_T:  NO   
   REPLAY_QV: NO    
   REPLAY_O3: NO    
   REPLAY_TS: NO    

This will perform the replay but the increments constructed in the replay procedure will be zero since you have told it not to replay to any of the possible variables. In principle the results of the run should be identical to the result you obtained from running without replay since you are adding "0" increments during the corrector loop.