How to control output times for CGRID files

Simulations are writing the CGRID files every 24 hours. For example, a simulation with a noon starting time will write a CGRID file at 12 UTC every day. I am wondering if there is any parameter controlling this output as I’d like CMAQ to write 2 CGRID files per day at 06 and 18 or if the solution to accomplish this goal would require the modification to any of the CMAQ subroutines.

CMAQ writes out the CGRID file at the end of each model execution. Our standard script executes the model for 24 hours at a time, beginning at 0 UTC and ending at 0 UTC, but the model can be started at any time and can be run for a much longer period if desired (theoretically multiple years in a single execution), and I have run it for as little as a single 5-minute time step when debugging.
To execute the model for a period that differs from 24 hours, change the value of the shell variable NSTEPS. If NSTEPS is set to 120000, then each model execution will be 12 hours.
It may be tricky to set up the script so that you can string together multiple 12-hour executions, but I cannot think of any reason in principle why it would not work.
Why do you want to run the model for 12 hours at a time?

Hello Chris,
I’m running several setups but the 2 at hand run for 10 days and 84 hours (it might sound odd but it is not an arbitrary choice). The former is OK even though the starting time is not 00 UTC as it will save the CGRID files every 24 hours as you mention. The latter is the one that I’m tinkering around. I’ve only tried with NSTEPS=240000 and the CGRID file was named CCTM_CGRID_v54_gcc_“simulation_name” (or similar) and the data are stored every 24 hours -ncdumping it shows the timestamp to be 2024055(120000) as the starting time was 12 PM-. If NSTEPS is changed to 120000, how would CMAQ annotate this? I’ll perform the test this afternoon, but it’s always better to know what to expect.

The CGRID file will have SDATE and STIME set according to the time the data are written. If you start at 2024055:120000 and run for 12 hours, then the resulting time will be 2024056:0.
The difficulty I was referring to is in getting your run script set up so that each execution finds the appropriate beginning time stamp and has an appropriate [distinct] name for the output S_CGRID file, and all the other output files. Alternatively you can append data to the output files, but you will need to modify the CLOBBER_DATA sections of the script to neither delete existing output files nor bomb if they exist.