The differences between using hybrid and branch to start the CESM2.1.5 BHSIT restart case

[xli585]

Hello! I am currently running a BHIST case using the cesm2.1.5 version, with the 2000-year restart file. The expectation is to run the simulation with fixed 2000-year emissions and external conditions until a stable equilibrium state is reached. My first startup settings are as follows:

./xmlchange RUN_REFCASE=b.e21.BHIST.f09_g17.CMIP6-historical.010
./xmlchange RUN_REFDATE=2000-01-01
./xmlchange RUN_STARTDATE=2000-01-01
RUN_TYPE=hybrid
Then I set: ./xmlchange STOP_OPTION=nyears
./xmlchange STOP_N=10.

After running for every ten years, I would stop to check, and then continue the run until reaching the equilibrium position. For the second run,
I set: ./xmlchange CONTINUE_RUN=TRUE.

I would like to know if, when running in such a intermittent manner, the output would be continuous or if it would always re-hybridize? Would there be any climate drift? And what would be the difference if using branch to spin up to the climate equilibrium state?

 

[manishvenu]

Hello @xli585,

Let me take a stab at these questions!

when running in such a intermittent manner, the output would be continuous or if it would always re-hybridize?

CONTINUE_RUN takes precendence over any other argument! So, you will get bit-for-bit output when you run this simulation further. After the first run, the RUN_TYPE=hybrid is not relevant and won't re-hybridize.

Would there be any climate drift?

Could you elaborate on what you mean by this?

And what would be the difference if using branch to spin up to the climate equilibrium state?

A branch takes the exact restart files from the previous case (so it is very strict), while hybrid allows you more flexibility, including changing the compset and ref date vs. start date. This link is a good documentation page in the CESM tutorial to learn more about the differences. It seems like you need some flexibility with your setup (and are not trying to reproduce a run), so hybrid likely makes more sense.

One other thing to note is that the "BHIST" might not be the best compset for your usecase of running to climate equilibrium. "HIST" has data from 1850 to 2014 (I'm not sure, but you may error out on the second 10-year run). Spin-up is often done with B1850, which repeats the year 1850 for the forcings. I would recommend searching B2000 in the CESM forums for some insight.

Please let me know if you have any additional questions about this.