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ACME and CESM were designed to provide bit-for-bit identical answers regardless of PE layout (though this doesn't happen in practice unless certain debug options are used - see comment 4 by Patrick Worley (Unlicensed) below). This means that the information used by e.g. the ocean model needs to be available at the time it runs regardless of whether the ocean is running in in parallel with the other components or whether it is run serially after all the other components are finished. This necessitates passing one component information which is lagged relative to the current state of the other processes. It also imposes some limitations on which PE layouts are acceptable:
- ATM must run serial with LND, ROF, and ICE
- everything can (
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- kind of) run concurrent with OCN
- LND and ICE can run concurrently
- ROF must run serial with land
- coupler is complicated -
The PE layout for a run is encapsulated in the env_mach_PES.xml file which is created by the create_newcase command during the model build step. This page focuses on changing this .xml file for a particular run; changing the default behavior for all runs on a particular machine requires muddling around with $ACME_code/cime/machines-acme/ stuff which is outside the scope of this page.
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