changes to building a case, be advised: ctsm's nuopc and cmake

[rgknox]

Both CTSM and E3SM have recently been going through some changes as to how the models are built and how cases are created.

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CMAKE:

Both models, since they both use CIME, now make use of "cmake" to build the model. More on cmake can be found here: https://cmake.org/
Here is a thread where they have been tracking issues with E3SM: E3SM-Project/E3SM#3919

If you use a supported machine, it is likely that you are not affected by these changes. But if you maintain your own machine, or your machine has a configuration maintained by CIME but is not tested regularly, you should know the following:

Previously, there were two really important config files that did most of the configuring for your maching: config_machines.xml and config_compilers.xml. Now, each machine and/or machine compiler also needs its own cmake configuration file. Here are the folders with the configuration files, which you can follow as a template for your machine:

For E3SM: https://github.com/E3SM-Project/E3SM/tree/master/cime_config/machines/cmake_macros
For CTSM: https://github.com/ESMCI/ccs_config_cesm/tree/main/machines/cmake_macros

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NUOPC (which deprecates MCT):

CTSM has a readme here: https://github.com/ESCOMP/CTSM/blob/master/README.NUOPC_driver.md

There was also a discussion that was recorded, here:
https://drive.google.com/file/d/1NtkxyvuOH-dX1SAOU4p-DfMMMKWKuQEG/view

If you just want your old/current scripts to work, you can add the following flag to your execution of "./create_case" :

./create_case --driver=mct

[glemieux]

Thanks @rgknox!

For quick reference, the update to make NUOPC default came in with CTSM tag ctsm5.1.dev062. The upcoming FATES API 18 update will utilize the NUOPC update. As a reminder, please see this wiki page for determine which fates API is compatible with which host land model tag: https://github.com/NGEET/fates/wiki/Table-of-FATES-API-and-HLM-STATUS. Or see the FATES User's Guide reference section: https://fates-users-guide.readthedocs.io/en/latest/user/Table-of-FATES-API-and-HLM-STATUS.html

[glemieux]

Update: As of ctsm5.1.dev075, the cime external was updated to cime6.0.13 which itself moved all the cime config files out to a new external called ccs_config_cesm which contains the cmake_macros directory. The link in the original issue above has been updated to https://github.com/ESMCI/ccs_config_cesm/tree/main/machines/cmake_macros.

[glemieux]

New update: specifying a set of nthreads for E3SM cases is necessary for unsupported machines now as this pr stripped out setting these by default: E3SM-Project/E3SM#4928