================================== The Community Earth System Model

See the CESM web site for documentation and information:

http://www.cesm.ucar.edu

The CESM Quickstart Guide is available at:

http://escomp.github.io/cesm

This repository provides tools for managing the external components that make up a CESM tag - alpha, beta and release. CESM tag creation should be coordinated through CSEG at NCAR.

This repository is also connected to slack at http://cesm2.slack.com

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Software requirements

Software requirements for installing, building and running CESM

Installing, building and running CESM requires:

• a Unix-like operating system (Linux, AIX, OS X, etc.)

• git client version 1.8 or newer

• python3 version 3.8 or newer

• perl version 5

• build tools gmake and cmake

• Fortran and C compilers

  • See Details on Fortran compiler versions_ below for more information

• LAPACK and BLAS libraries

• a NetCDF library version 4.3 or newer built with the same compiler you will use for CESM

  • a PnetCDF library is optional, but recommended

• a functioning MPI environment (unless you plan to run on a single core with the CIME mpi-serial library)

Details on Fortran compiler versions

The Fortran compiler must support Fortran 2003 features. However, even among mainstream Fortran compilers that claim to support Fortran 2003, we have found numerous bugs. Thus, many compiler versions do not build or run CESM properly (see https://wiki.ucar.edu/display/ccsm/Fortran+Compiler+Bug+List for more details on older Fortran compiler versions).

CESM2 is tested on several different systems with newer Fortran compilers: Please see CESM Compiler/Machine Tests <https://docs.google.com/spreadsheets/d/15QUqsXD1Z0K_rYNTlykBvjTRt8s0XcQw0cfAj9DZbj0/edit#gid=0>_ for a spreadsheet of the current results.

More details on porting CESM

For more details on porting CESM to a new machine, see https://esmci.github.io/cime/versions/master/html/ccs/model-configuration/support-a-new-machine.html

Obtaining the full model code and associated scripting infrastructure

CESM is now released via github. You will need some familiarity with git in order to modify the code and commit these changes. However, to simply checkout and run the code, no git knowledge is required other than what is documented in the following steps.

To obtain the CESM code you need to do the following:

#. Clone the repository. ::

  git clone https://github.com/escomp/cesm.git my_cesm_sandbox

This will create a directory my_cesm_sandbox/ in your current working directory.

#. Go into the newly created CESM repository and determine what version of CESM you want. To see what cesm tags are available, simply issue the git tag command. ::

  cd my_cesm_sandbox
  git tag

#. Do a git checkout of the tag you want. If you want to checkout cesm3_0_beta02, you would issue the following. ::

  git checkout cesm3_0_beta02

(It is normal and expected to get a message about being in 'detached HEAD' state. For now you can ignore this, but it becomes important if you want to make changes to your Externals.cfg file and commit those changes to a branch.)

#. Run the script bin/git-fleximod update. ::

  ./bin/git-fleximod update

The git fleximod script is a git extension that will populate the cesm directory with the relevant versions of each of the components along with the CIME infrastructure code.

At this point you have a working version of CESM.

To see full details of how to set up a case, compile and run, see the CIME documentation at http://esmci.github.io/cime/ .

More details on git fleximod

The file .gitmodules in your top-level CESM directory tells git fleximod which tag/branch of each component should be brought in to generate your sandbox.

NOTE: git fleximod will always attempt to make the working copy exactly match the externals description. For example, if you manually modify an external without updating .gitmodules, (e.g. switch to a different tag), then rerunning git fleximod will warn you and can restore the original version by using the --force option below documentation Customizing your CESM sandbox_ for more details.

You need to rerun git-fleximod update whenever .gitmodules has changed (unless you have already manually updated the relevant external(s) to have the correct branch/tag checked out). Common times when this is needed are:

• After checking out a new CESM branch/tag

• After merging some other CESM branch/tag into your currently checked-out branch

To see more details of git-fleximod, issue ::

./bin/git-fleximod --help

Customizing your CESM sandbox

There are several use cases to consider when you want to customize or modify your CESM sandbox.

Switching to a different CESM tag

If you have already checked out a tag and HAVE NOT MADE ANY MODIFICATIONS it is simple to change your sandbox. Say that you checked out cesm3_0_beta01 but really wanted to have cesm3_0_beta02 you would simply do the following::

git checkout cesm3_0_beta02 ./bin/git-fleximod update

You should not use this method if you have any ongoing CESM cases that were created from this sandbox. In these cases, it is often easiest to do a second git clone.

Pointing to a different version of a component

Each entry in .gitmodules has the following form (we use CAM as an example below)::

[submodule "cam"] path = components/cam url = https://www.github.com/ESCOMP/CAM fxDONOTUSEurl = https://www.github.com/ESCOMP/CAM fxtag = cam6_4_016 fxrequired = ToplevelRequired

Each entry specifies either a tag or a hash. To point to a new tag:

#. Modify the relevant fxtag entry/entries in .gitmodules (e.g., changing cam6_4_016 to cam6_4_017 above)

#. Checkout the new component(s)::

 ./bin/git-fleximod update cam

Keep in mind that changing individual components from a tag may result in an invalid model (won't compile, won't run, not scientifically meaningful) and is unsupported.

Committing your change to .gitmodules

After making this change, it's a good idea to commit the change in your
local CESM git repository. First create a CESM branch in your local
repository, then commit it.  For example::

  git checkout -b my_cesm_branch
  git add .gitmodules components/cam
  git commit -m "Update CAM to cam5_4_144"

Modifying a component
---------------------

If you'd like to modify a component via a branch and point to that
branch in your CESM sandbox, use the following procedure (again, using
CAM as an example):

#. Create a CAM branch. Let's assume you have created this branch and called it
   **my_branch**.

#. Update **.gitmodules** to point to a hash on your branch. You can replace the
   **tag** entry with a **hash** entry, as follows, note that we have also changed the url to
   point to a personal fork::

    [submodule "cam"]
      path = components/cam
      url = https://www.github.com/mycamfork/CAM
      fxDONOTUSEurl = https://www.github.com/ESCOMP/CAM
      fxtag = 94eaf83
      fxrequired = ToplevelRequired


#. Checkout your branch::

     ./bin/git-fleximod update cam

It's a good idea to commit your **.gitmodules** file changes. See the above
documentation, `Committing your change to .gitmodules`_.

Developer setup
===============

Developers who have not already done so should follow the recommended
`one-time <https://github.com/esmci/cime/wiki/CIME-Git-Workflow#configure-git-one-time>`_
setup directions for git. Developers may also want to set up
`ssh <https://help.github.com/articles/connecting-to-github-with-ssh/>`_
keys and switch to using the ``git@github.com:ESCOMP/cesm.git`` form of the github URLs.