On-the-fly FATES parameter file generation

[glemieux]

I'm starting up this discussion thread to capture some discussions happening across different repositories and issues about how to enable on-the-fly parameter file generation for FATES. @ekluzek has started up a design document which will attempt to detail the solution to going about this (note the file access is currently restricted). The motivation for this effort is presented below.

Motivation

The motivation for enabling on-the-fly parameter file generation partially comes out of efforts to reduce the number of API update to the host land model by removing the need to update the default parameter file. The current process requires the following steps:

1. FATES default parameter file (fates_parameter_default.cdl) is updated on the FATES repo through a PR
2. A new netcdf default parameter file is generated
3. The netcdf file is copied to a machine and imported to the cesm svn repository
4. The netcdf file is manually copied to e3sm ANL servers
5. Pull requests are created for each host land model to update the namelist default file with the new default netcdf file name

This main downside of this process is that the cadence at which the default parameter file is updated during model development is affected by the host land model pull request queue process. As such, there is an incentive to package together netcdf parameter file updates an wait for an opportune time in the host land model queue to update the default (typically with a different API update such as a refactor). This makes parameter file update coordination more complicated as the timing of netcdf updates are out of step in time with the actual FATES pull request parameter file update. Allowing the host land model to generate the FATES parameter on-the-fly based on the cdl file associated with a specific fates tag would alleviate this complexity.

It should be noted that this would not remove the need to update the API when there is a breaking change to the parameter file that affects the hlm-fates interface in some manner.

Relevant Discussions

• Have the FATES parameter file created at runtime in the buildnml (ctsm#2126)
• Improve the API to not require as many updates in HLM's (#1073)