Saving LND->GLC coupler history files in future NorESM runs as a default option

[mpetrini-norce]

Many relevant future NorESM2.5/NorESM3 experiments (e.g. CMIP7) will be done keeping the ice sheet component (GLC) in data mode (DGLC-NOEVOLVE) or in stub mode (SGLC). Since saving annual LND->GLC coupler history files does not depend neither on the GLC mode nor on the GLC domain or resolution (see Section 3.3.1 in https://escomp.github.io/cism-docs/cism-in-cesm/versions/release-cesm2.0/html/t-compsets.html), I would like to propose that saving GLC coupler history files becomes a default option in future NorESM versions. I think there are three main reasons supporting this:

1. Annual LND->GLC coupler history files are relatively small in size (one year worth of data is 15 MB; even in the case of a 400 years-long simulation, annual coupler history files printed out every year would not exceed 6 GB);

2. Saving coupler history files requires to (a) properly set up the GLACIER_REGION variable in the surface dataset, which should specify the domain(s) you wanted to calculate the SMB over (typically Greenland and Antarctica; this is already implemented in the NorESM2 surface dataset), (b) set in the user_nl_clm namelist glacier_region_behavior = virtual and glacier_region_melt_behavior = replaced_by_ice in the regions you want to calculate the SMB over (GLACIER_REGION takes integer values from 0 onwards. For example, if the Greenland region has a value of 1, then the second entry for the above-mentioned namelist variables should be 'virtual' and 'replaced_by_ice', and (c) set in the user_nl_cpl namelist histaux_l2x1yrg = .true. All these changes are highly case-independent, and once there it is unlikely that are going to be modified (even in the case of GLC_EVOLVE).

3. Coupler history files would allow to force CISM in T-compset mode (see https://escomp.github.io/cism-docs/cism-in-cesm/versions/release-cesm2.0/html/t-compsets.html), which is equivalent to a one-way coupled NorESM-CISM run (i.e., where ice sheet changes are not feeding back on the climate). This would open, for any future NorESM2.5/NorESM3 experiment, to assess the response of the Greenland and Antarctic ice sheets.

@mvertens @hgoelzer