ACCESS CM3 technical meeting minutes

Earth System Coupled Model
1

Date: 22 Oct 2025

Continuing CM3 O025 run.

To avoid repeating some of the initial adjustment and to save compute, it would be preferable to continue the run with updates rather than restarting (approx 2 MSU to repeat current length run). The Baltic fixes in OM3 look promising Baltic sea bathymetry hand edits · Issue #802 · ACCESS-NRI/access-om3-configs · GitHub. However the OM3 grid has been changed with an extra 10 land only rows in the south 25km: update ygrid from 1142 to 1152 - regenerate input files and update configurations · Issue #722 · ACCESS-NRI/access-om3-configs · GitHub so the new bathymetry can’t be used directly in a continued run. @kieranricardo suggests workaround is to cut off the extra rows. Eventually we’ll either want to do a fresh restart or will fix the CM3 restarts to use the new grid. @cbull noted that there were still some OM3 bathymetry updates coming but they’re less urgent for CM3.

Plan to continue run with Baltic fixes and UM GC5 central tuning (from @jemmajeffree). @sofarrell suggests checking these changes in an AMIP run too.

@sofarrell is preparing notes on penetrating SW radiation.

Evaluation

Using the multi variable atmosphere files is very slow. @MartinDix to look at switching to single variable files in the netCDF conversion and to creating a catalog of the CM2 0.25 runs (which used OM3 diag table).

Discussion on how to best handle OM3 scripts modified for CM3. Currently on a branch in OM3 repo (some variable names had to be changed) @dougiesquire keen to avoid divergence and propagating errors. Papermill allows switching datastores. CM3 should use OM3 diag table and file naming so that output is compatible.

Evaluation figures list Evaluation metrics for CM3 Mega-Issue · Issue #1 · ACCESS-Community-Hub/access-cm3-paper-1 · GitHub

New config docs repo GitHub - ACCESS-NRI/access-cm3-config-docs

2

Date: 5 Nov 2025

Continuing CM3 O025 run

Run with updated Baltic bathymetry (truncated OM3 file to avoid having to update model grid) and GC5 central atmospheric parameters restarted. Run shows a ~ 1.5 W/m^2 worsening of the net TOA radiation balance GC5 central configuration · Issue #53 · ACCESS-NRI/dev_coupling · GitHub. Possibly due to missing some GC5 central parameters that are not available in the standard vn13.0 code (Met Office used tests that made some internal parameters namelist variables).

TODO: Check whether this run did improve Baltic salinity drift and which components of the radiation balance changed.

Code version updates

ACCESS rAM3 uses UM vn13.5 and there’s now an update of both AM3 and CM3 to the same version. Runs ok but not fully tested yet. With this version we should be able to use the full set of GC5 central tunings.

TODO: @MartinDix to test GC5 parameters in AM3 AMIP experiments

Datastore:

@CharlesTurner’s work on zarr virtualisation has improved speed of reading atmospheric data enormously, though sea ice still seems slow. @anton suggests checking whether CM3 writes grid information every time (OM3 doesn’t). Virtualisation requires a higher level of file consistency and is not yet automated.

TODO: Look at creating catalog from model rose/cylc suite at end of each year. OM3 does this with a post-script triggered by payu om3-scripts/payu_config/postscript.sh at main · ACCESS-NRI/om3-scripts · GitHub. Would also require copying output to /g/data.

CM2 datastore

Question of which CM2 runs to use. Ideally use @wghuneke’s run but need to check where it started. 100 km ocean runs also worth adding (PI control will aready be in ESGF catalog but could add PD control).

@cbull to look at updating the evaluation template to include CM2 runs.

@MartinDix to create a list of key experiments like this OM3 list Experiments - ACCESS-OM3-configs