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ACCESS UKCA Coupled Model

Scott Wales, ARCCSS CMS


ACCESS UKCA Coupled Model (ACCESS-CCM) is a global earth-system model aimed at representing interactions between atmospheric chemistry and the oceans. It is based on a pre-release version of ACCESS-CM 2, with the atmospheric component replaced by a UM 8.4 model configured for GA4 with UKCA enabled.

Model Components

The model components are:

The UM8.4 source code has been modified to use the ACCESS coupling, the other models are unchanged from the ACCESS-CM2 versions

Model Configurations

The atmosphere and land surface configuration is based on the Accessdev UMUI job ‘vabqa: GA4.0 UKCA StratTrop+MODE’, which corresponds to the Monsoon UMUI job xhmaj [1].

The ocean and sea ice configuration comes from the ACCESS-CM2 development model


Using the Model

Run Scripts

The coupled model run scripts are held in version control, at the URL If you want to modify the model you should create your own branch of this and commit back regularly, if you just want to run the model check out the scripts with:

svn co access-cm2

There are two directories of interest here - ‘submodels’, where the model executables are built, and ‘exp’, which contains experiment configurations. The runscripts themselves are at exp/n96-ukca}}, the file {{initial can be changed to set the following information:

Run Dates

The model uses a 360 day calendar, as was used in the original UKCA configuration. To set the start and end dates of the model, change the following variables in the file ‘initial’

  • Start date: iniyear, inimonth, iniday
  • End date: finalyear, finalmonth, finalday


To change how long each restart segment runs for change the following variables in the file ‘initial’. Ensure UM restart dumps are output at least this often

  • Restarts: nyear, nmonth, nday

Initial Conditions

To change the initial conditions file change the variable ‘um_astart’ in the file ‘initial’. The initial conditions file should be reconfigured according to the instructions in the ‘Atmosphere Reconfiguration’ section

  • UM Initial Conditions: um_astart
  • MOM & CICE Initial conditions: inputdir2b (~access/access-cm2/input_b)


To build the model run the ‘’ script in the top-level directory. This needs to be run on Raijin’s login node so it can download the model source codes, but it will then submit the actual build jobs to the express queue.

Atmosphere Namelists

The atmospheric settings can be changed by editing the namelist values, but it is recommended that they be set via the UMUI so that values are consistent.

On Accessdev, make a copy of the UMUI job valro, which is vabqa with coupling enabled. Make any modifications, then process the job. This will create a set of namelists in the directory ~/umui_jobs/RUNID, where RUNID is the UMUI’s job identifier. Copy the contents of this directory to your experiment’s ‘umui’ directory on Raijin, eg.

rsync -rv accessdev:~/umui_jobs/vabcd/ ~/access-cm2/exp/n96-ukca/umui

Note that if you change the model’s start date in the UMUI you’ll also need to change the run scripts to reflect this, see ‘Job Submission’

Atmosphere Reconfiguration

The reconfiguration process ensures that fields required for atmospheric chemistry and ocean coupling are present in the initial conditions file. To perform a reconfiguration process & submit the UM job being used for the coupled run in the UMUI.

Reconfiguring produces a file on Raijin named /short/$PROJECT/$USER/UM_ROUTDIR/$USER/$RUNID/$RUNID.astart, where $RUNID is the UMUI job identifier. The path to this file should be set in the coupled model initialisation script exp/n96-ukca/initial as the variable um_astart so that it gets picked up by the model run.

Job Submission

To submit the job as-is go to the experiment directory (e.g. access-cm2/exp/n96-ukca) and run

qsub access-cm2

Model Output

Model output may be found in the directory /short/$PROJECT/$USER/archive

Coupling Changes

The ACCESS-CCM coupling sequencing is slightly different to ACCESS-CM, primarily to minimise code changes. The atmosphere reads ice and ocean variables from the previous timestep, while the ice and ocean models read atmospheric variables from the current timestep. In the original ACCESS-CM both the atmosphere and ice fields were read from the previous timestep.

Resource Requirements

Resources for different decompositions are as listed:

Atmosphere CPUs Ice CPUs Ocean CPUs Walltime per Model Month SU per Model Month Memory
224 16 16 ~ 2 hr ~ 500 < 30 GB

Load balance: (0-223 ATM, 224-239 ICE, 240-255 OCN) Load hpm rank.png

Atmosphere CPUs Ice CPUs Ocean CPUs Walltime per Model Month SU per Model Month Memory
288 16 16 ~ 1.5 hr ~ 500 < 30 GB

Load balance: (0-287 ATM, 288-303 ICE, 304-319 OCN) Load hpm rank 320.png

Atmosphere CPUs Ice CPUs Ocean CPUs Walltime per Model Month SU per Model Month Memory
352 16 16 ~ 1.3 hr ~ 650 < 30 GB

Load balance: (0-351 ATM, 352-367 ICE, 368-383 OCN) Load hpm rank 384.png