Restarts

ESM1.6 restart files contain copies of complete model states, allowing for experiments to be stopped and restarted at a later time. This page outlines their structure and details basic procedures for manipulating them.

Structure of the ESM1.6 restart directory¶

An ESM1.6 restart directory contains separate restart files for each component. These are organised into the following directories:

atmosphere  coupler  ice  ocean  README

An example restart directory is available in /g/data/vk83/prerelease/configurations/inputs/access-esm1p6/modern/pre-industrial/restart/2025.11.2.

Atmosphere¶

The atmosphere restart directory contains the following files

restart_dump.astart  um.res.yaml

restart_dump.astart is the main UM restart file, containing the atmospheric and land state in addition to static boundary information such as the land-sea mask, vegetation maps and orography.

um.res.yaml is a separate calendar file which holds the date and time associated with the restart. Information in this file is copied by Payu into the model namelist files at runtime.

Ocean¶

The ocean restart files are organised into separate groups of variables:

ocean_age.res.nc  ocean_density.res.nc  ...  ocean_solo.res

These netCDF files contain snapshots of the model state while ocean_solo.res is a text file specifying the model date and time.

Ice¶

The ice restart directory contains the following files:

cice_in.nml iced.2145-01-01-00000.nc  ice.restart_file  mice.nc

The iced.YYYY-MM-DD-00000.nc holds the model state and also carries the date and time in the global attributes. ice.restart_file is a pointer file used by CICE when finding the correct restart file to read. mice.nc contains ice coupling data to be sent to the ocean model at the beginning of the next run.

Info

The cice_in.nml file in the restart directory is not used by the model. This file will be removed from the restart directory when Payu version 1.2.1 is released.

Coupler¶

The coupler restart directory contains data sent from each component at the termination of the previous run, allowing for the submodels to restart with the same boundary conditions at the beginning of the next run.

a2i.nc  i2a.nc  o2i.nc

Common restart manipulations¶

Changing the date of restart files¶

It's commonly required to change the date for a restart file. For example when setting up a historical experiment, a restart might be taken from a pre-industrial simulation and the date changed to 1850. Another example is when combining ocean and atmosphere restarts from two different experiments into a single restart, where it is necessary to make sure the dates for the different components match.

The following instructions outline how to modify the date for each component. It's important to apply the updates to all components consistently, as inconsistencies in the model dates can cause crashes.

Info

Before starting any manipulation, copy the restart folders you want to modify to a new location of your choice.

Atmosphere:¶

Navigate to the restart/atmosphere directory:

module use /g/data/xp65/public/modules module load conda/analysis3 # Update the date in the UM dump file python ~access/apps/pythonlib/umfile_utils/access_cm2/change_dump_date.py restart_dump.astart <<< "YYYY MM DD", # Update the date in the calendar file sed -i "s/end_date:.*/end_date: YYYY-MM-DD 00:00:00/" /atmosphere/um.res.yaml

Ocean:¶

In the restart/ocean directory, edit ocean_solo.res:

     3        (Calendar: no_calendar=0, thirty_day_months=1, julian=2, gregorian=3, noleap=4)
     1     1     1     0     0     0        Model start time:   year, month, day, hour, minute, second
  2145     1     1     0     0     0        Current model time: year, month, day, hour, minute, second

Replace the date in the 3rd line, taking care to preserve the column alignment. The entries should not be zero-padded.

Ice:¶

In the restart/ice directory:

Rename the iced.YYYY-MM-DD-00000.nc with new date. The year, month, and day should be zero padded.

Edit the ice.restart pointer file to use the new file name: echo ./RESTART/iced.YYYY-MM-DD-00000.nc > ice.restart_file

Replace the year, nyr, month, and mday global attributes using nco (in most cases, only the year will need to be changed): module load nco ncatted -O -a year,global,o,l,YYYY iced.YYYY-MM-DD-00000.nc ncatted -O -a nyr,global,o,l,YYYY iced.YYYY-MM-DD-00000.nc ncatted -O -a month,global,o,l,MM iced.YYYY-MM-DD-00000.nc ncatted -O -a mday,global,o,l,DD iced.YYYY-MM-DD-00000.nc

Replace the time global attribute to equal the total number of seconds between 1/1/1 and the new date using the proleptic Gregorian calendar. This can be calculated using the cftime python library. E.g. using 2105-03-01 as the new date: python >>> import cftime >>> start = cftime.datetime(1,1,1, calendar="proleptic_gregorian") >>> end = cftime.datetime(2105,3,1, calendar="proleptic_gregorian") >>> (end-start).total_seconds() 66400905600.0

Then add this to the restart file using nco: ncatted -O -a time,global,o,d,66400905600 iced.YYYY-MM-DD-00000.nc

Update the configuration¶

Update the payu configuration file, config.yaml, with the path to the new restart files:

restart: <path_to_new_restarts>