Brief User's Guide to restarts with cm1.

Last updated: 20 April 2022 (see new section "Changing resolution on a restart" below)

Background:

Restart files allow the model simulation to "start again" (i.e., restart) from a previous point during the model simulation. The model automatically saves all information that is carried from one timestep to the next in the cm1rst* files.

Please note that major changes on a restart ... like changing the microphysics or PBL parameterization ... may not work, and could cause very bad things to happen. However, minor changes in model configuration, like the advection scheme or time step, could probably be changed without any problems. Also, in principle, users could add parcels and passive fluid tracers on a restart.

Bottom line: if you are relying on restart files to do something other than simply continuing a simulation from before ... as it was configured before ... then you may have to "get your hands dirty" and modify code in the restart_read.F and/or restart_write.F files.

To save restart files:

Simply set a value for "rstfrq" in "namelist.input". The model will generate a series of restart files, named (for example) cm1rst_000001_*.dat. The set of numbers (000001 in the case) is the restart file number: it starts at 1, and counts upward; use this number to start a subsequent restart simulation (see next section).

To restart the model:

In "namelist.input", set irst = 1, and then specify the restart file number (rstnum). For example, if a user wanted to use the file set cm1rst_000004_*.dat, then specify rstnum = 4 in namelist.input.

WARNING: restarts from CM1 may overwrite files from the previous simulation. For example, if you first ran CM1 for 2 days, and then restarted from restart files written after 1 day, then all files from 1-2 days from the previous simulation will be overwritten. To avoid this potential problem, it is usually best to move all output files from a previous simulation to a separate subdirectory to make sure they are not lost.

Changing resolution on a restart:

Starting with cm1r21.0, users can change the resolution on a restart. This process is started automatically when the code recognizes that the current grid is different from the grid saved in the restart file.

Note: the new domain must be equal in size, or smaller than, the domain from the previous simulation. In other words, you cannot make the domain larger in any direction on a restart.

Note: this option only works for restart_format=1 (binary-format output files) at the moment.

Note: this option is new and relatively untested, so please send comments and questions to the email address below if you encounter problems.

Questions, comments, suggestions: send email to gbryan@ucar.edu