Snapshots of the model state¶

Sometimes you want to check the model state every $1000$ years, for example. One possible solution is to run PISM for a thousand years, have it save all the fields at the end of the run, then restart and run for another thousand, and etc. This forces the adaptive time-stepping mechanism to stop exactly at multiples of $1000$ years, which may be desirable in some cases.

If saving exactly at specified times is not critical, then use the -save_file and -save_times options. For example,

pism -i foo.nc -y 10000 -o output.nc -save_file snapshots.nc \
     -save_times 1000:1000:10000

starts a PISM evolution run, initializing from foo.nc, running for 10000 years and saving snapshots to snapshots.nc at the first time-step after each of the years $1000$ , $2000$ , …, $10000$ .

We use a MATLAB-style range specification, $a : Δ t : b$ , where $a, Δ t, b$ are in years. The time-stepping scheme is not affected, but as a consequence we do not guarantee producing the exact number of snapshots requested if the requested save times have spacing comparable to the model time-steps. This is not a problem in the typical case in which snapshot spacing is much greater than the length of a typical time step.

It is also possible to save snapshots at intervals that are not equally-spaced by giving the -save_times option a comma-separated list. For example,

pism -i foo.nc -y 10000 -o output.nc \
     -save_file snapshots.nc \
     -save_times 1000,1500,2000,5000

will save snapshots on the first time-step after years $1000$ , $1500$ , $2000$ and $5000$ . The comma-separated list given to the -save_times option can be at most $200$ numbers long.

If snapshots.nc was created by the command above, running

pism -i snapshots.nc -y 1000 -o output_2.nc

will initialize using the last record in the file, at about $5000$ years. By contrast, to restart from $1500$ years (for example) it is necessary to extract the corresponding record using ncks

ncks -d t,1500years snapshots.nc foo.nc

and then restart from foo.nc. Note that -d t,N means “extract the $N$ -th record” (counting from zero). So, this command is equivalent to

ncks -d t,1 snapshots.nc foo.nc

Also note that the second snapshot will probably be around $1500$ years and ncks handles this correctly: it takes the record closest to $1500$ years.

By default re-startable snapshots contain only the variables needed for restarting PISM. Use the command-line option -save_size to change what is saved.

Another possible use of snapshots is for restarting runs on a batch system which kills jobs which go over their allotted time. Running PISM with options -y 1500 -save_times 1000:100:1400 would mean that if the job is killed before completing the whole 1500 year run, we can restart from near the last multiple of $100$ years. Restarting with option -ye would finish the run on the desired year.

When running PISM on such a batch system it can also be useful to save re-startable snapshots at equal wall-clock time (as opposed to model time) intervals by adding the “-backup_interval (hours)” option.

Caution

If the wall-clock limit is equal to $N$ times backup interval for a whole number $N$ PISM will likely get killed while writing the last backup.

It is also possible to save snapshots to separate files using the -save_split option. For example, the run above can be changed to

pism -i foo.nc -y 10000 -o output.nc -save_file snapshots \
     -save_times 1000,1500,2000,5000 -save_split

for this purpose. This will produce files called snapshots-year.nc. This option is generally faster if many snapshots are needed, apparently because of the time necessary to reopen a large file at each snapshot when -save_split is not used. Note that tools like NCO and ncview usually behave as desired with wildcards like “snapshots-*.nc”.

Parameters¶

file Snapshot (output) file name (or prefix, if saving to individual files).
size (small) The “size” of a snapshot file. See parameters output.sizes.medium, output.sizes.big_2d, output.sizes.big
split (no) Save model state snapshots to separate files (one per time record).
times List or a range of times to save model state snapshots at.