Initialization from a saved model state

“Initialization” has the specific, simple meaning in PISM that option “-i” was used. If a previous PISM run has saved a NetCDF file using “-o” then that file will contain complete initial conditions for continuing the run. The output file from the last run can be loaded with “-i”:

pisms -eisII A -y 100 -o foo.nc
pisms -eisII A -i foo.nc -y 100 -o bar.nc

As noted verification tests (section Verification) and simplified-geometry experiments (section Simplified geometry experiments) do not need input files at all because they initialize from formulas in the source code. They can, however, be continued from saved model states using -i. Specifying the simplified geometry experiment or verification test is, however, necessary if the run is to continue with the climate inputs for that experiment or test. For example, based on the above pisms runs, it is valid to do

pismr -i foo.nc -y 100 -o bar.nc

but the climate and other parameters use PISM default values, and thus are not (necessarily) the values specified in EISMINT II.

As a technical note about saved states, a PISM run with -stress_balance ssa also saves the last SSA velocities to the output file in variables u_ssa and v_ssa. The presence of these velocities adds efficiency in restarting because an initial estimate speeds up the solution of the SSA stress balance equations. If you want to use -i but also ignore these velocities then set stress_balance.ssa.read_initial_guess to false.

-i file format

PISM produces CF-1.5 compliant NetCDF files. The easiest way to learn the output format and the -i format is to do a simple run and then look at the metadata in the resulting file, like this:

pisms -eisII A -y 10 -o foo.nc
ncdump -h foo.nc | less

Note that variables in the output file have a pism_intent attribute. When pism_intent is diagnostic, the variable can be deleted from the file without affecting whether PISM can use it as a -i input file. Variables with pism_intent is model_state, by contrast, must be present when using -i.

The automatically-produced time variable has a units attribute like "seconds since 1-1-1" because the CF metadata conventions require a reference date.

By default PISM ignores this reference date except when it is used in unit conversions based on a calendar (see below).


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