PISM’s configuration parameters and how to change them

PISM’s behavior depends on values of many flags and physical parameters (see Configuration parameters for details). Most of parameters have default values 1 which are read from the configuration file pism_config.nc in the lib sub-directory.

It is possible to run PISM with an alternate configuration file using the -config command-line option:

pismr -i foo.nc -y 1000 -config my_config.nc

The file my_config.nc has to contain all of the flags and parameters present in pism_config.nc.

The list of parameters is too long to include here; please see the Configuration parameters for an automatically-generated table describing them.

Some command-line options set configuration parameters; some PISM executables have special parameter defaults. To examine what parameters were used in a particular run, look at the attributes of the pism_config variable in a PISM output file.

Managing parameter studies

Keeping all PISM output files in a parameter study straight can be a challenge. If the parameters of interest were controlled using command-line options then one can use ncdump -h and look at the history global attribute.

Alternatively, one can change parameter values by using an “overriding” configuration file. The -config_override command-line option provides this alternative. A file used with this option can have a subset of the configuration flags and parameters present in pism_config.nc. Moreover, PISM adds the pism_config variable with values used in a run to the output file, making it easy to see which parameters were used.

Here’s an example. Suppose we want to compare the dynamics of an ice-sheet on Earth to the same ice-sheet on Mars, where the only physical change was to the value of the acceleration due to gravity. Running

pismr -i input.nc -y 1e5 -o earth.nc <other PISM options>

produces the “Earth” result, since PISM’s defaults correspond to this planet. Next, we create mars.cdl containing the following:

netcdf mars {
    byte pism_overrides;
    pism_overrides:constants.standard_gravity = 3.728;
    pism_overrides:constants.standard_gravity_doc = "m s-2; standard gravity on Mars";

Notice that the variable name is pism_overrides and not pism_config above. Now

ncgen -o mars_config.nc mars.cdl
pismr -i input.nc -y 1e5 -config_override mars_config.nc -o mars.nc <other PISM options>

will create mars.nc, the result of the “Mars” run. Then we can use ncdump to see what was different about mars.nc:

ncdump -h earth.nc | grep pism_config: > earth_config.txt
ncdump -h mars.nc | grep pism_config: > mars_config.txt
diff -U 1 earth_config.txt mars_config.txt
--- earth_config.txt 2015-05-08 12:44:43.000000000 -0800
+++ mars_config.txt  2015-05-08 12:44:51.000000000 -0800
@@ -734,3 +734,3 @@
                pism_config:ssafd_relative_convergence_units = "1" ;
-               pism_config:constants.standard_gravity_doc = "acceleration due to gravity on Earth geoid" ;
+               pism_config:constants.standard_gravity_doc = "m s-2; standard gravity on Mars" ;
                pism_config:constants.standard_gravity_type = "number" ;
@@ -1057,3 +1057,3 @@
                pism_config:ssafd_relative_convergence = 0.0001 ;
-               pism_config:constants.standard_gravity = 9.81 ;
+               pism_config:constants.standard_gravity = 3.728 ;
                pism_config:start_year = 0. ;

Saving PISM’s configuration for post-processing

In addition to saving pism_config in the output file, PISM automatically adds this variable to all files it writes (snap shots, time series of scalar and spatially-varying diagnostic quantities, and backups). This may be useful for post-processing and analysis of parameter studies as the user has easy access to all configuration options, model choices, etc., without the need to keep run scripts around.



For pismr, grid parameters Mx, My, that must be set at bootstrapping, are exceptions.

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