# 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 {
variables:
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 = "scalar" ;
@@ -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.

Footnotes

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

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