How do I…?¶

Contents

How do I…?

Create and use configuration flags and parameters ¶

Edit src/pism_config.cdl. Each flag or parameter is stored as a NetCDF attribute and should have a corresponding “_doc” attribute describing its meaning and the “_type” defining the type. All scalar parameters have to have “_units” set as well.

pism_config:constants.standard_gravity = 9.81;
pism_config:constants.standard_gravity_doc = "acceleration due to gravity on Earth geoid";
pism_config:constants.standard_gravity_type = "scalar";
pism_config:constants.standard_gravity_units = "meter second-2";

One can access these parameters using the Config class. IceModel and all classes derived from Component have an pointer to an instance of this class as a data member m_config, so no additional code is necessary to initialize the configuration database.

To use a scalar parameter, do

double g = m_config->get_number("constants.standard_gravity");

To use a flag, do

bool compute_age = config->get_flag("age.enabled");

Note

It is best to avoid calling m_config->get_...() from within loops: looking up a parameter by its name is slow.
Please see Configuration parameters for a list of flags and parameters currently used in PISM.

Create and use additional variables ¶

Creating IceModelVec instances ¶

PISM uses the following classes to manage 2D and 3D fields, their I/O and metadata:

`IceModelVec2S`	scalar 2D fields
`IceModelVec2V`	vector 2D fields such as horizontal velocities; corresponds to 2 NetCDF variables
`IceModelVec2Int`	2D masks, such as the grounded/floating mask
`IceModelVec2T`	2D time-dependent fields (used to read and store forcing data)
`IceModelVec3`	scalar 3D fields (usually within the ice)

Please see the documentation of these classes for more info. The base class IceModelVec is a virtual class, so code should use the above derived classes.

To create a scalar field, for example, one needs to create an instance of IceModelVec2S and then set its metadata:

// land ice thickness
IceModelVec2S ice_thickness(grid, "thk", WITH_GHOSTS, 2);
ice_thickness.set_attrs("model_state", "land ice thickness",
                        "m", "land_ice_thickness");
ice_thickness.metadata().set_number("valid_min", 0.0);

Here grid is an IceGrid instance, thk is the name of the NetCDF variable, WITH_GHOSTS means that storage for “ghost” (“halo”) points will be allocated, and “2” is the number of ghosts (in other words: required stencil width).

The IceModelVec::set_attrs() call sets commonly used NetCDF variable attributes seen in PISM output files:

`pism_intent`	variables that are a part of the model state of a sub-model should have `pism_intent` set to “model_state”
`long_name`	the (descriptive) long name used for plotting, etc (a free-form string)
`units`	units used in the code; does not have to match units in a file.
`standard_name`	CF standard name, if defined, or an empty string.

The ice_thickness.metadata() call above allows accessing variable metadata and adding arbitrary attributes. See VariableMetadata for details.

The CF convention covers some attribute semantics, including valid_min in this example.

PISM will automatically convert units from ones present in an input file into internal units defines by the set_attrs() call above.

If you want PISM to save data in units other than internal ones, first set these “glaciological” units:

ice_thickness.metadata().set_string("glaciological_units", "km");

Read data from a file ¶

There are at least three cases of “reading data from a file”:

reading a field stored in an input file on a grid matching the one used by the current run (restarting a run),
reading a field stored in an input file on a different grid, interpolating onto the current grid (bootstrapping), and
reading a field stored in a file other than the input file using interpolation (assuming that grids are compatible but not identical)

// case 1, using a file name
unsigned int time = 0;
std::string filename = "filename.nc";
ice_thickness.read(filename, time);

// case 1, using an existing File instance (file is already open)
File file(communicator, "guess_mode", filename, PISM_READONLY);
ice_thickness.read(file, time);

RegriddingFlag flag = OPTIONAL;
double default_value = 0.0;
// cases 2 and 3 (interpolation)
ice_thickness.regrid(filename, flag, default_value);

// cases 2 and 3 (interpolation) using an existing File instance
ice_thickness.regrid(file, flag, default_value);

When interpolating (“regridding”) a field, the flag specifies whether a variable is required (flag is CRITICAL or CRITICAL_FILL_MISSING) or optional (flag is OPTIONAL or OPTIONAL_FILL_MISSING). PISM will stop with an error message if a required variable is not found in an input file.

Flag values of CRITICAL_FILL_MISSING and OPTIONAL_FILL_MISSING replaces “missing” values matching the _FillValue attribute by the default value.

If flag is OPTIONAL or OPTIONAL_FILL_MISSING PISM will fill the variable with default_value if it was not found in the file.

Write data to a file ¶

IceModelVec::define() will define all spatial dimensions used by a variable. However, we usually need to “prepare” a file by defining the time dimension and appending a value to the time variable.

File file(m_grid->com, m_config->get_string("output.format"),
         filename, PISM_READWRITE_CLOBBER, m_grid->ctx()->pio_iosys_id());

io::define_time(file, *m_grid->ctx());
io::append_time(file, *m_grid->ctx()->config(), current_time);

When a file is opened with the mode PISM_READWRITE_CLOBBER, PISM checks if this file is present overwrites if it is; to append to an existing file, use PISM_READWRITE. To move the file aside (appending “~” to the file name), use PISM_READWRITE_MOVE.

A newly-created file is “empty” and contains no records. The io::append_time() call creates a record corresponding to the current_time (in seconds).

To write a field to an already “prepared” file, call

precip.write("filename.nc");
// or, if the file is already open and a File instance is available:

precip.write.(file);

Read scalar forcing data ¶

PISM uses instances of the ScalarForcing class to read scalar forcing data; please see pism::surface::Delta_T for an example.

Read 2D forcing fields ¶

PISM uses instances of the IceModelVec2T class to read 2D forcing fields that vary in time; please see pism::surface::Given for an example.