# Model time¶

Table 7 gives the command-line options which control PISM time. If option -ys is absent then the start year is read from the input file (if present) or it defaults to zero. The default value for the end of the run is the start time plus the given (-y) run length. If both -ys and -ye are used then the run length is set to the difference. If both -y and -ye are set PISM will use -ye and ignore -y.

Table 7 Command-line options controlling PISM time

Option

Description

-y (years)

Number of model years to run (see time­.run_length).

-ys (years)

Model year at which to start the run (see time­.start). Also resets the model time, ignoring any time in the input file.

-ye (years)

Model year at which to end the run (see time­.end).

Times specified by -ys and -ye are interpreted as years since the reference date in the chosen calendar (see time­.reference_date and time­.calendar), so by default -ys 0 will correspond to January 1 of year 1.

It is also possible to provide units: for example, -ys 1day -ye 2weeks will start the run from January 2, 1 and run for 14 days (until January 15).

In addition to this, -ys and -ye (as well as -extra_times, etc) can take dates of the form Y-M-D, so -ys 2000-1-1 -ye 2100-1-1 would tell PISM to run for 100 years from January 1, 2000.

The run length given by -y is interpreted as the number of $$365$$-day years. For more precise control of the run duration, provide units (e.g. -y 1s for a very short run or -y "1000 360day" to run for one thousand $$360$$-day years) or use -ye instead.

Note

The fact that -y is not calendar-aware can lead to some confusion. Note, for example, that -ys 2000-1-1 -y 100 -calendar 360_day will end the run on 2101-05-21: this is the date in the $$360$$-day calendar that is one hundred $$365$$-day years from 2000-1-1.

It is also possible to run PISM for the duration of the available forcing data using the -time_file option. The command

pismr -calendar gregorian -time_file forcing.nc


will extract the reference date and run length from forcing.nc, respecting time bounds.

Note

When preparing input files it is important to use time units that are a fixed multiple of “seconds”, such as “minutes since 1989-1-1” or “days since 1999-12-31” and avoid “months” and “years”. (PISM uses UDUNITS-2 to convert units, and in UDUNITS one month is always interpreted as $$\frac{1}{12}\cdot 365.242198781$$ days.) Please see the CF Conventions document for details.

## Calendars¶

Most of PISM, and its ice dynamics core in particular, only needs to know the length of the current time-step. Internally PISM stores time in “seconds since a specified moment” and thus PISM generally does not use or need a calendar.1 We refer to PISM internal time as model time.

One can select a calendar for more precise control of the model time, however (see time­.calendar). A “calendar” is a concept that is part of the CF Conventions. Choosing a calendar is appropriate for runs for specific temporal periods like “the 18th-century” or “1989–2010”. The calendar is generally needed because specific knowledge of lengths of months and years is required to use climate data properly or to facilitate model validation.

PISM uses CalCalcs by David W. Pierce to perform calendric computations. This lets us support all the calendars defined by the CF Metadata Conventions document except for the 366_day (all_leap) calendar.

By default PISM uses the 365_day calendar. This is appropriate for runs that do not require precise application of forcing data or reporting on particular dates (paleo-climate runs, for example).

In addition to setting run start and end times, the calendar setting also affects the interpretation of “monthly” and “yearly” reporting with -extra_times, -ts_times, and -save_times (see Spatially-varying diagnostic quantities, Scalar diagnostic quantities, and Snapshots of the model state).

Note

This does not affect unit conversion: the factor used to convert $$m/s$$ to $$m/\text{year}$$ does not depend on the calendar choice.

Table 8 Calendars supported by PISM. Please see CalCalcs documentation for details

Keyword

Meaning

gregorian or standard

Mixed Gregorian/Julian calendar used today.

proleptic_gregorian

Gregorian calendar extended to dates before 1582-10-15.

noleap or 365_day

Calendar with fixed-length 365-day years

360_day

Calendar with fixed-length 360-day years divided into 30-day months

julian

Julian calendar

## Re-starting an interrupted run using -time_file¶

If a run using -time_file gets interrupted but manages to save a backup, re-starting with -time_file will attempt to re-do the entire run because options -y, -ys, and -ye are ignored:

# This run gets killed but leaves backup.nc:
pismr -i input.nc -time_file time.nc -o output.nc
# This WILL NOT start from the time saved in backup.nc
# and continue until the end time in time.nc
pismr -i backup.nc -time_file time.nc -o output.nc


In this case we want to set the start time of the run from backup.nc, but use the end time from time.nc. To achieve this, use the option -time_file_continue_run.

# This run gets killed but leaves backup.nc:
pismr -i input.nc -time_file time.nc -o output.nc
# This WILL continue until the end time in time.nc, starting from backup.nc
pismr -i backup.nc -time_file time.nc -o output.nc -time_file_continue_run


## Diagnostic computations¶

A “diagnostic” computation can be defined as one where the internal state does not evolve. The internal state of PISM is the set of variables read by “-i”. You can ask PISM to do a diagnostic computation by setting the run duration to a small number such as $$1$$ hours (-y 10hours). The duration to use depends on the modeling setup, but should be smaller than the maximum time-step allowed by PISM’s stability criteria. Such short runs can also be used to look at additional fields corresponding to the current model state.

As an example, consider these two runs:

pismr -eisII A -y 6000 -o foo.nc
pismr -i foo.nc -y 10hours -o bar.nc -o_size big


The result of the second (short) run is a NetCDF file bar.nc which contains the full three-dimensional velocity field in the scalar NetCDF variables uvel, vvel, and wvel, as well as many other variables. The file foo.nc does not contain many of these fields because it was written with the default output size of medium. The “-y 10hours” run has diagnostically “filled-in” all the fields which PISM can model at a time step, but the run duration was chosen so as to avoid significant model state evolution during the run.

This diagnostic mode is often associated to the modeling of ice shelves and ice streams. section An SSA flow model for the Ross Ice Shelf in Antarctica describes using a short “diagnostic” run to model the Ross ice shelf [68]. Verification tests I and J, section Verification, are diagnostic calculations using the SSA.

The NetCDF model state saved by PISM at the end of an evolution run (i.e. with “-y Y” for $$Y>0$$) does not, under the default -o_size medium output size, contain the three-dimensional velocity field. Instead, it contains just a few more variables than those which are needed to restart the run with -i. One can force PISM to save all the supported diagnostic quantities at the end of a time-stepping run using the option -o_size big. Or one can go back and do a “-y small_number” diagnostic run using -o_size big.

Footnotes

1

Note seconds are part of SI units.

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