The NEMS application is highly configurable. During build-time, i.e. when the NEMS executable is being compiled and linked, choices are made as to which model and mediator components are built into the system. The built-in components become accessible during run-time, allowing the execution of different run configurations without the need to rebuild NEMS.

Often the build and run configurability of NEMS is hidden from the user by application or test scripts. However, there are times where it is useful to understand the technical details on the level of the NEMS executable.

Run Configuration

During run-time of the NEMS executable, it accesses a file called nems.configure, which it expects to find in the run directory. This file specifies the dynamic component selection, and the exact run sequence to be used. Only models built into the executable at build-time are accessible during run-time. An error will be triggered if an unavailable model is selected in nems.configure. The component selection is based on two variables:

xxx_model:          abc
xxx_petlist_bounds: lower upper

Here xxx can be atm, ocn, ice, ipm, med. The abc stands for the actual instance name, e.g. fv3 or mom6. The lower and upper bounds of the petList specification are integer PET numbers.

The specification of the run sequence provides the flexibility needed to cover different coupling scenarios. The format is line based between special tags:

runSeq::
    line1
    line2
    ...
    lineN
::

There are a number of format options for each line:

A time loop is introduced by a @ symbol, followed immediatly by the number of seconds of the associated time step.
A time loop is closed by a single @ symbol on a line.
The RUN method of model component xxx is called by specifying just xxx on a line. The supported values of xxx are the same as for the model specification above. A specific RUN phase can be provided by adding the phase label to the same line, following the model string.
A connector going from component xxx to component yyy is specified by a line like this: xxx -> yyy. An additional argument on the same line can be used to specify connection options for all of the field handled by the connector. The format and supported values of the connection options is documented in the NUOPC reference manual.

A very simple run sequence is running only the atmospheric uncoupled model. The run sequence looks like:

# Run Sequence #
runSeq::
    ATM
::

A more complex example is when components are exchanging information at two time scales:

# Run Sequence #
runSeq::
    @7200.0
    OCN -> MED
    MED MedPhase_slow
    MED -> OCN
    OCN
    @3600.0
        MED MedPhase_fast_before
        MED -> ATM
        ATM
        ATM -> MED
        MED MedPhase_fast_after
      @
    @
::

Anything on a line after the # symbol is treated as a comment and ignored. Indentation in the formatting does not change the meaning of a line, but is purely used to increase readability.