source: lmdz_wrf/WRFV3/README.io_config @ 13

Run time control of IO stream variable sets

For WRF releases prior to V3.2, state variables were associated with I/O
streams at compile time via their entries in the Registry.  That will
still work, and should still be considered the primary method for defining
stream variable sets in WRF. This new capability allows users to add or
exclude state variables from a history or input stream at run-time without
recompiling.  Also, this can be done on a per-domain basis as well.

Here's a quick sketch of how it is used:

a) There are two new namelist variables in the time_control section
of the namelist.input file.  The first is iofields_filename. This is a
character variable, settable separately for each domain and defaulting
to "NONE_SPECIFIED".  This is where a user can list the names of text
files containing lists of variables to added/removed from history and
input streams for that domain.  As with other namelist variables that
have max_domains in their rconfig definitions, each domain does have
to have an entry in the namelist.input file. You can set each entry to
the name of separate definitions file for each domain, or some or all
of them can be the name of the same file.

The second namelist variable is a logical, ignore_iofields_warning,
which tells the program what to do if it encounters an error in these
user-specified files.  The default value, .TRUE., is to print a warning
message but continue the run.  If set to .FALSE., the program will
abort if there are errors in these user-specified files.

b) The user-specified files, named as settings to the iofields_filename
   in the namelist.input file.  The files contain lines associating
   variables with streams.  Each line has the form:

  op:streamtype:streamid:variables

    op:          either + (plus) or - (minus) for adding or removing
                 a variable
    streamtype:  either i (input) or h (history) indicating which type
                 of stream is being affected
    streamid:    an integer between 0 and the number of streams (0 
                 represents main input or history)
    variables:   comma separated list of variables

Example.  The line:

    +:i:5:u,v,w

will add the U, V, and W variables to auxinput5.  Note for this example,
these are 2 time-level variables so that, as with Registry-defined
stream-associations, the designations apply to variables representing
the highest numbered time level (U_2). See the file test/em_real/sample.txt

These lines cannot be longer than 256 characters, but you can associate
long lists of variables using multiple lines with the same op, streamtype,
and streamid. You can have comments in these files by putting # on the
line, causing the entire line to be a comment (even characters before
the #).

An implementation note: the mechanism is not very efficient at this
point. Each variable listed involves a search through all the state
variables in the grid.  That is still relatively quick - even a very
long list shouldn't take more than a couple seconds -- and the lists are
processed only once when a grid is allocated.  Even so, super-long lists
of these run-time stream associations are mildly discouraged. In other
words, use this as a mechanism to tweak the Registry I/O definitions or to
get domain by domain specific behavior, but don't use it as a substitute
for defining I/O definitions with state variables in the Registry.