GfsSourceControlField is similar to GfsSourceControl but can be used to control the value of a scalar at a given spatial scale (typically coarser than the finest spatial resolution of the simulation). This can be used for example to relax the coarse-scale solution toward a known coarse-scale field while allowing finer-scale details to develop.
The syntax in parameter files is
[ GfsSourceControl ] 3 60
3" is the level of the coarse-scale field defined in GfsSourceControl and "
60" is a relaxation timescale.