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The source code changes to run OpenIFS 43r3 in nudged configuration are under version control in the OpenIFS Git repository in branch feature/nudging.

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In order to control the model's nudging configuration the following source code files are relevant:

  • namrlx.nam.h  h  –  the namelist block which appears in the fort.4 file. These variables allow to customise the model's nudging behaviour without recompiling the model source code.
  • yomrlx.F90  – in this module the  variables in the namelist block are defined.
  • surlx.F90  F90  – this is the setup routine for the namelist variables, which includes their initialisation with default values and consistency checks.
  • updrlxref.F90  F90  – this subroutine updates the reference fields for the relaxation.

The code in the nudging branch of OpenIFS 43r3 differs from that in IFS insofar that the variables CLPSHRLX and CLPGGRLX have been added to the namelist. These two variables allow to specify the location of the directory with the forcing files on the file system.  If these are not set then the model will expect to find the forcing files in the experiment directory.

Further, the relaxation to analyses is implemented with further routines (e.g. yomsrlx.F90, relaxgp.F90, stepo.F90, stepo_oops.F90, su0yomb.F90, and updtim.F90) however the code therein does not need to be changed if the existing nudging capability of the model is to be used.

To enable nudging the namelist NAMRLX will need to be included in the namelist file fort.4. The values below show one possible configuration reflecting standard settings. As described previously, the question of which variables should be relaxed to the external analysis, the model grid domain where relaxation is applied, and the tightness of the nudging, controlled by the relaxation coefficient, are very much dependent on the objective of the model experiment.

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