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Section

In OpenIFS 43r3v1 the source code required for nudging is included by default. This section points out some of the relevant files within the source code, however in order to simply enable nudging this part is not essential to understand as the only changes that need to be applied are in the namelist file. The following part How to Enable Nudging describes the required namelist changes.

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

  • namrlx.nam.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  – this is the setup routine for the namelist variables, which includes their initialisation with default values and consistency checks.
  • updrlxref.F90  – this subroutine updates the reference fields for the relaxation.
  • relaxgp.F90 – this subroutine performs the relaxation in grid point space (in src/ifs/transform/). 

The nudging code in OpenIFS 43r3v1 differs from that in IFS CY43R3 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.

The relaxation to the analyses involves code in further routines (e.g. yomsrlx.F90, relaxgp.F90, stepo.F90, stepo_oops.F90, su0yomb.F90, and updtim.F90) however this part does not need to be changed if the existing nudging capability of the model is to be used. The code in relaxgp.F90 performs the actual relaxation in grid point space. 

How to Enable Nudging

To enable nudging the namelist NAMRLX will need to be included in the namelist file fort.4. No further modifications should be necessary. 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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