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In OpenIFS 43r3v1 the source code required for nudging is included by default. This section points out the relevant files of 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.

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.

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.