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For every 6-hour period the IFS suite will create one gridpoint and one spectral forcing file from ERA-Interim analyses at T255L60 reduced linear Gaussian horizontal grid. It is essential that the forcing files have the same horizontal grid and vertical levels as the OpenIFS experiment.

Forcing File Examples

Example for a spectral and a gridpoint forcing file are available for download from http://download.ecmwf.int/test-data/openifs/nudging/examples/  (no login required for access)

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rlxmlgg201308251200   12-Sep-2019 10:00   48312000
rlxmlsh201308251200   12-Sep-2019 10:00   24109200

These files were created using the IFS experiment described above. They are in GRIB format and they can be examined using the command line tools available from the ecCodes library (e.g. grib_ls or grib_dump).

Model Code Changes

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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  –  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 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.

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.

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