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Newtonian Relaxation

OpenIFS uses initial and boundary conditions to calculate its own model dynamics, i.e. meteorological variables that are resolved on the grid scale. It is however possible to constrain the model dynamics with external data. Newtonian relaxation, sometimes referred to as "nudging", is a simple form of data assimilation which allows to constrain or force the model's meteorological fields with reanalysis data. This is sometimes referred to as running the model in "offline" mode. In nudged configuration the model's dynamics is continually specified by the meteorological reanalysis independent of the run length of the experiment. 

This method can be useful, for instance, in sensitivity studies which aim to isolate the model physics or chemistry while preventing feedbacks to the model dynamics. Another example for its use is to align a climate model simulation closer to historic meteorology for comparison with measurements. 

In this article we explain how to prepare and configure OpenIFS for a nudged simulation. Therein the model needs to read meteorological parameters at the grid scale from pre-computed external forcing files. These forcing files have to be created prior to the nudged OpenIFS model experiment and this process is also described here.


Further information

Following reference describes the Newtonian relaxation techniques in atmospheric models:

  • Jeuken, A., Siegmund, P., Heijboer, L., Feichter, J., and Bengtsson, L.: On the Potential of assimilating meteorological analyses in a global climate model for the purposes of model validation, J. Geophys. Res., 101, 16,939–16,950, 1996. doi:10.1029/96JD01218.

Please note that nudging in OpenIFS is an experimental research tool and therefore may change between model versions.

For further assistance with configurations of OpenIFS, please contact: openifs-support@ecmwf.int.





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Preparation of Forcing Files

Forcing files are generated using an IFS CY43R3 experiment. For this to succeed you will need to be able to submit an IFS suite using prepIFS. You will further need to make a local copy of a scripts branch using Git.

Script Branch

A local copy of scripts branch damk_CY43R3_getrelax is required. This branch (gen/getrelax) copies the forcing files from the /fws working directory to $SCRATCH/rlxml/. At present the directory rlxml must exist in the user's scratch space with global writing permissions.

Tasks - Prepare scripts branch
  1. Load ifs git tools:   module load ifs-git-tools
  2. Add repository of user damk :  git ifsremote damk
  3. Make a copy of the scripts branch:  git ifsbranch -B damk_CY43R3_getrelax -b mygetrelax
  4. In script file getrelax (line 183) assign your username to variable ME
  5. Commit the changes in git and push the branch to your repository (git add, git commit, git push)
  6. On the ECMWF HPCF create the destination directory for the forcing files:   mkdir -p -m 777 $SCRATCH/rlxml

IFS Experiment

IFS experiment h7z9 is set up for nudged configuration and the inigroup task family (shown in XCdp workflow GUI) will retrieve ERA-Interim re-analyses from the MARS archive and generate the required forcing files. You will need to make a copy of this suite, modify it as described below, and submit it to the ECMWF HPCF.

Note: There is no need to run the model task family. The suite can be aborted after inigroup has completed. 

In prepIFS select the start date and run length in order to produce forcing files. This is done by adjusting variables INIBEGINDATE, INIENDDATE and INITIME to all reflect the same start date (first day of the month), e.g. 2013070100.

FCLENGTH should be set to 744 for one month or 1488 for two months of forcing files to be created.

Tasks - Prepare IFS experiment
  1. Start prepIFS
  2. In menu "File" click on "Add user" and include experiments from user damk
  3. Copy experiment h7z9
  4. Set INIBEGINDATE and INIENDDATE to 2013070100
  5. Set INIBY to zero
  6. The INITIME list of individual initial dates should have just one entry, 2013070100
  7. Set FCLENGTH to 744 to create forcing files for one calendar month
  8. Replace the scripting branch with your own copy
  9. Check and submit the suite to the HPCF

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.

Model Code Changes

git repository branch feature/nudging

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