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In this article we explain how to prepare and configure OpenIFS 43r3 for 43r3v1 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. 

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

The description on this page refers to nudging in gridpoint space which also allows regional nudging. This differs from nudging in spectral space which constrains the model in the longer wave numbers on a global scale. 

For further assistance on nudging and configuring OpenIFS please post your question in the OpenIFS User Forums or alternatively email openifs-support@ecmwf.int

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 the user 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 nudged towards the meteorological reanalysis independent of the run length of the experiment. 

This method relaxes the model state towards gridded re-analysis data (or towards output from another atmospheric model, or gridded observational data) by adding a non-physical relaxation term to the model equations (Jeuken et al., 1996). In the equation below X represents a prognostic model variable and Fmodel the model forcing which determines the evolution of X. The relaxation term G (Xobs - X) includes the relaxation coefficient G (in s-1) which determines the "tightness" of the nudging.

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\frac{\partial X}{\partial t} = F_{model}(X) + G(X_{obs} - X)

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. 


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The current model version permits nudging of the following 9 prognostic variables that are stored in two external forcing files (one gridpoint file and one spectral file) for each nudging step: 

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  • Vorticity,
  • divergence,
  • temperature,
  • surface pressure,
  • specific humidity,
  • ice water contents,
  • liquid water contents,
  • cloud fraction,
  • and stratospheric ozone.

The decision on which variable requires nudging of these variables should be nudged will depend on the scientific objectives of the model experiment. Often it is not necessary to nudge all the variables, however as a minimum we recommend to constrain vorticity, divergence and temperature.

In this context disk space usage will become a consideration as an increasing number of nudged variables will result in larger forcing files. At T255L60, for instance, forcing files that contain all 9 variables require 70 MB for each time step (47 MB for the grid point file 47 and 23 MB , spectral file 23 MB).for the spectral file). This amounts to approximately 8.5 GB for one month with 6-hourly nudging. 

Relaxation coefficients

As a standard all relaxation coefficients are initially set to 0.5 which results in a relatively "tight" nudging to the external data. These values should be adjusted for each experiment according to its objectives. Frequently the best results are obtained when different relaxation coefficients are used, specific to each nudged variable. The external analysis data is updated every six hours and the model linearly interpolates in time between these data points. Too tight nudging can result in unrealistic behaviour in the freely calculated model variables. 

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One of the strengths of OpenIFS is its ability to operate with a wide range of grid resolutions. The model is however not capable of spatial interpolation of the gridded forcing file data. Therefore the forcing files need to be prepared at the horizontal and vertical resolution of the model experiment. ERA-Interim re-analyses have a horizontal resolution of T255 (79 km globally) with 60 levels and ERA-5 re-analyses are at T639 (31 km globally) with 137 levels. The IFS experiment described below is able to generate forcing files at either of these two resolutions. In order to nudge the model when using other resolutions the forcing files need to be first interpolated to the model grid. 

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Note: At present we only generate forcing files at the horizontal resolutions T255L60 and T639L137. 

Data storage

The spectral and gridpoint forcing files need to be accessible to the model at runtime, i.e they must not be read from an offline tape archive. Depending on the length of the model experiment (e.g. for a seasonal forecasts or in climate model configuration) it is conceivable that several months or even years of 6-hourly forcing files need to be prepared. This will have implications for the available disk space. At T255 with 60 levels one year of forcing files can require up to 100 GB disk space (depending on the number of variables the files contain). Once the forcing files have been created their long-term storage may need to be considered.

Preparation of Forcing Files

This section describes a possible way to generate forcing files for nudged experiments. It is only necessary to carry out these steps if no nudging files are available and need to be created.

In its default nudging configuration the model requires a spectral and a gridpoint forcing file every six hours, at 0000, 0600, 1200, and 1800 UTC.

The default naming convention for these files are rlxmlshYYYMMDDHHHH for the spectral files, and rlxmlggYYYYMMDDHHHH for the gridpoint files. YYYY, MM, DD, HHHH represent the year, month, day and hour of the forcing data. The model will expect to find these files in the experiment directory unless the path variables CSPSHRLX CLPSHRLX and CSPGGRLX CLPGGRLX in the namelist are set.

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The name strings of the forcing files are generated by the model in  src/ifs/utility/updrlxref.F90

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titleTasks - 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 bb-damk/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 (cca or ccb) create the destination directory for the forcing files:   mkdir -p -m 777 $SCRATCH/rlxml

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These two forcing files for 25-08-2013 0012:00 were created using the IFS experiment described above. They are in GRIB format and can be examined using the command line tools available from the ecCodes library.

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$ grib_ls rlxmlgg201308251200 | head
rlxmlgg201308251200
edition      centre       typeOfLevel  level        dataDate     stepRange    dataType     shortName    packingType  gridType     
1            ecmf         hybrid       1            20130825     0            an           q            grid_simple  reduced_gg  
1            ecmf         hybrid       1            20130825     0            an           ciwc         grid_simple  reduced_gg  
1            ecmf         hybrid       1            20130825     0            an           clwc         grid_simple  reduced_gg  
1            ecmf         hybrid       1            20130825     0            an           cc           grid_simple  reduced_gg  
1            ecmf         hybrid       1            20130825     0            an           o3           grid_simple  reduced_gg  
1            ecmf         hybrid       2            20130825     0            an           q            grid_simple  reduced_gg  
1            ecmf         hybrid       2            20130825     0            an           ciwc         grid_simple  reduced_gg  
1            ecmf         hybrid       2            20130825     0            an           clwc         grid_simple  reduced_gg

$ grib_ls rlxmlsh201308251200 | head
rlxmlsh201308251200
edition      centre       typeOfLevel  level        dataDate     stepRange    dataType     shortName    packingType  gridType     
1            ecmf         hybrid       1            20130825     0            an           lnsp         spectral_complex  sh          
1            ecmf         hybrid       1            20130825     0            an           vo           spectral_complex  sh          
1            ecmf         hybrid       1            20130825     0            an           d            spectral_complex  sh          
1            ecmf         hybrid       1            20130825     0            an           t            spectral_complex  sh          
1            ecmf         hybrid       2            20130825     0            an           vo           spectral_complex  sh          
1            ecmf         hybrid       2            20130825     0            an           d            spectral_complex  sh          
1            ecmf         hybrid       2            20130825     0            an           t            spectral_complex  sh          
1            ecmf         hybrid       3            20130825     0            an           vo           spectral_complex  sh

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Relevant Source Code

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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/nudgingIn 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 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 nudging code in the nudging branch of OpenIFS 43r3 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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  • Jeuken, A., et al. (1996)  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.  doi:10.1029/96JD01218.


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