You are viewing an old version of this page. View the current version.

Compare with Current View Page History

« Previous Version 62 Next »

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

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.

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 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.

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

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. 

On this page...

Planning the Experiment

For the preparation of a nudged model experiment the following points should be given consideration:

Nudged variables

The current setup permits the nudging of the following 9 prognostic variables: 

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

Dependent on the scientific objectives of the model experiment not all variables will require nudging. As a minimum it is recommended to use vorticity, divergence and temperature. In this context disk space usage should also be a consideration as an increasing number of nudged variables will result in larger forcing files. At T255L60 forcing files that contain all 9 variables require 70 MB for each time step (grid point file 47 MB, spectral file 23 MB).

Relaxation coefficients

As a standard all relaxation coefficients are initially set to 0.5 which represents a relatively "tight" nudging to the external data. These values should be adjusted according to the objectives of the experiment. Frequently the best results are obtained with different relaxation coefficients 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. A too tight nudging can result in unrealistic behaviour in the freely calculated model variables. 

Grid resolution

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. 

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). Once the forcing files have been created their long-term storage may need to be considered.

Preparation of Forcing Files

This section describes how forcing files can be generated from ERA-Interim reanalyses (at T255L60) 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 IFS Git commands.

Scripts Branch

A local copy of scripts branch damk_CY43R3_getrelax is required. This branch 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 (cca or ccb) create the destination directory for the forcing files:   mkdir -p -m 777 $SCRATCH/rlxml

IFS Experiment

IFS experiment h7z9 is set up to run in nudged configuration and the inigroup task family (shown in the 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 in this section, 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 such that all show the same start date (usually the 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 user damk
  3. Copy experiment h7z9 from user damk
  4. Set INIBEGINDATE and INIENDDATE to 2013070100 (or your chosen start date)
  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 created in the previous section
  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

The source code changes to run OpenIFS 43r3 in nudged configuration are under version control in the OpenIFS Git repository in branch feature/nudging. The code in OpenIFS 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. 

To enable nudging the namelist NAMRLX will need to be included in the namelist file fort.4.

Default settings in the NAMRLX namelist
    &NAMRLX
      LRLXG=true,    ! Master switch to enable nudging
      LRLXVO=true,   ! vorticity
      LRLXDI=true,   ! divergence
      LRLXTE=true,   ! temperature
      LRLXQ=true,    ! specific humidity
      LRLXQL=true,   ! liquid water
      LRLXQI=true,   ! ice water
      LRLXQC=true,   ! cloud fraction
      LRLXLP=true,   ! log surf pressure
      LRLXO3=true,   ! ozone
      XRLXVO=0.5,    ! relaxation coefficient
      XRLXDI=0.5,
      XRLXTE=0.5,
      XRLXQ=0.5,
      XRLXO3=0.5,
      XRLXLP=0.5,
      ALATRLX1=90,   ! latitude domain for nudging
      ALATRLX2=-90,
      ALONRLX1=0,    ! longitude domain for nudging
      ALONRLX2=360,
      AXRLX=-0.5,
      AYRLX=-0.5,
      AZRLX=1.0,
      NRLXLMIN=1,    ! top model level for nudging
      NRLXLMAX=60,   ! bottom model level for nudging
      CLPSHRLX='../rlxml/sh/',  ! relative path to spectral forcing files
      CLPGGRLX='../rlxml/gg/',  ! relative path to gridpoint forcing files
    /

References

  • 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.
  • No labels