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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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The decision on which 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 , for the spectral file 23 MB).). 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.
Data storage
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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 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 |
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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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Files
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In 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.
In order to control the model's nudging configuration the following files of the source code files are relevant:
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. |
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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