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Please note that nudging in OpenIFS is an experimental research tool and therefore may change between model versions. We are describing on this page the use of the NAMRLX nudging scheme in the IFS. This page has not yet been updated for OpenIFS 48r1, however the process to enable nudging is very similar and makes use of the same namelist. For further assistance on nudging and configuring OpenIFS please post your question in the OpenIFS User Forums or alternatively email openifs-support@ecmwf.int |
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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 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
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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.
The NAMRLX code in the IFS
The IFS contains several codes that facilitate nudging/relaxation. The code which is described here is controlled by the NAMRLX namelist in fort.4.
Some of the variables that the models can use for nudging are in spectral space, others are in grid point space. Since cycle 43R1 of the IFS the scheme makes use of NRLXSMAX, which limits the nudging of spectral fields up to a certain spectral wave number after which there is a sharp cut-off.
It is possible to impose a geographical and vertical regional limitation to where nudging is applied. Given the global nature of spectral data, the way it works is that first the nudging is applied in spectral space (up to NRLXSMAX). Then the nudged fields are transformed to grid point space, where the original un-nudged fields are also available. The final field is then a combination of the nudged field within the defined geographical nudging region, and the original (un-nudged) field elsewhere. There is some smoothing applied across a transition zone, and namelist variables control the width of this transition.
By default, IFS reads nudging information every 6 hours during the model experiment.
Planning the Experiment
For the preparation of a nudged model experiment the following points should be considered:
Nudged variables
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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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.
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The decision on which of these variables should be nudged will depend on the scientific objectives of the experiment. In most applications, it will not be necessary to nudge all the variables, however as a minimum we recommend to constrain vorticity 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 and 23 MB for the spectral file). This amounts to approximately 8.5 GB for one month with 6-hourly nudging.
Relaxation coefficients
The relaxation coefficient is set in the NAMRLX model namelist as parameter XLRX??, with ?? referring to the nudged variable (see the relevant Section below). The units for the relaxation coefficient are "per hour". Therefore a value of 0.5 would correspond to 2 hours, and 0.0833 would be 12 hours as a timescale during which the model is nudged to the forcing value.
As a default all relaxation coefficients are initially set to 0.5 (2 hours) which results in a relatively "tight" nudging to the external data. Satisfactory results have previously been achieved with 0.0833 (12 hours) when nudging temperature and vorticity.
The relaxation values can be adjusted for each parameter and 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.
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 native horizontal resolution of T255 (79 km globally) with 60 levels and ERA-5 re-analyses are at T639 (31 km globally) with 137 levels. ECMWF can provide a workflow to generate relaxation forcing files at the desired horizontal resolution and vertical level number. Please contact OpenIFS support if you need forcing files generated.
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.
Relevant Source Code 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 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
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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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&NAMRLX
LRLXG=true, ! Master switch to enable nudging
LRLXVO=true, ! vorticity
LRLXDI=false, ! divergence
LRLXTE=true, ! temperature
LRLXQ=false, ! specific humidity
LRLXQL=false, ! liquid water
LRLXQI=false, ! ice water
LRLXQC=false, ! cloud fraction
LRLXLP=false, ! log surf pressure
LRLXO3=false, ! ozone
NFRLXG=2, ! number of analysis time steps in memory
NFRLXU=6, ! interval between two references fields (in hours) - this is equal to NFRHIS in nudging
NRLXLMIN=1, ! top model level for nudging
NRLXLMAX=60, ! bottom model level for nudging (for entire vertical domain set either to 60, 91, or 137)
NRLXLMINU=-1, ! uppermost model level for velocity variables (-1 sets to NRLXLMIN)
NRLXLMAXU=-1, ! lowermost model level for velocity variables (-1 sets to NRLXLMAX)
XRLXVO=0.0833, ! relaxation coefficients
XRLXDI=0.5,
XRLXTE=0.0833,
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, ! smoothing parameter for lon, lat and in the vertical
AYRLX=-0.5,
AZRLX=1.0,
CLPSHRLX='../rlxml/sh/', ! relative path to spectral forcing files
CLPGGRLX='../rlxml/gg/', ! relative path to gridpoint forcing files
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Note: When using OpenIFS versions prior to CY43R3 the available namelist parameters can differ from the ones shown above. For instance, nudging of ozone is not available in CY40R1. The namelist used in OpenIFS 48r1 has also small differences to this example. |
References
- Jeuken, A., et al. (1996) On the potential
Following reference describes the Newtonian relaxation techniques in atmospheric models:
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- of assimilating meteorological analyses in a global climate model for the purposes of model validation, J. Geophys. Res., 101, 16,939–16,950
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- . doi:10.1029/96JD01218.
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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
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The information in the remaining sections below is outdated and will be removed in due course. Please contact OpenIFS support for the generation of relaxation forcing files. |
OUTDATED: 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 CLPSHRLX
and CLPGGRLX
in the namelist are set.
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The name strings of the forcing files are generated by the model in |
The following two sections describe a method to generate forcing files using an IFS experiment on the ECMWF HPCF and examples for two forcing files are available for download below.
Using IFS – Scripts Branch
This section describes how forcing files can be generated from ERA-Interim reanalyses (at T255L60)
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using an IFS CY43R3 experiment. For this to succeed you will need to be able to submit an IFS suite
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using prepIFS. You will further need to make a local copy of a scripts branch using IFS Git commands.
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.
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IFS Experiment
IFS experiment h7z9 is set up to run in nudged configuration and the inigroup task family
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(shown in the XCdp
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workflow GUI) will retrieve ERA-Interim re-analyses from the MARS archive and generate the required forcing files
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. You will need to make a copy of this suite, modify it as described in this section, and submit it to the ECMWF HPCF.
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Note: There is no need to run the model task family |
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. 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
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such that all
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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
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1488 for two months of forcing files to be created.
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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.
Model Code Changes
Forcing File Examples
Examples 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 [ md5: cc7428b4016637aee63277d14b6149e9 ]
rlxmlsh201308251200 12-Sep-2019 10:00 24109200 [ md5: 3722948745891d1de0ae5e5b8c1a7e33 ] |
These two forcing files for 25-08-2013 12:00 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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