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For further assistance with configurations of OpenIFS, please contact: openifs-support@ecmwf.int.
Newtonian Relaxation
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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:
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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.
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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.
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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.
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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
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To enable nudging the namelist NAMRLX will need to be included in the namelist file fort.4.
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&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, ! smoothing parameter for lon, lat and in the vertical
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
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References
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