There are currently two possible ways to create reference files for atmospheric nudging 1) by interpolating ECMWF re-analysis data or 2) by storing the atmospheric field of an contiunous EC-Earth simulation.
1) Generating nudging files at ECMWF can be done using the same procedure that is required to generate initial conditios Generating atmospheric initial conditions for IFS at ECMWF. However instead of defining a list of dates a 6-hourly output has to be defined in INIBY in prepIFS. The range of the output INIBEGINDATE and INIENDDATE can not exceed one year (or just little more) because the list of dates gets too long.
2) Since only jobs generating one year can be launched the generation of nudging files can take a very long time. A way to increase the efficiency of this process is to introduce a parallel loop in the work flow of prepIFS. This has currently to be done manually. Start by creating and launching a new experiment and suspend the part of the part of initdata.
3) Get the git-repository ifs_initial_forcing and copy the script
modify_play.sh to your home at ECMWF. Open the script and adapt the parameters by changing the experiment name and the number of additional loops that you want to run. This will add to the experiment the subsequent years which will run in parallel. The next step has to be performed to update the experiment in XCDP
4) Open a terminal, go to the directory where experiment suite is unfolded: /vol/ifs_sms/es/c3o/XXXX and type
cdp. Enter after this the following commands, while
XXXX is the experiment name you have create
set SMS_PROG 992030 login ecgate xes 1 play -r /XXXX XXXX.play
Your experiment should after that include now in the loop the additional dates.
5) After having finished all the 6-hourly steps of the period of interest the data has to be processed and downloaded to BSC. A pre-processing script can be found at https://earth.bsc.es/gitlab/cp/ifs_initial_forcing that retrieves the file from the tape archive, preprocesses it and moves it to the $SCRATCH folder of the user. Mind the space limit, you can't preprocess more than 10 years due to the limitations on scratch.
6) While the files are being placed at the $SCRATCH folder you can launch at BSC the shell script copy_nudging_files.sh which will simulatanously copy the files to BSC and put them in the correct format. The script allows to launch parallel ecaccess requests on the data machine and your desktop. Copy the files to the remote HPC to use them in an experiment.
The generation of nudging files from a an experiment is simple. Just put the flag CREATE_ATM_NUDGING=TRUE in the proj.conf file and also make sure that you use the outclass create_nudging. A section of the ecearth3.post template and the atmospheric_nuding.sh plugin will reformat the model output and put the files to the nudging repository of the HPC you are running at. Copy the files after finishing the experiment to /esarchive to keep them here.