Custom Tracers in FESOM2

[alxthor]

Is there a way to add tracers in a custom region in FESOM2 to observe where they are transported?
I am running FESOM2 standalone without esm-tools and changing the following things in the namelists:

In namelist.oce i changed:

num_tracers=3       !number of all tracers
tracer_ID  =0,1,105 !their IDs (0 and 1 are reserved for temperature and salinity)
n_ic3d   = 3                              ! number of tracers to initialize
idlist   = 1, 0, 105                      ! their IDs (0 is temperature, 1 is salinity, etc.). The reading order is defined here!
filelist = 'ClimateDataPath/phc3.0_winter.nc', 'ClimateDataPath/phc3.0_winter.nc', 'phc3.0_winter.nc.modified' ! list of files in ClimateDataPath to read (one file per tracer), same order as idlist
varlist  = 'salt', 'temp', 'salt'     ! variables to read from specified files

in namelist.io I added:
'otracers ',1, 'd', 8,

This is the tree of the ClimateDataPath specified in namelist.config:

.
├── ClimateDataPath -> /work/ollie/projects/clidyn/FESOM2/hydrography/phc3.0/
├── phc3.0_winter.nc
└── phc3.0_winter.nc.modified

I was hoping that this would read in the 'salt' variable of the modified Climatology NetCDF file, but I get the following output (similar error if I use 2 instead of 105 as tracer ID):

Arrays are set
 FCT is initialized
           3  tracers will be used in FESOM
 tracer IDs are:            0           1         105
 read Temperatur climatology from:ClimateDataPath/phc3.0_winter.nc
 read Salt       climatology from:ClimateDataPath/phc3.0_winter.nc
 Start: Initial conditions  for tracers
 reading input tracer file for tracer ID=            1
 input file: /home/ollie/althor001/fesom2/ClimateData/ClimateDataPath/phc3.0_winter.nc
 variable  : salt
 reading input tracer file for tracer ID=            0
 input file: /home/ollie/althor001/fesom2/ClimateData/ClimateDataPath/phc3.0_winter.nc
 variable  : temp
 reading input tracer file for tracer ID=          105
 input file: /home/ollie/althor001/fesom2/ClimateData/phc3.0_winter.nc.modified
 variable  : salt
 converting insitu temperature to potential...
 DONE:  Initial conditions for tracers
   |-> gobal max init. temp. =   30.0510326153043     
   |-> gobal min init. temp. =  -2.06008970141432     
   |-> gobal max init. salt. =   41.1176835670450     
   `-> gobal min init. salt. =   5.62836183480000     
 invalid ID 105 specified for   3 th tracer!!!
 the model will stop!
 fesom should stop with exit status = 0

After having a brief look into the source code, it seems that FESOM2 only supports predefined tracers (101, 301, 302, 303). Is that correct, or is there a way around this?

[JanStreffing]

This should be possible. I think @mbutzin would know more.

[mbutzin]

Yes, it is possible but you have to modify the source code. Depending what do want to include this will have to be done (at least) in oce_ale_tracer.F90, oce_setup_step.F90, io_meandata.F90, and io_restart.F90. It may also be necessary to modify fvom_main.F90, gen_modules_config.F90, gen_model_setup.F90, and oce_modules.F90.