ICB namelist confusing

[mathiot]

Context

By looking at SETTE, Seb spotted that ORCA2 LONG/SHORT have different icb settings and magically gave the same answer. After looking at it in more detailed, this was normal but I found it very confusing.

Analysis

Namelist ref controlling the icb initiialisation:

    nn_test_icebergs        =  10     ! Create test icebergs of this class (-1 = no)
    !                                 ! Put a test iceberg at each gridpoint in box (lon1,lon2,lat1,lat2)
    rn_test_box             = 108.0,  116.0, -66.0, -58.0
    ln_use_calving          = .false. ! Use calving data even when nn_test_icebergs > 0

So by default if you switch of the iceberg, you use the test functionality and you don't read a calving file.

In icbini, nn_test_icebergs = -1 and ln_use_calving have a similar effect => trigger the reading of the calving file:

       ! when not generating test icebergs we need to setup calving file
       IF( nn_test_icebergs < 0 .OR. ln_use_calving ) THEN

In icbini, if we want to read the icb restart, we need to set nn_test_icebergs <= 0.

       IF( .NOT.ln_rstart ) THEN
          IF( nn_test_icebergs > 0 )   CALL icb_ini_gen()
       ELSE
          IF( nn_test_icebergs > 0 ) THEN
             CALL icb_ini_gen()
          ELSE
             CALL icb_rst_read()
             l_restarted_bergs = .TRUE.
          ENDIF
       ENDIF

And if you want to initialized with icebergs (test), and let the model go using restart without a calving file defined it seems you need to set nn_test_icebergs = 0.

Extra suggestion: being able to read an initial file (basically an old restart even is NEMO does not start using restart), this will decrease the spinup time of the icebergs

Suggestion

All of this looks very confusing and possible source of unwanted behavior. I suggest to change the namelist_ref (and the code will need to be change accordingly):

   ln_use_test = .false.    ! use the testing capacity
      nn_test_icebergs = 0  ! Create test icebergs of this class (need to be larger than 1)
      rn_test_box = 0.0,  0.0, -90.0, -90.0 ! Put a test iceberg at each gridpoint in box (lon1,lon2,lat1,lat2)
      ln_test_rst = .false. ! do not read a restart file when ln_use_test = .true., seed icb in the testing box instead.

   ln_use_calving = .false. ! read a calving file
      sn_icb = '', '', ...  ! calving file definition

   ln_icb_init = .false.      ! read icb from a previous restart (only used when the master flag ln_rstart is false)
      cn_icbini= 'icbinit.nc' ! icb restart name

By default both flag ln_use_test and ln_use_calving are false and NEMO should STOP.
Then you choose what you want, and if you activate both, you initialized the icb as defined by the test and read a calving file

[mathiot]

Other ticket about icb namelist: #2642.