'''Note on system simplification''' [[BR]] [[BR]]

'''Motivation''':

We need to simplify the code for maintenance, robustness and future evolution of the dynamical core. This means fewer options, and a consensus on the default (and thus well validated) set of numerics. The idea is to change the current most used choices from : [[BR]]

  linear filtered free surface zps ocean with TVD, and standard iso-neutral diffusion ;  levitating sea-ice with virtual salt fluxes

possibly to :[[BR]]

  Non linear (vvl), split-explicit (ts) free surface, mixte zps/s-coordinate ocean with high order advection schemes,  triad iso-neutral diffusion (Griffies operator) ; embedded multi-category sea-ice (exact fluxes).

And this with a removing of obsolescent features and a cleaning of the code.

Timing: achievement within 2 years from now for NEMO v4.0 (to be ready for CMIP6, and on time for the foreseen evolution of the dynamical core)

Notations:

  '''  *'''    core action (supported by all system team)

  '''  -'''    subject to discussion

  '''  ?'''    feasible but all conditions required are not yet met

[[BR]] [[BR]] '''I. CHANGE in options used by default'''

'''*'''        '''non-linear free surface''': by default ('''WP 2014''')

'''*'''  '''filtered free surface''': remove it as well as all its associated solvers (if yes when? 2014?)[[BR]]

  (required AGRIF with time-splitting, '''done''' in 2013) and deeply tested ('''WP 2014''')

'''*'''  '''vertical coordinate''' : default option mixte zps/s-tilde coordinate (thus with variable volume level), with mixte zps*/s*, mixte z/s, zps, z and s coordinate system as sub-option (same code used) (2014-2015 ?)

  !-  tracer solved as thickness weighted tracer: D,,  t,,  (e,,  3t,,   T) ('''WP 2014''')

  !-  thickness weighted quantities in the outputs ('''WP2014''')

  !-  no more substitute (domzgr_substitute.h90): all e3 always defined at before, now and after time steps ('''partly done'''   in 2013, '''to be achieved'''   in 2014).

  !- adapte OFF line for tracer to variable volume (2015?)

'''*''' '''linear ssh''' option set without optimization (i.e. starting from the non-linear case with minimal changes, not optimal changes), or linear case suppressed ?? We should probably keep a linear case for test cases. (2014-2015 ?)

'''*'''        '''sea-ice''' : embedded sea-ice as default

  !-  suppress levitating sea ice with virtual salt fluxes. (2014 ?)

  !-  Suppress levitating sea ice with mass exchange (?)  CICE issue…

  !-  need land-fast ice ('''WP 2014''')

  !- need BDY compatibility with embedded sea-ice …. (who when?)

'''-'''         '''tracer advection scheme''': diminish their number (?).

  !-  Typically one module for 2^  nd^  /4^  th^   order and one for a 3^  rd^   order scheme, both associated with an optional flux limiter to ensure positivity. To be discussed

  !- Addition of the rotation of the diffusive part of 3^rd^   order scheme (2014-2015)

'''-'''        '''vertical physics''':  simplification versus diversity

  !- keep only one (GLS) ? or maintain some diversity?

  !- KPP remove unless if-less KPP is implemented (Lemarié et al 2012). Keeping a validated KPP scheme in the system make sense for comparison purposes.

  !- suppress or keep the explicit vertical diffusion?

[[BR]] [[BR]]'''II.OBSOLESCENT features''' [[BR]]

'''mains''':

  '''  *'''          remove '''LIM2'''   : only LIM3 remains in the system ('''WP 2014''')

  '''  *'''          remove '''OBC'''   : only BDY remains in the system  ('''==>> done in v3.6''')

  '''  *'''          remove '''OASIS'''   3 and 4 :  OASIS-MCT instead ('''WP 2014''')

  '''  -'''   duplicate '''  isoneutral operator'''  : keep only the triad operator (Griffies' one) ? Tests needed before decision. Those tests are planed in 2014 NOCS+LOCEAN)

'''smaller ones'''

  '''  *'''          remove '''  acceleration of convergence'''   (notably rdttra=F(k) ==> unique rdt)

  '''  *'''          remove '''Neptune effect''' : ORCA1 runs shows its inefficiency ('''  WP 2014'''  ).

  '''  *'''          remove '''no-slip accurate''' option (key_noslip_accurate). Module divcur.F90 (==>> done in branch [https://forge.ipsl.jussieu.fr/nemo/browser/branches/2014 2014/dev_CNRS0_NOC1_LDF​] )

  '''  *'''           suppress '''cross-land advection''' : this option is hard coded for ORCA_R2_L31 and is nomore used.

  '''  *'''           suppress '''key_traldf_ano''' and the associated possibility of computing lateral diffusion on tracer using only the departure from a given profile.  (==>> done in branch [https://forge.ipsl.jussieu.fr/nemo/browser/branches/2014 2014/dev_CNRS0_NOC1_LDF​]

  '''  -'''          '''Smagorinsky''' (?) : use 3rd order operator (UBS) instead and remove it ?

  '''  -'''          '''Non-penetrative convection''' : with its correction and improvement using alpha & beta (work done by L. Brodeau, Stockholm), it seems now vbetter to keep it in the system.

  '''  -'''          '''runoffs''' : only introduced as lateral flow: remove the option of runoff as surface boundary condition as well as specific treatment in vicinity of river mouths.



'''I/O'''

  '''  -'''          remove Dimg format (?) (agreement needed from DRAKKAR group)

  '''  ?'''          remove IOIPSL : 2 issues, still use in some places, and XIOs portability… (''  probably much too early to decide that''  )

  '''  *'''          use of XIOS logical to control the computation of diagnostics (2014-2015, already done LIM3, see [https://forge.ipsl.jussieu.fr/nemo/browser/branches/dev_r4028_CNRS_LIM3_MV2014 dev_r4028_CNRS_LIM3_MV2014])

'''CODE cleaning'''

  '''  *'''          LDF: remove 1D and 2D cases, keep only 3D ; laplacian and bilaplacian coefficients defined at the same points (T and F) ;  time varying coefficients (function of growth rate of baroclinic instabilities, of the local velocity...): better integration rather than configuration dependent cases. This is also associated with the re-writing of bilaplacian operator on tracer and on dynamics as re-entrant laplacian operator. As a results, curl and div are no more required in the restart file. On going work, see #1260  and  the associated wiki page [https://forge.ipsl.jussieu.fr/nemo/wiki/ticket/1260_CNRS0_NOC1_LDF 1260_CNRS0_NOC1_LDF]

  '''  *'''          OBS operator : clean the stand-alone implementation ('''WP 2014''')

  '''  *'''          avm defined at t-point ==> suppress avmu, avmv and ~10 lbc_lnk calls (2015 ?)

  '''  *'''          suppress most of the CPP keys, if not all of them (2014-2015 ?)

  '''  *'''          dynamical allocation: return back to standard allocation (?) (no more '''  CALL '''  wrk_alloc and '''  CALL '''  wrk_dealloc)  Evaluation to decided what to do ('''WP 2014''')

  '''  *'''          no more potential density (zdfnpc and MLD computed from alpha & beta) (''' WP 2014''')

  '''  *'''          remove all hard coded specific alterations associated with configurations (ORCA,…) while introducing associated input files (2014-2015 ?)

  '''  *'''           same for the definition of coordinate system (s, mixed z-s): only a simple case hard coded. For more sophisticated cases, the required information should be read in an input file. (2015 ?)