'''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 2015''')

'''*'''  '''filtered free surface''': remove it as well as all its associated solvers (2015?)

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

'''*'''  '''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 2015''' )

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

  !-  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).

  !- adapt OFFLINE for tracer to variable volume (2015)

  !- For OFFLINE, compute the grid in the same way than in ONLINE.

'''*''' '''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 the management of grounded-ice & land-fast ice (''' WP 2014''' )

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

'''-'''         '''tracer advection scheme''':'''     DONE in ''' [https://forge.ipsl.jussieu.fr/nemo/browser/branches/2015/dev_r5721_CNRS9_NOC3_LDF ​2015/dev_r5721_CNRS9_NOC3_LDF]

'''-'''        '''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 better 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.

  '''   -'''          ''' passive tracers'''  : use trasbc routines for passive tracers to add the river inputs of nutrients

'''I/O'''

  '''   *'''           remove Dimg format (agreement obtained from the DRAKKAR group) (2014)

  '''   ?'''           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 but only for 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.'''                 DONE in ''' [https://forge.ipsl.jussieu.fr/nemo/browser/branches/2015/dev_r5721_CNRS9_NOC3_LDF ​2015/dev_r5721_CNRS9_NOC3_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)

  '''   *'''           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 ?)

  '''   -'''            remove the key_esopa option that allow to compile and test all code option that are controlled by a cpp key. With the removal of nearly all cpp keys, it should become useless...'''     DONE in ''' [https://forge.ipsl.jussieu.fr/nemo/browser/branches/2015/dev_r5721_CNRS9_NOC3_LDF ​2015/dev_r5721_CNRS9_NOC3_LDF]