'''Note on system simplification'''
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'''Motivation''':

We need to simplify the code for maintenance, robustness and future evolution (change in time stepping, …etc). 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, "Madec" iso-neutral diffusion, and standard equation of state ;  levitating sea-ice with virtual salt fluxes

possibly to :[[BR]]
   Non linear (vvl), split-explicit (ts) free surface, mixte zps/s-coordinate ocean with 3rd order advection (UBS with rotation of the diffusive part along iso-neutral surfaces),  "Griffies" iso-neutral diffusion ; 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 time stepping)

Notations: 

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

   '''-'''  desired, but subject to discussion

   '''?'''  feasible but all conditions required are not yet met
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'''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, '''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 (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
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'''OBSOLESCENT features'''
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'''mains''':

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

   '''*'''	remove '''OBC''' : only BDY remains in the system (planed 2013, early 2014)

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

   '''-''' duplicate '''isoneutral operator''': keep only the Griffies' one (tests needed before decision, tests planed in 2014 NOCS+LOCEAN) 
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'''smaller ones'''

   '''-'''	'''Non-penetrative convection''' : correct it (using alpha & beta) (L. Brodeau, Stockholm)

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

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

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

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

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

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

   '''*'''	LDF: remove 1D and 2D cases, keep only 3D ; laplacian and bilaplacian coefficients defined at the same points (T and F) ;  time varying coefficients (Smagorinsky, function of growth rate of baroclinic instabilities): better integration rather than configuration dependent cases ('''on going work''', 2015 ?)