= '''Note on possible system simplification''' =
Last edited [[Timestamp]]

We need to simplify the code for both maintenance and future evolution. This means fewer options, and a consensus on the default (and thus well validated) set of numerics.

The idea is to change the used choices from :

linear filtered free surface ocean with TVD, "Madec" isoneutral diffusion, and standard equation of state ;  levitating sea-ice with virtual salt fluxes

to :

Non linear (vvl) split-explicit (ts) free surface with 3rd order advection (UBS),  "Griffies" isoneutral diffusion and TEOS10 ; embedded multi-category sea-ice (exact fluxes).

== '''Environment :''' ==
  • suppress most of the CPP keys, if not all of them 

  • dynamical allocation: return back to standard allocation (no more '''  CALL '''  wrk_alloc and '''  CALL '''  wrk_dealloc)


== '''Free surface & vertical coordinate :''' ==
  • remove filtered free surface and all associated solvers

  • default : non-linear free surface

  • linear option set without optimization (i.e. starting from the non-linear case with minimal changes, not optimal changes), or linear case suppressed ??

  • 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)

  Needs :

 * split-explicit free surface adapted to AGRIF refinements and TAM

 * tracer solved systematically as thickness weighted tracer: D,,t,,(e,,3t,, T)

 * no more substitute (domzgr_substitute.h90): all e3 always defined at before, now and after time steps.

 * sea-ice : embedded sea-ice only, so management of land-fast ice and especially sea-ice approaching the ocean-bottom.[[BR]]

== '''Equation of State (eos) :''' ==
  • EOS10 as default + 1 simplified eos. Suppression of the now used EOS.

  • EOS provides systematically alpha and beta (used in ldf_iso, bbl, mld, npc…)

  • no more potential density (mixed layer depth computed from alpha & beta)

  • efficient transfer function from conservative temperature to potential temperature and vice versa. [[BR]]

== '''Coupled interface to the atmosphere :''' ==
  • OASIS-MCT only: remove OASIS 3 interface.

  • Multi-category sea-ice interface [[BR]]

== '''Physics :''' ==
  • avm defined at t-point ==> suppress avmu, avmv and ~10 lbc_lnk calls

  • 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 see 

  • TKE remove (use GLS instead?)

  • 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.

  • Non-penetrative convection : either correct it (using alpha & beta) or remove it • remove the explicit vertical diffusion

== '''Configurations :''' ==
  • remove all hard coded specific alterations associated with configurations (ORCA,…) while introducing associated input files

  • 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.[[BR]]

== '''Obsolescent features : '''to be suppressed ==
  • remove LIM2 : only LIM3 remains in the system

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

  • acceleration of convergence (in particular, rdttra function of k ==> unique rdt)

  • remove Neptune effect : ORCA1 run shows its inefficiency.

  • duplicate isoneutral operator: keep only the Griffies' one

  • Smagorinsky (?) : use 3^rd^ order operator (UBS) instead and remove it ?

  • tracer advection schemes: 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.[[BR]]

  • suppress cross-land advection

== '''Outputs :''' ==
  • remove Dimg format ? (agreement needed from DRAKKAR group)

  • remove IOIPSL: the key issue here is do we believe XIOs is able to work on all existing and foreseen platform?