Trough grid refinement (AGRIF nesting & coarsening)

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Coarsening of physics for biogeochemistery model

Presentation

A capacity of coarsening physics to force a BGC model coupled to NEMO has been developped in NEMO_3.6. This capacity allow to run 'online' a BGC model coupled to OPA-LIM with a lower resolution, to reduce the CPU coast of the BGC model, while preserving the effective resolution of the dynamics.

This capacity is not included in the NEMO_3.6 , but is available in a specific branch of the 3.6 stable. This branch is regularly phazed with the NEMO_3.6. An associated ticket to follow evolution is here.

A presentation is available [xxx here], where the methodology is presented.

How to ?

To activate the coarsening, key_crs should be added to list of cpp keys. This key will only activate the coarsening of dynamics.

Some parameters are available in the namelist_cfg:

               !   passive tracer coarsened online simulations
!-----------------------------------------------------------------------
   nn_factx    = 3         !  Reduction factor of x-direction
   nn_facty    = 3         !  Reduction factor of y-direction
   nn_msh_crs  = 0         !  create (=1) a mesh file or not (=0)
   nn_crs_kz   = 3         ! 0, volume-weighted MEAN of KZ
                           ! 1, MAX of KZ
                           ! 2, MIN of KZ
                           ! 3, 10^(MEAN(LOG(KZ)) 
                           ! 4, MEDIANE of KZ 
   ln_crs_wn   = .false.   ! wn coarsened (T) or computed using horizontal divergence ( F )
                           !                           !
   ln_crs_top = .true.     !coarsening online for the bio
/

• Only nn_factx = nn_facty = 3 is available and the coarsening only works for grids with a T-pivot point for the north-fold lateral boundary condition( ORCA025, ORCA12,ORCA36,...).

• nn_msh_crs = 1 : will activate the generation of the coarsened grid meshmask.

• nn_crs_kz : is the operator to coarsene the vertical mixing coefficient.

• ln_crs_wn : when key_vvl is activated, this logical has no effect; the coarsened vertical velocities are computed using horizontal divergence.

when key_vvl is not activated, coarsened vertical velocities are computed using horizontal divergence ( ln_crs_wn = .false )

or coarsened vertical velocities are computed with an average operator ( ln_crs_wn = .true. )

• ln_crs_top = .true: should be activated to run BCG model in coarsened space; so only works when key_top is in the cpp list and eventually key_pisces or key_my_trc.

Choice of operator to coarsene KZ

A sensiblity test has been done with an Age tracer to compare the differents operagtor. 10(MEAN(LOG(KZ)) or MEDIANE of KZ seems to provide the best resultats.

Some results can be found [xxx here]