Debugging variable volume option

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This page gives a non exhaustive description of the work done to restore VVL option from NEMO version 3.1
At this stage (april 2009) :

• variable volume option can be used with either z, zps and s coordinate
• time splitting issue has not been really investigated

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1. Getting the code

At this time (04-20-09), the work has been done on a branch starting from svn rev1359.

• Outside modipsl :
  

svn co --username nemo_user http://forge.ipsl.jussieu.fr/nemo/svn/branches/dev_004_VVL/NEMO

• Using modipsl :
  

Edit modipsl/util/mod.def and replace
tags/nemo_v3_1
by
branches/dev_004_VVL in NEMO section

Note that this branch has been created some time after nemo_v3_1 release (svn rev1332) so it includes intermediate developments and correction not dealing with vvl, including:

• complete work on time origin in outputs (ticket:335) + downward vertical axis (ticket:357)
• Update lib_mpp, see ticket #379
• add s-sigma coordinates option, ticket #378
• Update lib_mpp, see ticket #379 -first implementation of iom_put, see ticket:387
• update of diaptr

In addition, the VVL has been synchronized with the trunk :

• dev_004_VVL:sync: see ticket #417 : corrections for IOM
• dev_004_VVL:sync: synchro with trunk, see ticket #361 : correction in diaptr
• dev_004_VVL:sync: synchronisation with the trunk, see ticket #388 : bound salt exchange

2. Code organisation

2.1 Flow chart

(original from Mathieu Leclair)

2.2 Sea surface height repartition

Repartition on the whole water column, so key_sigma_vll has been suppressed. Reference coordinate is referred as e3t_0 for instance, and pre-processing (in domzgr_substitute.h90) is used to define before, now, after scale factors, for instance :
# define fsdept_n(i,j,k) (fsdept_0(i,j,k)*(1+sshn(i,j)*mut(i,j,k)))

Note: we shouldn't need 3d array mut (muu, muv) , but the code blows up without.

3. Modified routines

• domvvl is now used only for initialisation (called from istate) (===> should be better to call it from domain....)
• dynspg_flt, dynspg_ts, dynspg_exp : no more update of ssh, just barotropic contribution for velocities
• trazdf_exp, trazdf_imp : ponderation by the correct scale factors
• wzvmod : suprresion of wzv_mod routine, addition of ssh_wzv to compute ssh after from the ssh equation, update the vertical coordinate and compute vertical velocity, addition of ssh_nxt for ssh time stepping
• dynnxt, tranxt : rewritting, take vvl case into account
• zdfevd : for stability issue, test to apply enhanced diffusion has to be performed on now Brunt-Vaisala frequency

4. Cautions

• use s-coordinate pressure gradient when using variable volume
  
• the time splitting is minimalistic
  
• sea ice and vvl
• vertical velocity in output is shifted by 1 time step

5. To be done

• embedded sea-ice (Gurvan and Yevgeni)
• in vector invariant formulation, temporal derivatif of h is not treated (for both baroclinic and barotropic mode of momentum equations)
• a simplification for code and diagnostic : work on the tracer content tendancy everywhere, not on the tracer tendency
• for global ocean, resolve the damping force using time splitting routine
• use a predictor-corrector shceme for time splitting (as Jérome does)
• suppress mu* 3D arrays, and anly use instead 2D arrays
• we should used correct before, now, after fse3 everywhere in the code
• develop a pressure gradient algorithm specific and more accurate for zps-vvl (not only the use of the standard s-coord pressure gradient)
• put the call to domvvl in domain.F90 (not in istate)

6. Tests

6.1 Paris

• ORCA2_LIM zps fixed volume and filtred free surface
• ORCA2_LIM zps variable volume and filtred free surface
• ORCA2_LIM zps fixed volume and time splitting free surface
• ORCA2_LIM zps variable volume and time splitting free surface
• restartability
• Agrif test case
• parallelisation
• conservation (with Asselin filter to 0 and ice tracer fluxes to 0)

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