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Last edited [[Timestamp]]

[[BR]]

'''Author''' : Matthieu Leclair

'''ticket''' : #863

'''Branch''' : [https://forge.ipsl.jussieu.fr/nemo/browser/branches/2011/dev_r2739_LOCEAN8_ZTC      z_tilde coordinate ] 
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=== The z_tilde vertical coordinate ===
Implementation of the z_tilde Arbitrary Lagrangian Eulerian vertical coordinate discribed in Leclair & Madec, Ocean Modelling, 2011

Baroclinic high frequency vertical motions are followed by the coordinate (Lagrangian part) while barotropic and baroclinic low frequency ones are discribed through cross-level velocities (Eulerian part).

=== Structure ===
z_star and z_tilde vertical coordinates are now two possible "sub-options" of the vvl key.
A third one, the full layer coordinate has been introduced for academic and debuging purposes.

Next sea surface height is first computed. The latter is necessary before computing next vertical scale factors.
And now vertical velocity (eulerian cross-level velocity) can then be calculated. [[BR]]
After this sequence, tracers and dynamics terms can be computed. Next tracers and dynamics fields are updated.[[BR]]
Finally, after the swap of those fields, sea surface height and vertical scale factors can be swaped.

Calling sequence in {{{step.F90}}} before tracers and dynamics:[[BR]]
{{{
               CALL ssh_nxt       ( kstp )
IF( lk_vvl )   CALL dom_vvl_sf_nxt( kstp )
               CALL wzv           ( kstp )
}}}
Calling sequence in {{{step.F90}}} after tracers and dynamics:[[BR]]
{{{
               CALL ssh_swp       ( kstp )
IF( lk_vvl )   CALL dom_vvl_sf_swp( kstp )
}}}

=== Core description ===

== Step 1 : Initialisation ==

== Step 2 : Next sea surface height ==

== Step 3 : Next vertical scale factor ==

== Step 4 : Now vertical velocity (Eulerian cross-level velocity) ==

== Step 5 : Thickness diffusion velocity in tracers advection ==

== Step 6 : Swap of sea surface height ==

== Step 7 : Swap of vertical scale factors ==

=== Side modified modules ===

== sbc_oce.F90 ==

== sbcssm.F90 ==

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=== Testing ===
Testing could consider (where appropriate) other configurations in addition to NVTK].

||NVTK Tested||!'''YES/NO!'''||
||Other model configurations||!'''YES/NO!'''||
||Processor configurations tested||[ Enter processor configs tested here ]||
||If adding new functionality please confirm that the [[BR]]New code doesn't change results when it is switched off [[BR]]and !''works!'' when switched on||!'''YES/NO/NA!'''||

(Answering UNSURE is likely to generate further questions from reviewers.)

'Please add further summary details here'

 * Processor configurations tested
 * etc----

== Bit Comparability ==
||Does this change preserve answers in your tested standard configurations (to the last bit) ?||!'''YES/NO !'''||
||Does this change bit compare across various processor configurations. (1xM, Nx1 and MxN are recommended)||!'''YES/NO!'''||
||Is this change expected to preserve answers in all possible model configurations?||!'''YES/NO!'''||
||Is this change expected to preserve all diagnostics? [[BR]]!,,!''Preserving answers in model runs does not necessarily imply preserved diagnostics. !''||!'''YES/NO!'''||

If you answered !'''NO!''' to any of the above, please provide further details:

 * Which routine(s) are causing the difference?
 * Why the changes are not protected by a logical switch or new section-version
 * What is needed to achieve regression with the previous model release (e.g. a regression branch, hand-edits etc). If this is not possible, explain why not.
 * What do you expect to see occur in the test harness jobs?
 * Which diagnostics have you altered and why have they changed?Please add details here........

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== System Changes ==
||Does your change alter namelists?||!'''YES/NO !'''||
||Does your change require a change in compiler options?||!'''YES/NO !'''||

If any of these apply, please document the changes required here.......

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=== Resources ===
!''Please !''summarize!'' any changes in runtime or memory use caused by this change......!''

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=== IPR issues ===
||Has the code been wholly (100%) produced by NEMO developers staff working exclusively on NEMO?||!'''YES/ NO !'''||

If No:

 * Identify the collaboration agreement details
 * Ensure the code routine header is in accordance with the agreement, (Copyright/Redistribution etc).Add further details here if required..........