= Name and subject of the action

Last edition: '''[[Wikinfo(changed_ts)]]''' by '''[[Wikinfo(changed_by)]]'''

The PI is responsible to closely follow the progress of the action, 
and especially to contact NEMO project manager if 
the delay on preview (or review) are longer than the 2 weeks expected.

[[PageOutline(2, , inline)]]

== Summary

||=Action       || optimisation of the vertical scale factor e3 computation          ||
||=PI(S)        || Techene, Madec                                                 ||
||=Digest       || compute e3 on the fly from e3_0(:,:,:,Ktl) * ( 1 + ssh(:,:,Ktl) / h_0( :,: ) * mask( :,:,: ) instead of storing e3t/u/v/w/f... ||
||=Dependencies || If any                                                ||
||=Branch       || source:/NEMO/branches/2020/dev_r{REV}_{ACTION_NAME} ||
||=Previewer(s) || Madec                                                 ||
||=Reviewer(s)  || Madec                                                 ||
||=Ticket       || #2385                                                 ||

=== Description


The current e3[P] at P-point computation uses interpolation of the r3t 4D table at P = {u-, v-, w-, f-, uw-, vw-} points. This means 7 4D tables stored in memory. 
The proposed optimisation consists in computing e3[P](ji,jj,jk,Ktl) on the fly using the r3[P] = ssh[P] / h_0 and the e3[P]_0. r3[P] is a 2D table, then this means only 4 2D tables stored in memory.
z-tilde management is done through e3[P]_0 that may varies with time in the z-tilde case.
Asselin filter management is done recomputing r3[P] directly with the filtered ssh.


=== Implementation

{{{#!box width=35em help
Describe flow chart of the changes in the code. \\
List the Fortran modules and subroutines to be created/edited/deleted. \\
Detailed list of new variables to be defined (including namelists), \\
give for each the chosen name and description wrt coding rules.
}}}

1/ REFERENCE version change the way to compute the e3 using ssh instead of interpolating \\
For this we need to modify : \\
- cfgs/GYRE_PISCES/cpp_GYRE_PISCES.fcm (top de-activation) \\
- src/OCE/DOM/dom_oce.F90 (declaration and allocation of new variable) \\
- src/OCE/DOM/domain.F90 (initialisation of h new variables) \\
- src/OCE/DOM/dommsk.F90 (initialisation of mask new variable) \\
- src/OCE/DOM/domvvl.F90 (dom_vvl_interpol and dom_vvl interpol_3d) \\
- src/OCE/DYN/dynatf.F90 (change but no effect since ln_dynadv_vec = .true.)
- src/OCE/TRA/traatf.F90 (change the e3t update with Asselin filter)
- src/OCE/step.F90 (change the ordering ssh >> tra >> dyn) \\


2/ 


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=== Documentation updates

{{{#!box width=55em help
Using previous parts, define the main changes to be done in the NEMO literature 
(manuals, guide, web pages, …).
}}}

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== Preview 

{{{#!box width=50em info
[[Include(wiki:Developers/DevProcess#preview_)]]
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== Tests

{{{#!box width=50em info
[[Include(wiki:Developers/DevProcess#tests)]]
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== Review

{{{#!box width=50em info
[[Include(wiki:Developers/DevProcess#review)]]
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