ENHANCE-5_Gurvan — RK-3 time stepping

Summary

Action	ENHANCE-5_Gurvan — RK-3 time stepping
PI(S)	Gurvan Madec
Digest	Introduce an optional RK3 time-stepping scheme. The scheme will: be a valuable alternative to the current Modified Leap-Frog (MLF, Leclair and Madec OM2009) scheme, prepare the futur introduction of a compensated time-space scheme, allow ​AGRIF to be exactly conservative, and make much more easier to use time coarsening of TRC and OFF-line variable volume calculation make feasible to properly implement an implicit computation of top and bottom friction (properly means consistent with the ssh time splitting scheme) As such a change will impact many module of OPA, it has been preferred to develop the RK3 version in a separate directory, RK3_SRC, put at the same level as OPA_SRC and being a copy of OPA_SRC as starting point. In addition to the RK3 scheme, a Forward-Bacward Scheme (FBS) will be introduced. The goals associated with FBS are the following: (i) FB scheme is also a two levels time-stepping scheme but much simpler than a RK3. Its implementation will prepare all the required environment for the RK3 scheme, while been much easier to develop and test. (ii) FB scheme may be a valuable alternative for low resolution simulations (in particular ORCA 2° and 1°) which do not necessarily need a high order time-stepping scheme, although FB scheme requires to used UBS scheme on advection of both dynamics and tracers. In practice the work is to : start from the trunk, updated with HPC09 branch (i.e. ZDF modifications) update with the ICEMODEL branch (i.e. new sea-ice model) Postponed to 2018 workplan: create RK3_SRC directory a a copy of OPA_SRC implement the FB scheme in a stp_fbs routine (found either in step.F90 or in stpfbs.F90), i.e. changes in most of SBC, TRA, DYN and ZDF modules implement the RK3 scheme in a stp_rk3 routine (found either in step.F90 or stprk3.F90)
Dependencies	no
Target	2017 merge party
Trac Ticket	#1911
SVN branch	dev_r7881_ENHANCE09_RK3
Previewer(s)	Names
Reviewer(s)	Names
Status	To be developed
Link	ExtractUrl(.)?

Abstract

This section should be completed before starting to develop the code, in order to find agreement on the method beforehand.

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Once the PI has completed this section, he should send a mail to the previewer(s) asking them to preview the work within two weeks.

Preview

Since the preview step must be completed before the PI starts the coding, the previewer(s) answers are expected to be completed within the two weeks after the PI has sent his request.
For each question, an iterative process should take place between PI and previewer(s) in order to reach a "YES" answer for each of the following questions.

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Once all "YES" have been reached, the PI can start the development into his development branch.

Tests

Once the development is done, the PI should complete this section below and ask the reviewers to start their review in the lower section.

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Review

A successful review is needed to schedule the merge of this development into the future NEMO release during next Merge Party (usually in November).

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Once review is successful, the development must be scheduled for merge during next Merge Party Meeting.