VALID-02_GeorgeN-evalOSMOSIS

[acc]

Summary

Action	${ACTION_NAME} Subject
PI(S)	George Nurser
Digest	The OSMOSIS surface boundary layer scheme introduced in 2017 was tested in fully global configurations in 2018 and some issues uncovered in some circumstances (for example in places where the mixed layer extends to the seabed). Improvements to the parameterisation have been implemented and will be retested in 2019.
Dependencies
Branch	NEMO/branches/2019/dev_r11078_OSMOSIS_IMMERSE_Nurser
Previewer(s)
Reviewer(s)
Wiki	wiki:2019WP/VALID-02_GeorgeN-evalOSMOSIS

Abstract

The OSMOSIS OBL code work well in summer conditions, but gives too-deep mixed-layers in winter. It is also written assuming a linear equation of state, and has not yet been applied to passive tracers.
Under-ice behaviour has not been checked. All these issues need to be addressed.

Description

he OSMOSIS OBL code has shown promise in giving more realistic, deeper, mixed layers (MLs) in the Southern Ocean in the southern summer. However there are various issues that must be settled before it can be used as an OBL code for global integrations.

1. ML are too deep in winter, especially over the ACC in southern winter. This problem is being addressed by a) taking account of the effect of the Coriolis force on the turbulence when the eddy-turnover scale approaches the inertial timescale and b) taking account of the lesser effectiveness of Langmuir turbulence in driving mixing when the ML depth becomes very deep relative to the depth to which the Stokes drift penetrates.
   

2. Non-local fluxes of passive tracers need to be defined.
   

3. The non-linearity of the equation of state needs to be better taken account of.
   

4. The behaviour of the model under ice requires detailed examination.
   

5. The code has been written for simplicity and ease of understanding rather than performance, and needs to be optimized.

Implementation

Changes will mostly be restricted to the zdfosm.F90 module.

Namelists will need also to be modified.

New variables are not fully known yet.

Reference manual and web pages updates

Chapter will be written for reference manual and text for the user guide

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.

[agn]

23 May. Added description of work on this ticket and progress as of 23 May 2019
Branch will be placed in public NEMo repository in early June
--George Nurser

[smueller]

In 12928:

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[smueller]

In 12934:

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[acc]

Osmosis improvements arising from this work were merged onto the trunk at changeset:14045 . Some further code restructuring is planned for the 2021WP but this ticket can be closed.