Opened 4 years ago

Closed 18 months ago

#1713 closed Bug (wontfix)

Bugs in split explicit diffusion

Reported by: jchanut Owned by: jchanut
Priority: low Milestone:
Component: OCE Version: release-3.6
Severity: minor Keywords:
Cc:

Description (last modified by nicolasmartin)

Description

Few users may be concerned since implicit diffusion is the default in NEMO. Nevertheless, setting ln_zdfexp=.true. (ie split explicit vertical diffusion) will inevitably give wrong results.

Analysis

1) Surface boundary condition (i.e. wind stress) is wrong.
2) (Explicit) bottom friction is not implemented in sub-stepping.
A 1d simulation of wind forced ekman dynamics in shallow water emphasizes the problem (see attached plot).

1d experiment of shallow water Ekman problem. 10 meter depth, 41 levels, viscosity=13 cm2/s, latitude=45°N, westerly wind stress =0.04 N/m2, linear bottom friction=0.02m/s

After bug correction it also shows the equivalence between implicit and split-explicit formulations. Small deviations from theory may come from the approximate no-slip bottom boundary conditions in the model.

Recommendation

Do not use split explicit diffusion for the time being unless you neither have wind nor bottom stresses…

Commit History (0)

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Attachments (1)

NEMO_zdf_ekman_shallow.jpg (147.7 KB) - added by jchanut 4 years ago.
1d experiment of shallow water Ekman problem. 10 meter depth, 41 levels, viscosity=13 cm2/s, latitude=45°N, westerly wind stress =0.04 N/m2, linear bottom friction=0.02m/s

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Change History (6)

Changed 4 years ago by jchanut

1d experiment of shallow water Ekman problem. 10 meter depth, 41 levels, viscosity=13 cm2/s, latitude=45°N, westerly wind stress =0.04 N/m2, linear bottom friction=0.02m/s

comment:1 Changed 3 years ago by nicolasmartin

  • Owner changed from nemo to jchanut
  • Status changed from new to assigned

comment:2 Changed 3 years ago by nicolasmartin

  • Description modified (diff)

comment:3 Changed 3 years ago by nicolasmartin

  • Milestone Miscellaneous deleted
  • Severity set to minor

comment:4 Changed 2 years ago by jchanut

  • Version changed from trunk to release-3.6

comment:5 Changed 18 months ago by jchanut

  • Resolution set to wontfix
  • Status changed from assigned to closed
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