Opened 3 years ago

Last modified 2 years ago

#1812 assigned Defect

Surface stresses at wrong time step in zdftke/zdfgls

Reported by: jchanut Owned by: rbourdal
Priority: low Milestone:
Component: OCE Version: trunk
Severity: Keywords: GLS TKE mixing
Cc: rbourdal

Description

Context

Gls and tke vertical mixing schemes are discretized in time thanks to backward Euler time stepping (over rdt), but shifted one time step in the past. Hence turbulent quantities are integrated from "kstep-1" to "kstep" leapfrog time steps. This leads to "now" diffusivities/viscosities at kstep. However, surface stresses are obtained from taum which is the surface stress module at kstep+0.5. It should be taken one step in the past at kstep-0.5. I encountered some instabilities due to that.
There is a similar problem with bottom stresses, taub being computed from "now" bottom friction parameter. This is mitigated by now velocities entering the stress which is in agreement with implicit bottom friction.

Analysis


Fix

Would be far better if tke/gls schemes would be integrated from now to after time steps. Then taum would be at the right time step and this would greatly simplify interaction with AGRIF. Sticking with the current implementation, simply recomputing squared surface velocity scales from before stresses could solve the problem.

This said, and in contrary with what is stated in the documentation, I don't think that mixing a backward Euler time stepping over rdt with backward Euler time stepping over 2 rdt does give the correct energetic transfer (through vertical shear) from momentum equations to TKE/GLS schemes.

Commit History (0)

(No commits)

Change History (5)

comment:1 Changed 3 years ago by nemo

  • Keywords GLS added; gls removed

comment:2 Changed 3 years ago by nemo

  • Keywords TKE added; tke removed

comment:3 Changed 2 years ago by clevy

  • Owner changed from jchanut to gm

comment:4 Changed 2 years ago by clevy

  • Cc rbourdal added
  • Owner changed from gm to rbourdal
  • Status changed from new to assigned

comment:5 Changed 2 years ago by nicolasmartin

  • Type changed from Bug to Defect
  • Version changed from nemo_v3_6_STABLE to trunk
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