#1832 closed Defect (fixed)
TKE source for vertical mixing must be reduced under the sea-ice (TKE turbulent closure scheme)
Reported by: | ctlod | Owned by: | gm |
---|---|---|---|
Priority: | normal | Milestone: | |
Component: | OCE | Version: | v3.6 |
Severity: | minor | Keywords: | |
Cc: | gm |
Description (last modified by nemo)
Context
2 options in zdftke.F90 module are concerned:
- TKE due to surface and internal wave breaking (namzdf_tke: nn_etau = 1 in the ocean namelist)
- TKE source term due to Langmuir circulation (namzdf_tke: ln_lc = True in the ocean namelist)
Analysis
When using these 2 TKE sources or one of them, by default each of them is weighted by ( 1. - fr_i(:,:) );
so as soon as the sea-ice fraction is strictly below 1, which is the case almost everywhere under the
sea-ice, the vertical mixing will be slightly increased leading to a seasonal cycle on both temperature
and salinity under sea-ice; this should not occur since:
- for a Langmuir circulation to develop, the surface ocean should be directly or partially in contact with the wind,
- mixing induced by waves could occur under sea-ice but "relatively close" to the MIZ, while the "relatively close" term is still not clear and is an ongoing research topic.
Fix
Reduce the threshold so these 2 TKE sources will be active only when sea-ice fraction will be lower than 25%.
This is an ad hoc choice which does not rely on either observations nor a specific study.
So it has to be adjusted more precisely later.
Proposed modification in zdftke.F90 module:
Use '''( MAX(0.,1._wp - 4.*fr_i(ji,jj) ) )''' instead ( 1._wp - fr_i(ji,jj) ) weight.
For the surface and internal wave breaking:
IF( nn_etau == 1 ) THEN !* penetration below the mixed layer (rn_efr fraction) ..... en(ji,jj,jk) = en(ji,jj,jk) + rn_efr * en(ji,jj,1) * EXP( -fsdepw(ji,jj,jk) / htau(ji,jj) ) '''* ( MAX(0.,1._wp - 4.*fr_i(ji,jj) ) )''' * wmask(ji,jj,jk) * tmask(ji,jj,1) .....
For the Langmuir circulation parameterization:
en(ji,jj,jk) = en(ji,jj,jk) + rdt * '''( MAX(0.,1._wp - 4.*fr_i(ji,jj) ) )''' * ( zwlc * zwlc * zwlc ) / zhlc(ji,jj) * wmask(ji,jj,jk) * tmask(ji,jj,1)
Commit History (1)
Changeset | Author | Time | ChangeLog |
---|---|---|---|
7785 | cetlod | 2017-03-10T17:29:27+01:00 | Bugfix on TKE vertical mixing under sea-ice, see ticket #1832 |
Change History (7)
comment:1 Changed 8 years ago by jchanut
comment:2 Changed 8 years ago by ctlod
Just to make things clear: the effect of the bug is more obvious in the seasonal cycle it creates for the vertical mixing than T or S. The problem is that it creates a seasonal cycle of the vertical heat flux (Kz *dT/dz) that results in an artificial dominant timescale (seasonal) for the interaction between the ocean and the sea ice.
comment:3 Changed 8 years ago by clevy
- Owner changed from nemo to rbourdal
comment:4 Changed 7 years ago by gm
- Owner changed from rbourdal to gm
- Type changed from Bug to Defect
comment:5 Changed 7 years ago by clevy
- Cc gm added
- Status changed from new to assigned
comment:6 Changed 7 years ago by gm
- Resolution set to fixed
- Severity set to minor
- Status changed from assigned to closed
This ad hoc fix has been add to the 2017 merged version (branch: dev_merge_2017), and so will be available in the forthcoming versions.
In the v3.6_stable, a less restrictive factor is used : MAX(0.,1._wp - 2.*fr_i(ji,jj) ) is used (see revision 7785).
comment:7 Changed 7 years ago by nemo
- Description modified (diff)
one more thing:
one has to revert to a constant surface roughness below ice if ln_mxl0=T (in place of Charnock formula).