[MITgcm-support] Bryan-Lewis vertical diffusivity for passive tracers

[David Wang]

Hi there,

I have been using Bryan and Lewis (1979) type spatially varying vertical
diffusivity together with pkg/kpp and pkg/gmredi for both potential
temperature and salinity in my model runs. Now I need to put in some passive
tracers (with pkg/ptracers) of the same vertical diffusivity. I wasn't sure
if passive tracers simply inherit that from potential temperature and
salinity or not. So I looked into the code, but was left confused even for
the treatment of active tracers.

The key routine to calculate vertical diffusivity with implicitDiffusion on,
I believe, is calc_3d_diffusivity.F. In this routine, Bryan-Lewis
diffusivity and pkg/kpp are apparently mutually exclusive, as Bryan-Lewis
parameters contribute KappaRTr only when trUseKPP is false. This routine is
called by both model/src/thermodynamics.F and pkg/kpp/kpp_calc.F. In the
former,

          CALL CALC_3D_DIFFUSIVITY(
     I         bi,bj,iMin,iMax,jMin,jMax,
     I         GAD_TEMPERATURE, useGMredi, useKPP,
     O         kappaRk,
     I         myThid)

which, since useKPP is true, ignores the Bryan-Lewis contribution, but take
the kpp contribution (CALL KPP_CALC_DIFF_T); while in the latter,

      CALL CALC_3D_DIFFUSIVITY(
     I        bi,bj,1-Olx,sNx+OLx,1-Oly,sNy+OLy,
     I        GAD_SALINITY, .FALSE., .FALSE.,
     O        KPPdiffKzS(1-Olx,1-Oly,1,bi,bj),
     I        myThid)

which, since the flag is hard-coded false, takes the Bryan-Lewis
contribution, but ignores the kpp contribution.

Now it's unclear to me how different contributions to the total vertical
diffusivity add up (even though for the kpp model output, I believe the
model correctly adds the kpp contribution upon the Bryan-Lewis background
values). Without this understanding, I'm not sure I can implement
Bryan-Lewis diffusivity for passive tracers if it's not readily available.

Any clarification and explanation appreciated!

David




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