[MITgcm-support] how to output vertical diffusivity？

[Martin Losch]

Hi,

in short, GMRedi (with your parameters) computes a diffusive flux along isopycnals and wherever these isopycnals are inclined this also leads to a projection of the diffusive flux onto the vertical. So yes, the vertical diffusive flux changes. The diagnostics DFrE_TH, DFrI_TH give you the explicit and implicit part of the diffusive flux. The explicit part is the one that is used in computing the divergence of fluxes in gad_calc_rhs.F and should include the gmredi part (if I am not mistaken). The implicit part is diagnosed after it has been applied. Both fluxes contain the grid cell area so you could estimate the diffusivity by F/grad_z(theta)/RAC (where RAC is the grid cell area).

The same field diffKr is used to compute the total diffusivity for both temperature and salinity. If your salinity diffusive fluxes are zero, then it could mean that either you gradients are zero (constant salinity?), or that you are not even stepping salinity (saltStepping = .False.). What does your namelist file “data” look like?

Martin

> On 5. Sep 2019, at 03:38, 李 家宁 <li.jia.ning at outlook.com> wrote:
> 
> Hello everyone,
> 
>   
> 
>   I’m running an ocean model with a spatial varying vertical diffusivity field, by setting “diffKrFile=diffKr.bin”. Meanwhile, The KPP package is disabled and the GMRedi (code is attached below) package is enabled.
> 
>   I wonder if the vertical diffusivity field would change with the model run, or it just keeps being its preset value. And how to output the vertical diffusivity field?
> 
>   I tried to look into the diffusive flux (DFrE_TH, DFrI_TH, DFrE_SLT and DFrI_SLT) produced by diagnostics package, but they have no pattern of the preset diffusivity. Is there any way to calculate the diffusivity by diffusive flux (As the unit of diffusive flux is ℃·m^3·s, it seems the diffusivity cannot be calculated by k = F/grad_z(theta))? Besides, I noticed the DFrE_SLT and DFrI_SLT are all zeros. Does that mean the salinity field has no diffusion? If so, how to set the salinity diffusivity?
> 
>  
> Many thanks for any advice!
> 
>  
> Jianing Li
> 
>  
> data.gmredi:
> 
> # GM+Redi package parameters:
> 
>  
> #-from MOM :
> 
> # GM_background_K:      G & Mc.W diffusion coefficient
> 
> # GM_maxSlope    :      max slope of isopycnals
> 
> # GM_Scrit       :  transition for scaling diffusion coefficient
> 
> # GM_Sd          :       half width scaling for diffusion coefficient
> 
> # GM_taper_scheme: slope clipping or one of the tapering schemes
> 
> # GM_Kmin_horiz  : horizontal diffusion minimum value
> 
>  
> #-Option parameters (needs to "define" options in GMREDI_OPTIONS.h")
> 
> # GM_isopycK     : isopycnal diffusion coefficient (default=GM_background_K)
> 
> # GM_AdvForm     :      turn on GM Advective form       (default=Skew flux form)
> 
>  
>  &GM_PARM01
> 
>   GM_background_K    = 1.e+3,
> 
>   GM_taper_scheme    = 'gkw91',
> 
>   GM_maxSlope        = 1.e-2,
> 
>   GM_Kmin_horiz      = 0.,
> 
>   GM_Scrit           = 4.e-3,
> 
>   GM_Sd              = 1.e-3,
> 
>  &
> 
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