[MITgcm-support] Vertical diffusion for non-hydrostatic ocean model with a stretched grid

[Jean-Michel Campin]

Hi Paul,

There is some confusion about Smagorinsky in MITgcm since we have 2 implementations:
1) a Smagorinsky like horizontal viscosity that is function of horizontal shear,
  with the choice of harmonic (viscC2smag) or bi-harmonic (viscC4smag) viscosity.
2) a simple 3-D Smagorinsky harmonic viscosity that work best when the grid is
  near isotropic (mostly for Non-Hydrostatic simulation) and does not acount for
  stratification. 
  Need ALLOW_SMAG_3D_DIFFUSIVITY to be defined and uses run-time parameter: 
   useSmag3D = .TRUE., with smag3D_coeff,
  see experiment: tutorial_deep_convection, with input.smag3d/data

You may want to try this second implementation.
Alternatively, you could always set a vertical profile of (background) 
vertical viscosity using Nr values to set vector "viscArNr" in main parameter
input file "data" (note that the fist value corresponds to the very top, 
i.e., surface, vertical index k=1, and has no effect).

Cheers,
Jean-Michel

On Wed, Feb 01, 2023 at 12:55:22PM +0000, Burns, Paul wrote:
> Hi,
> 
> I was hoping that someone could give me some advice about the best way to define the vertical viscosity (and diffusivity) in MITgcm for a non-hydrostatic model using a stretched vertical grid.  I've used a stretched vertical grid to resolve very small features of interest (~0.2 m) in the upper ocean and to keep the simulation practical.
> 
> The viscosity should vary with the grid resolution (e.g., Smagorinsky, 1993), however, the MITgcm documentation says that the Smagorinsky scheme isn't implemented in the vertical. So would a reasonable approach be to compute a vertical profile of viscosities using a vertical Reynolds number and use that to set viscArNr?
> 
> 
> Many thanks,
> Paul
> 
> 
> 
> 
> Dr Paul Burns MInstP FRMetS
> 
> Research Fellow
> 
> Geophysical and Astrophysical Fluid Dynamics Group
> 
> Department of Mathematics
> 
> Harrison Building
> 
> University of Exeter
> 

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