[MITgcm-support] GMRedi parameterization

[Ryan Abernathey]

>
> I'm not sure about the best combination of diffusivities defined between
> data and data.gmredi files.
>

Don't worry, no one is. First, make sure you have read the manual:
http://mitgcm.org/public/r2_manual/latest/online_documents/node240.html


> For climate simulation, in z-coordinate model (such as MITgcm), diffusion
> should be primarily along isopycnal rather than horizontal.  Therefore, I
> guess diffusivities defined in data file (e.g., diffKh and diffKr) should
> be set to zero to prevent spurious diapycnal mixing.
>

You are correct about diffKh. However diffKr is the the vertical
diffusivity, which is effectively a diapycnal diffusivity. So you put your
desired diapycnal diffusivity into diffKr.


> GMRedi package is used in MITgcm to diffuse tracer in local isopycnal
> coordinates through some kind of rotation.  However, I cannot find
> definitions of isopycnal and diapycnal diffusivities in data.gmredi file.
> In the test case of tutorial_global_oce_latlon, there is a GM_isopycK,
> which is commented, that may be isopycnal diffusivity but no diapycnal
> definitions (or even a ratio between them).  There is also a
> GM_background_K in data.gmredi which I don't understand why we need it.
>

You are correct that GM_isopycK is the isopycnal (Redi) diffusivity.
GM_background_K is the "Gent McWilliams coefficient" (K_GM in the
documentation), which regulates the strength of the advective part of the
eddy parameterization. It is the single most important parameter in the
package.


> Besides, does the GMRedi scheme apply to the viscosities for momentum?
>

No. There are other options for viscosity though.
http://mitgcm.org/public/r2_manual/latest/online_documents/node86.html

GM sheme is proposed to parameterize effects of geostrophic eddies in
> non-eddy resolving model.  If the model is eddy resolving, do we still need
> to include this scheme?
>

If the model is truly eddy resolving, you should not use GM. If the model
is "eddy permitting," it is a tough call. You should read this paper by Bob
Hallberg.
http://www.sciencedirect.com/science/article/pii/S1463500313001601
Unfortunately MITgcm does not implement any of the fixes proposed by
Hallberg.


>
>
> ------------------
> Best regards
>
> Yu-Kun Qian (钱钰坤)
> Center for Monsoon and Environment Research
> Department of Atmospheric Sciences
> School of Environmental Science and Engineering
> Sun Yat-sen University
> No. 135 Xingang West Road, Haizhu District
> Guangzhou, 510275, P.R. China
> Tel; 020-84115227
> Email: qianyk at mail3.sysu.edu.cn <qianyk at mail2.sysu.edu.cn>
>
>
>
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