[MITgcm-support] non-hydrostatic pressure and KPP

[Ramón Casañas, Cintia]

Dear all,

I got the warning and found these lines about KPP and the use of the non-hydrostatic capabilities of MITgcm (see below).
I'm new with MITgcm. We are trying to run MITgcm in a small lake (2.5 km in length and 0.5 km wide) and we would like to resolve the convective cells developing during night-time cooling in the lake. The grid size that it's needed to resolve or at least partially resolve these cells (dx = dy = 5m), taking into account non-hydrostatic effects, is still coarse (compared to the grid size that would be needed for a LES), so, a turbulence closure is still needed to model the mixing.
So please, could you explain why the non-hydrostatic capabilities cannot be used with a turbulence closure scheme? Or is this only specifically related to the KPP formulation?

Thanks in advance

Cintia

________________________________

Hi Sherry,



thanks to version control, you can easily checkout old versions of the code and try to find, when (in "code-time") the instabilities occur. 2009 ended with checkpoint62 (get it with "cvs co -r checkpoint62 MITgcm", but you can also use dates, see cvs documentation, see MITgcm/doc/tag-index for record of what has been done over time). With "divide and conquer" (e.g. if 2009 code works try 2012 code, if that works 2014, if not 2010.5, etc), it doesn't take that long, but it's tedious anyway. Once you've found the point, where the model starts to have problems, you can use http://mitgcm.org/viewvc/MITgcm/MITgcm/ to look at differences.



KPP does not make too much sense together with a non-hydrostratic simulation, where you (usually) want to resolve the mixing processes, rather than parameterise them. I would take the warnings seriously.



Martin

> On 18 Aug 2016, at 07:59, Sherry <schou at hawaii.edu<http://mitgcm.org/mailman/listinfo/mitgcm-support>> wrote:

>

> Hi all,

>

> I have a 2-D nonhydrostatic configuration of an internal wave beam forced through specifying u, v, w, and temperature at the western boundary. At the moment I am trying to reproduce an experiment that was done in 2009 (Grisouard and Staquet, 2010), but for some reason I am getting instabilities after about 10 wave cycles, even though I am using the same input parameters. Could there be some changes in the model since 2009 that would affect default values? Any advice about how to track these down?

>

> To control the instabilities I am trying higher diffusion values, as well as turning on KPP. However, it seems there is some internal inconsistency with the configuration being nonhydrostatic, and I get the following warnings (from config_check.F):

>

> WARNING  CONFIG_CHECK: Implicit viscosity applies to provisional u,vVel

> WARNING  => not consistent withfinal vertical shear (after appling 3-D solver solution

> WARNING  CONFIG_CHECK: Implicit viscosity not implemented in CALC_GW

> WARNING  CONFIG_CHECK: Explicit viscosity might become unstable if too large

>

> Should I be concerned about these warnings? Is there any way around them since nonhydrostatic requires 3-D solver and KPP requires implicit viscosity?

>

> Thanks for reading!

>

> Sherry Chou

> University of Hawaii

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