[MITgcm-support] Extracting the non-Ekman component of vertical velocity

[Uchida Takaya]

Hi Martin,


Thank you very much for the clarification.

I understand that the model doesn’t differentiate dynamical regime components and simply solves the full governing equations of dynamics and thermodynamics but because the vertical velocity at Nr has relatively large values, I have been wondering whether I am missing processes other than the Ekman velocity due to the bottom friction and surface wind stress.
I’ve been trying to calculate the bottom Ekman velocity using bottom friction following the MITgcm code (https://github.com/MITgcm/MITgcm/blob/master/pkg/mom_common/mom_v_bottomdrag.F <https://github.com/MITgcm/MITgcm/blob/master/pkg/mom_common/mom_v_bottomdrag.F>) but the root-mean square of my estimated Ekman velocity is orders of magnitude smaller than the “true” vertical velocity I get as the model output at Nr.

The overall motivation here is that I am inverting the quasi-geostrophic Omega equation and I would like to remove all components that are not included in quasi geostrophy from the “true” vertical velocity.


Best,
Takaya Uchida
———————
PhD Candidate
Physical Oceanography
Columbia University in the City of New York
https://roxyboy.github.io/

> On Aug 9, 2018, at 5:32 AM, Martin Losch <Martin.Losch at awi.de> wrote:
> 
> Hi Takaya,
> 
> the vertical velocity in the C-grid model is at the top of the k-th grid cell (see model documentation and header of GRID.h for details). The vertical velocity at the bottom of a water colum is alway zero (no flow througth the bottom). Therefore for a flat-bottom domain with no topography the vertical velocity at Nr+1 is always assumed to be zero. In fact, this layer Nr+1 does not exist, that why it cannot be included in output. I am not sure if I understand your question about Ekman and non-Ekman components. The model does not differentiate between different dynamical regime components.
> 
> Hope that helps,
> Martin
> 
>> On 31. Jul 2018, at 04:27, Uchida Takaya <tu2140 at columbia.edu> wrote:
>> 
>> Dear MITgcm support.
>> 
>> I currently have the outputs for a flat-bottom zonally re-entrant channel model. Since it's a flat bottom, the vertical velocity at the bottom should be zero. 
>> The issue I’m facing is that MITgcm only outputs the vertical velocity down until to the grid point right above the very bottom, which has relatively large root-mean square values (figure attached below for a snapshot of the vertical velocity).
>> My current guess is that the vertical velocity goes to zero at the bottom in the calculation from Ekman contributions. Is there a simple and numerically consistent way to parse out the non-Ekman component of vertical velocity?
>> 
>> Thank you very much for your support in advance.
>> 
>> Best,
>> Takaya
>> ———————
>> PhD Candidate
>> Physical Oceanography
>> Columbia University in the City of New York
>> https://roxyboy.github.io/
>> 
>> <Screen Shot 2018-07-30 at 10.17.45 PM.png>
>> 
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