[MITgcm-support] Momentum Budget

[Emily Shroyer]

Thanks Matt,
For some reason things still are not adding up quite right. The balance 
is almost perfect, until I turn on wind forcing then the sum of terms 
differs significantly from the total tendency.  I attached a figure of 
the zonal balance at one grid cell as a function of time. Winds turn on 
around day 12, the black line and the red dashed line would line up if I 
was including everything correctly. This is one reason I thought that 
the vertical viscosity term might be missing, i.e., maybe I am not 
accounting for the flux of momentum from the surface properly. The other 
is that if I evaluate this term (nu*d^2u/dz^2) on my own, the difference 
is roughly correct. I tried manipulating the VISrE_Um and VISrI_Um terms 
but I am not sure exactly how these are scaled, the units appear to be 
in m^4/s^2, so surface area alone is not going to work. I will likely 
just return to estimating a momentum budget from the normal output 
fields, a little messy and it won't balance perfectly, but at least I 
know what it does. Thanks again, Emily


Matthew Mazloff wrote:
> Hi Emily,
>
> I believe there are a few tricks to this
>
> Disclaimer -- this info may be out of date or not apply to your 
> specific setup....
>
>
> I think the tendency term is in units per day, not per second, so 
> try rescaling from m/s/d to m/s^2 by dividing by 86400
>
> For vector invariant momentum formulation (not sure about flux 
> form) Um_Advec includes coriolis
>
> I believe Um_dPHdx does not include pressure forcing from SSH 
> gradient, you need to add that in
>
> Note that UBotDrag and USidDrag should be in Um_Diss
>
> If you want to look at the implicit vertical viscous flux (VISrE_Um) 
> then I believe this has units m^3/s^2 and so needs to be rescaled by 
> surface area ( divide by RAW and RAS for zonal, meridionaly momentum 
> balance respectively) 
>
>
> Hope this helps -- or at least gives you some ideas...
>
> -Matt
>
>
>
> Request to developers...maybe the momentum diagnostics could be 
> cleaned up?
>
>
>
>
> On Dec 7, 2010, at 1:46 PM, Emily Shroyer wrote:
>
>> I would like to output terms of the momentum budget using the diagnostic
>> package. My data.diagnostic file is set as:
>>
>> &diagnostics_list
>>  diag_mnc     = .TRUE.,
>>  frequency(1) = -43200.,
>>  filename(1) = 'UmomDiag',
>>  fields(1,1) = 'TOTUTEND','UBotDrag ','USidDrag
>> ','Um_Diss','Um_Advec','Um_Cori',
>>                'Um_dPHdx','Um_Ext','Um_AdvZ3','UDIAG1  ',
>>
>>  frequency(2) = -43200.,
>>  filename(2) = 'VmomDiag',
>>  fields(1,2) = 'TOTVTEND','VBotDrag ','VSidDrag
>> ','Vm_Diss','Vm_Advec','Vm_Cori',
>>                'Vm_dPHdy','Vm_Ext','Vm_AdvZ3','UDIAG2  ',
>>
>> &
>>
>> The terms Udiag1/2 were added to account for gradients in the sea
>> surface height. Unfortunately, the total tendency is not quite equal to
>> the sum of the other terms. I think that difference may be associated
>> with the implicit diffusivity term? I am using a KPP mixing scheme,
>> although I have also tried a MY scheme. I spent  a bit of time trying to
>> decide where best to output this term with a user defined diagnostic
>> without any result. My questions are 1) Is it likely that this is indeed
>> the term I am missing in the momentum budget or have a missed something
>> else? 3) If so, does anyone have a suggestion as to where best to output
>> this term with the user diagnostic?
>>
>> Thanks for any advice, Emily
>>
>> -- 
>> Emily Shroyer, Postdoctoral Scholar
>>
>> Woods Hole Oceanographic Institution
>> Clark 316A, MS #21
>> Woods Hole, MA 02543
>>
>> phone: 508 289 2525
>>
>>
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-- 
Emily Shroyer, Postdoctoral Scholar

Woods Hole Oceanographic Institution
Clark 316A, MS #21
Woods Hole, MA 02543

phone: 508 289 2525

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