[MITgcm-support] how to initialize a 2D geostrophic flow

[jklymak at uvic.ca]

Hi Li,

This looks about correct.  I'd expect that if you looking at the velocity
in the E-W direction  (assuming this is N/S you have plotted) you will
have a strong flow to the east near the surface after about 7 h as the
Coriolis effects starts to act on your un-balanced surface flow.  It will
set up a geostrophic flow relatively quickly, but you'll have a surface
wave rattling around for a long time.  Also you see that you lose your
initial condition because you are not in perfect geostrophic balance. 
Some kinetic energy has to be lost to make the potential energy of the
geostrophically balanced surface slope.

I would expect spindown to be very slow in this simulation unless you have
quite high A_z or C_d.
i.e. hundreds of years if k_z =10^{-4} or so.

Cheers,  Jody

> Hi Li,
>
> I'm a little confused regarding the setup.  Is there any forcing or
> do you want to do a spin-down problem?  And dt = 15s is extremely
> small considering dx = 5km.
>
> I guess I dont understand what you are trying to model...
>
> -Matt
>
>
>
>
> On May 16, 2009, at 5:23 PM, q li wrote:
>
>> Hi Jody and Matt,
>>
>> I am sorry for the late to show you the result. There was a
>> computer problem. I am running it now.
>>
>> Here it the first run: use geopotential to get meridional velocity
>> (v) but with zero initial Eta. Figure are in http://
>> www.po.gso.uri.edu/~qli/picST0.zip (~4.5Mb).
>>
>> Here is the configuration:
>>
>> 2D in a x-z domain
>> Vertical: 10m*300 grids
>> Horizontal: dx changes from 5km to 6km in the farfield, 512 grids.
>> ~2400 km total.
>> The temperature is from Levitus data, and the salinity is constant.
>> Flat bottom.
>> Velocity v is calculated from the geopotential method. A jet is in
>> the center.
>> dt=15 s. Output is every 3 hours. 5 day run.
>> Latitude: 21
>>
>> There is an oscillation. The estimated period is ~33 hr, "exactly"
>> equal to the inertial period at 21N. I am not sure if it is caused
>> by that I didn't set up initial surface elevation Eta.
>>
>> I am trying to set up the initial Eta to see any different or
>> improvement.
>>
>> Another question is that, if I have a topography featuer, for
>> example, a sill in the center, how I should get the geotrophic flow
>> from geopotential or thermal wind. The more basic question is how I
>> should set up the reference level (v=0).
>>
>> Li
>>
>>
>>
>>
>>
>> From: Matthew Mazloff <mmazloff at mit.edu>
>> To: mitgcm-support at mitgcm.org
>> Sent: Saturday, April 25, 2009 3:30:43 PM
>> Subject: Re: [MITgcm-support] how to initialize a 2D geostrophic flow
>>
>> Hi Jody,
>>
>> is this true -- I thought eta is solved for in first time step to
>> make flow non-divergent?  I need to check this?
>>
>> -Matt
>>
>>
>> On Apr 24, 2009, at 10:57 AM, Klymak Jody wrote:
>>
>>>
>>> Make sure that you set the sea surface eta properly too or you'll
>>> have some pretty nasty adjustment.  I think it is "pSurfInitFile".
>>>
>>> Cheers,  Jody
>>>
>>>
>>>
>>> --
>>> Jody Klymak
>>> http://web.uvic.ca/~jklymak/
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> _______________________________________________
>>> MITgcm-support mailing list
>>> MITgcm-support at mitgcm.org
>>> http://mitgcm.org/mailman/listinfo/mitgcm-support
>>
>>
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