[MITgcm-support] Zonal velocity bands

[Neil Swart]

The zonal velocity bands problem can be easy recreated in the verification experiment: tutorial_baroclinic_gyre.

In the default case with stratification, everything goes as expected. If however you set all T,S values to a constant, the velocity bands occur in the surface layer.

This occurs with the default wind forcing, that is zero on the southern boundary. It also happens regardless of whether you have a no-slip condition on the sidewalls. 
I believe this means the problem is not vigorous upwelling in the south of the domain as Jason suggested. 

Using time averaged fields makes no difference to the pictures.

Does anyone have any ideas/suggestions on how to sort this out? 

These are some images of how it develops in time:







Thanks in advance,
Neil


---------------------------------------------------
Neil Swart

Climate Modelling Group
School of Earth and Ocean Sciences
University of Victoria
Victoria, BC
Canada

http://climate.uvic.ca/

On Mar 26, 2010, at 12:03 PM, Neil Swart wrote:

> Hi Jason,
> 
> If I check  advcfl_wvel_max, it remains low. I have also tried to rectify the problem by tapering the windstress near the boundaries, or setting it to zero. A strong tapering does help, but in general does not solve the problem. I will try and pursue this more.
> 
> (PID.TID 0011.0001) %MON advcfl_wvel_max              =   9.5183183632057E-05
> 
> thanks,
> Neil
> 
> -----------------------------------
> Neil Swart
> Climate Modelling Group
> University of Victoria
> Victoria, BC, Canada
> 
> http://climate.uvic.ca/
> 
> On 26-Mar-10, at 11:20 AM, Jason Goodman wrote:
> 
>> On Mar 26, 2010, at 1:57 PM, Neil Swart wrote:
>> 
>>> I'm not sure I understand your comment on periodicity for the forcing.
>> 
>>> tauMax=0.1;
>>> x=((1:nx)-0.5)/(nx-1); % nx-1 accounts for a solid wall
>>> y=((1:ny)-0.5)/(ny-1); % ny-1 accounts for a solid wall
>>> [X,Y]=ndgrid(x,y);
>>> 
>>> tau=tauMax*sin(1.2*pi*Y);
>> 
>> With a nonzero zonal velocity at the southern boundary, you'll get a northward Ekman flow at the south wall: by continuity, this can only be balanced by a ferocious vertical upwelling.  I'm guessing you're getting some sort of numerical instability as a result.
>> 
>> Try checking the vertical advective CFL number in the monitor output (advcfl_wvel_max).  If it's a large fraction of 1, you're in trouble.
>> 
>> 
>> 
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