[MITgcm-support] [EXTERNAL] Warm start and cold start yielding same results

[Martin Losch]

This is the relevant section in the manual:

https://mitgcm.readthedocs.io/en/latest/phys_pkgs/obcs.html#obcs-balance-flow

you need to decide, what you want to do. If you have a net inflow through one boundary and a net outflow through another one (i.e. some large scale current going through your domain), then set -1 on each boundary will make this current go away (or a least look very strange). I am not familiar with the circulation pattern in your domain, so you’ll have to figure it out yourself. Typically, one would have 0 everywhere (no correction) and 1 on one of the boundaries (which then balances the flow through all other open boundaries). How you select which boundary should have which, is up to your judgement of the circulation at hand.

M.



> On 2. Nov 2021, at 10:44, kunal madkaiker <kunal.madkaiker02 at gmail.com> wrote:
> 
> Thank you Dimitris and Martin.
> 
> @Martin, I went through the documentation and tried setting each boundary (E,W,S) OB_balance as -1, so that it would balance the net flow at individual boundaries.
> What the OBCS_balanceFac* should be set? Could you please advise me?
> 
> Thanks
> Kunal
> 
> 
> On Tue, Oct 19, 2021 at 11:47 PM Dimitris Menemenlis <menemenlis at jpl.nasa.gov> wrote:
> Just for fun, this cold/warm start exercise is one of the very first study that I carried out using the MITgcm:
> https://journals.ametsoc.org/view/journals/atot/14/6/1520-0426_1997_014_1420_loaogc_2_0_co_2.xml
> 
> Dimitris
> 
> 
>> On Oct 19, 2021, at 5:58 AM, Martin Losch <Martin.Losch at awi.de> wrote:
>> 
>> Kunal,
>> 
>> basic ocean circulation theory tells us that density gradients drive the flow and that the dominant large scale mode is geostrophic. So it is not surprising that your initial hydrography immediately leads to a geostrophic balance, which is more or less independent of the initial conditions.
>> 
>> As for obcs balancing, please refer to the documentation. You options do not appear useful for the problem at hand.
>> 
>> Martin
>> 
>>> On 12. Oct 2021, at 14:02, kunal madkaiker <kunal.madkaiker02 at gmail.com> wrote:
>>> 
>>> Dear All,
>>> 
>>> I am trying to setup the physical model in north Indian Ocean in order to study the mesoscale eddies, using warm start as well as cold start.
>>> 
>>> In warm start, Initial conditions of T,S come from WOA18 and U,V from SODA. Boundary conditions of T,S,U,V are from SODA and ext. forcing from ERA5. 
>>> For cold start, setup is same except initial velocities are zero. Timestep is 120s and spatial resolution is ~5km. I am running model for 30 days as of now (doing monthly climatological simulations). Lateral boundary is from west, east and south.
>>> 
>>> Both the simulations: warm and cold start is yielding the same results in terms of current, temperature etc. and I am not seeing any significant effect of the initial velocities in the warm start simulation. My understanding is that somehow the boundary conditions are nullifying the effect of initial velocities, but I am not understanding why that might happen.
>>> 
>>> Kindly provide some suggestions. Let me know if anything is wrong from my end.
>>> 
>>> Also, I would appreciate some help in setting the OBCS balance values. Currently, I am balancing individual boundaries by giving value as -1. The flow pattern in my domain is such that, during summer monsoon (JJAS) water flows from Arabian sea to bay of bengal (west to east) and vice versa during winter monsoon (OND).
>>> 
>>> Attaching some plots and my namelists. Apologies for the long email.
>>> 
>>> Thanks and Regards
>>> Kunal
>>> <Cold_UV.png><Comp_warm_coldstart_SST.png><Warm_UV.png><data><data.exf><data.obcs>_______________________________________________
>>> MITgcm-support mailing list
>>> MITgcm-support at mitgcm.org
>>> http://mailman.mitgcm.org/mailman/listinfo/mitgcm-support
>> 
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