[MITgcm-support] Anamalous Decrease in Sea Surface Level and SPONGE Layer

[Martin Losch]

Himansu,

the most likely cause for "loosing sea level" with obcs is an imbalance of the horizontal velocities through the boundaries. To convince yourself, compute the fluxes from averaged output of UVEL*DYG*DR*HFACW or /VVEL*DXG*DR*HFACES. The divergence of those should match your sea level change.
With Orlanski BCs you cannot prescribe velocities so that the fluxes are correct. I suggest that you use the balancing option described here:
<http://mitgcm.org/public/r2_manual/latest/online_documents/node236.html#SECTION00731560000000000000>

M.

On Sep 21, 2012, at 9:42 PM, Himansu Pradhan wrote:

> Sorry for a belated answer.
> 
> i too also confused why i am LOOSING sea level. Except the values for "Eta" i am getting a good circulation , temperature and salinity fields using Orlanski boundary condition. 
> 
> i am attaching my "input" and "code" directory for checking and suggesting me anything to move forward. 
> 
> I am trying the suggestation regarding SPONGE layer. 
> 
> Thank you
> 
> 
> 
> On Thu, Sep 20, 2012 at 10:51 PM, Jody Klymak <jklymak at uvic.ca> wrote:
> 
> Himansu,
> 
> Not sure why you are losing sea level...
> 
> For a sponge, you need the spatial extent to be at least a large fraction of a mode-1 tidal wavelength.  Unless you are in shallow water, 40 km is not going to do it.  Second, your time constants are far too long to absorb internal waves.  They will bounce off that boundary no problem.  Use timescales that are closer to an hour at the outer boundary and 12 h at the inner side.
> 
> I do something like
> 
> # Sponge layer parameters
> &OBCS_PARM03
> Urelaxobcsinner=500.,
> Urelaxobcsbound=500.,
> Vrelaxobcsinner=500.0,
> Vrelaxobcsbound=500.0,
> spongeThickness=10,
> /
> 
> but my sponge is closer to 150 km wide because I telescope the grid on the boundaries.
> 
> Cheers,    Jody
> 
> On Sep 20, 2012, at  9:44 AM, Himansu Pradhan wrote:
> 
>> Hello MITgcm users,
>> 
>> I have configured MITgcm for my region (174*200*17) to look after circulation and internal waves. dx = 2.5 km and dy = 2.5km approximatly . Tides are included  into the  region through momenteum equations to take note of the thermal/density oscillations. Orlanski OBC were applied at the boundary to take care for physics/dynamics.
>> ... here starts the PROBLEM ???? their is an ANAMALOUS DECREASE in the Sea Surface Level with time. ?????
>> .
>> so finding myself NOWHERE i wanted to include SPONGE layer at the BOUNDARY .------> Is this OK ??? .( need some suggestation/help HERE ).
>> 
>> to proceed with I included :
>> #define ALLOW_OBCS_SPONGE   : ---> in ..../code/OBCS_OPTIONS.h
>> 
>> and edited "data.obcs" :
>> # Open-boundaries
>> &OBCS_PARM01
>> OBCSfixTopo=.FALSE.,
>> OB_Jnorth=174*-1,
>> OB_Jsouth=174*1,
>> OB_Ieast=200*-1,
>> OB_Iwest=200*1,
>> # useOrlanskiN/S/E/S=.FALSE.,
>> useOBCSbalance=.FALSE.,
>> useOBCSprescribe = .FALSE.,
>> useOBCSsponge=.TRUE.,
>> &
>> # Orlanski parameters
>> &OBCS_PARM02
>> Cmax=0.45,
>> cVelTimeScale=1000.,
>> &
>> # Sponge-layer parameters
>> &OBCS_PARM03
>> Urelaxobcsinner=864000,
>> Urelaxobcsbound=43200,
>> Vrelaxobcsinner=864000,
>> Vrelaxobcsbound=43200,
>> spongeThickness=15,
>> &
>> 
>> to start with. i had given : " # Sponge-layer parameters ;  &OBCS_PARM03 " .  values BLINDLY. it didn't HELP. The temperature and Circulation results were not satisfactory.
>> 
>> I have gone trough the manual , . But need a bit more explanation regarding the physics of these paramaters.
>> 
>> And supposingly if i want to run MITgcm with this current configuration for one month, ????? what should be the VALUES (of Sponge-layer parameters) to START with. ??????
>> 
>> Any Suggestation/Help is highly appreciated.
>> 
>> Thanking all in advance.
>> 
>> Himansu.
>> 
>> 
>> 
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> 
> --
> Jody Klymak
> http://web.uvic.ca/~jklymak/
> 
> 
> 
> 
> 
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