[MITgcm-support] mitgcm seaice

Fri Dec 2 09:30:41 EST 2022

Hi Mike,

the code is not very easy to read. The stress term is DWAN *  (uIce - uVel), but for stability reasons, uIce is treated implicitly. so that the equation looks like this:

(uIce^{n)-uIce^{n-1})/deltaT + DWATN * uIce^{n} = - DWATN (- uVel) + other terms
<=>
(uIce^{n)-uIce^{n-1})/deltaT + DWATN * uIce^{n} = - DWATN (- uVel) + other terms

uIce^{n} = uIce^{n-1} + deltaT * other terms / (1 + deltaT * DWATN ) ...

this is hidden in variables like denomU, betaFacP1U, etc., the sin-part needs to be fully explicity.

This is actually a little inconsistent, because the stress imparted on the ocean is then slightly different, when the momentum equations have no converged perfectly (which they never have).

Martin

> On 2. Dec 2022, at 15:06, Michael A Spall <mspall at whoi.edu> wrote:
> 
> Hi Martin,
> A followup question. Looking in seaice_evp (seaice_lsr has the same), the stress term is added to
> the rhs as:
>              FORCEY(I,J,bi,bj)=FORCEY0(I,J,bi,bj)+
>       &           ( 0.5 _d 0 * ( DWATN(I,J,bi,bj)+DWATN(I,J-1,bi,bj) ) *
>       &           COSWAT * vVel(I,J,kSrf,bi,bj)
>       &           + SIGN(SINWAT, _fCori(I,J,bi,bj)) * 0.5 _d 0 *
>       &           ( DWATN(I,J  ,bi,bj) * 0.5 _d 0 *
>       &            (uVel(I  ,J  ,kSrf,bi,bj)-uIce(I  ,J  ,bi,bj)
>       &            +uVel(I+1,J  ,kSrf,bi,bj)-uIce(I+1,J  ,bi,bj))
>       &           + DWATN(I,J-1,bi,bj) * 0.5 _d 0 *
>       &            (uVel(I  ,J-1,kSrf,bi,bj)-uIce(I  ,J-1,bi,bj)
>       &            +uVel(I+1,J-1,kSrf,bi,bj)-uIce(I+1,J-1,bi,bj))
>       &           )*locMaskV ) * areaS(I,J,bi,bj)
> 
> where DWATN comes from seaice_oceandrag_coeffs.F   by
>       &         CwatC(i,j,bi,bj) = dragCoeff*SQRT(tempVar)
> 
> My question is why does the first term in the ocean-ice stress calculated in seaice_evp
> use the ocean velocity instead of the difference between the ice and ocean velocity?
> DWATN is positive, so if vIce > vVel > 0, this would result in the ocean accelerating the
> ice even if the ice is moving faster than the water.  I see that the difference is used in
> seaice_ocean_stress but I think that only goes to modify the momentum equation for the
> top ocean layer. The difference is also used for the zonal velocity with SINWAT not zero.
> I must be missing something.
> 
> Mike
> 
> ==================================================================
> 
> Michael A. Spall
> Senior Scientist
> Clark 323A
> Department of Physical Oceanography
> Woods Hole Oceanographic Institution
> 360 Woods Hole Road MS #21
> Woods Hole, MA 02543
> 
> 508-289-3342
> mspall at whoi.edu <mailto:mspall at whoi.edu>
> https://www2.whoi.edu/staff/mspall <https://www2.whoi.edu/staff/mspall>
> 
> ________________________________________
> From: MITgcm-support <mitgcm-support-bounces at mitgcm.org <mailto:mitgcm-support-bounces at mitgcm.org>> on behalf of Martin Losch <Martin.Losch at awi.de <mailto:Martin.Losch at awi.de>>
> Sent: Monday, November 28, 2022 3:06 AM
> To: MITgcm Support
> Subject: Re: [MITgcm-support] mitgcm seaice
> 
> Hi Mike,
> 
> the non-zero drag used to be necessary to have a non-singular system matrix for the solver also where there there is no ice.
> 
> In a more recent version of the code I added a runtime parameter SEAICEdWatMin, which defaults to 0.25. This value can be set to zero, and the model should still run. After different (older) changes it is not longer required that there’s a minimum drag.
> 
> I am not sure about checkpoint 67c, but you can most likely reduce 0.25 to a much smaller value, if necessary.
> 
> Martin
> 
> 
>> On 23. Nov 2022, at 21:01, Jinlun Zhang <jlzhang at uw.edu> wrote:
>> 
>> Hi Mike,
>> 
>> The 0.25 value is used to prevent water drag from being too small in order to avoid possible instability. Water drag is often higher than 0.25, so the insensitivity might not come from this. Otherwise, the ice V or ocean V might be a bit too small.
>> 
>> Cheers,
>> 
>> Jinlun
>> 
>> On 11/23/2022 9:02 AM, Michael A Spall wrote:
>>> Hi All,
>>> I am doing a problem with the MITgcm and the seaice package. The results are nearly insensitive to the value of SEAICE_waterDrag.
>>> I see in the code that if a certain condition is met then the drag is set to 0.25, which I suspect is being met in my calculations (I am
>>> checking this).  Can someone explain the reasoning behind this limit?
>>> 
>>> from MITgcm_c67c, seaice_oceandrag_coeffs.F:
>>> 
>>>           IF ( SEAICE_waterDrag.LE.0. ) THEN
>>>            CwatC(I,J,bi,bj)=0.
>>>           ELSEIF ( TEMPVAR.LE.(0.25 _d 0/SEAICE_waterDrag)**2 ) THEN
>>>            CwatC(I,J,bi,bj)=0.25 _d 0
>>>           ELSE
>>>            CwatC(I,J,bi,bj)=SEAICE_waterDrag*SQRT(TEMPVAR)
>>>           ENDIF
>>> 
>>> Thanks,
>>> Mike
>>> 
>>> ==================================================================
>>> 
>>> Michael A. Spall
>>> Senior Scientist
>>> Clark 323A
>>> Department of Physical Oceanography
>>> Woods Hole Oceanographic Institution
>>> 360 Woods Hole Road MS #21
>>> Woods Hole, MA 02543
>>> 
>>> 508-289-3342
>>> mspall at whoi.edu
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