[MITgcm-support] mitgcm seaice

Michael A Spall mspall at whoi.edu
Fri Dec 2 09:32:34 EST 2022 

Ahh, perfect, thank you.

==================================================================

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
https://www2.whoi.edu/staff/mspall

________________________________________
From: MITgcm-support <mitgcm-support-bounces at mitgcm.org> on behalf of Martin Losch <Martin.Losch at awi.de>
Sent: Friday, December 2, 2022 9:30 AM
To: MITgcm Support
Subject: Re: [MITgcm-support] mitgcm seaice

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<mailto: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

________________________________________
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<mailto: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<mailto:mspall at whoi.edu>
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