[MITgcm-support] shelfice_thermodynamics.F crash

[Martin Losch]

Please have a look at PR #968
https://github.com/MITgcm/MITgcm/pull/968
Fix pkg/shelfice `saltFreeze=0`-bug by mjlosch · Pull Request #968 · MITgcm/MITgcm
github.com

@Dimitry, it would be great, if you could try this code in your little test, and also review it in github.

M

> On 21. Jan 2026, at 21:25, Martin Losch <Martin.Losch at awi.de> wrote:
> 
> I’ll take care of it.
> M
> 
>> On 21. Jan 2026, at 19:49, Dimitris Menemenlis <dmenemenlis at gmail.com> wrote:
>> 
>> Hi Martin, for time being I added your suggested modification to our local repo:
>> https://github.com/MITgcm-contrib/llc_hires/blob/master/trillium/llc_4320/code/shelfice_thermodynamics.F
>> 
>> But yes, I think this probably needs to make its way back to the main branch.
>> Will you take care of the PR?  Or shall I try to figure out how to submit one?
>> 
>> D.
>> 
>> 
>>> On Jan 21, 2026, at 1:04 AM, Martin Losch <Martin.Losch at awi.de> wrote:
>>> 
>>> Do you think that this should be a PR?
>>> 
>>> Martin
>>> 
>>>> On 20. Jan 2026, at 18:38, Dimitris Menemenlis <dmenemenlis at gmail.com> wrote:
>>>> 
>>>> Hi Martin, I just tested your modification below in my baby set-up and it works fine.
>>>> 
>>>> You just need to add a “-d 0” in "IF ( saltFreeze .EQ. 0. _d 0 ) THEN"
>>>> 
>>>> Thanks for quick response.
>>>> 
>>>> D.
>>>> 
>>>> P.S.: I still owe you a response for the “Supercooled waters” post.  I don’t know how you and Oliver and Jean-Michel are so efficient!
>>>> 
>>>> P.P.S.: Happy New Year and belated Merry Christmas to you and family.
>>>> 
>>>> 
>>>>> On Jan 20, 2026, at 5:37 AM, Martin Losch <Martin.Losch at awi.de> wrote:
>>>>> 
>>>>> Hi Dimitris and Kayhan,
>>>>> 
>>>>> great catch. I suggest a different fix: Even though it’s unlikely, we may want to allow saltFreeze=0. So If sLoc = 0, then we have freshWaterFlux = rUnit2mass * shiTransCoeffS(i,j,bi,bj) * ( 1. _d 0 - 0. / 0. ) and could drop the part in the parentheses, i.e.
>>>>> 
>>>>>             IF ( saltFreeze .EQ. 0. _d ) THEN
>>>>>              freshWaterFlux = rUnit2mass
>>>>>      &            * shiTransCoeffS(i,j,bi,bj)
>>>>>             ELSE
>>>>>              freshWaterFlux = rUnit2mass
>>>>>      &            * shiTransCoeffS(i,j,bi,bj)
>>>>>      &            * ( 1. _d 0 - sLoc(I,J) / saltFreeze )
>>>>>             ENDIF
>>>>> 
>>>>> What do you think? Does that work for you?
>>>>> 
>>>>> Martin
>>>>> 
>>>>>> On 20. Jan 2026, at 00:46, Dimitris Menemenlis <dimitris.menemenlis at sjsu.edu> wrote:
>>>>>> 
>>>>>> shelfice_thermodynamics.F can crash when the line
>>>>>>            saltFreeze = (- bqe - SQRT(discrim))*recip_aqe
>>>>>> returns 0, so that
>>>>>>            freshWaterFlux = rUnit2mass
>>>>>>     &           * shiTransCoeffS(i,j,bi,bj)
>>>>>>     &           * ( 1. _d 0 - sLoc(I,J) / saltFreeze )
>>>>>> returns NaN.
>>>>>> 
>>>>>> This appears to be a consequence of "sLoc(I,J)  = 0”
>>>>>> One possible fix is to add:
>>>>>>          sLoc(I,J) = MAX(salt(I,J,K,bi,bj), 0.1 _d 0)
>>>>>> 
>>>>>> >>>>>>>
>>>>>> 
>>>>>> Kayhan Momeni, from his ongoing llc4320 simulation has extracted all the model state variables
>>>>>> at the time step before (3205150) and during (3205151) the first NaN:
>>>>>> https://nasa-ext.box.com/s/5ah3cffma49vkvfwgjsown7ytgs8i5i1
>>>>>> 
>>>>>> I have created a small (17x17x173) regional set-up that (approximately) recreates this crash:
>>>>>> https://nasa-ext.box.com/s/varcd6iu3ilg44bzly9hnzee1wnigfod
>>>>>> https://nasa-ext.box.com/s/bpq9p9yksksdhgr2knofj9fsqj0ntv5x
>>>>>> https://nasa-ext.box.com/s/atgqgp43st6o4u4976gfr5not465zbdr
>>>>>> 
>>>>>> I have verified that the small set-up is able to reproduce the SHICE_fwFlux NaN, the white grid cell at center of:
>>>>>> https://nasa-ext.box.com/s/trhs4kd8vpt9tx4t8riq934olrks3h30
>>>>>> 
>>>>>> All of the above are under: https://nasa-ext.box.com/s/q4sx5w4bnwui4v1afec9k2kb41ejml3v
>>>>>> 
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>>>>> 
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