[MITgcm-support] shelfice_thermodynamics.F crash

[Dimitris Menemenlis]

Thank you Ian.  Your notes are super helpful.  Kayhan and I did run into an ocean-salinity = 0 case during initialization of ice shelf cavities for an llc4320 set-up.  This is probably excessive initial melting and undershoot, as you suggest.

Dimitris


> On Jan 23, 2026, at 10:46 AM, Fenty, Ian G (US 329B) <ian.fenty at jpl.nasa.gov> wrote:
> 
> Martin and Dimitris,
> 
> I may have missed this if the discussion, but if the ocean salinity is zero and the salinity of the ice is zero, then the assumptions of the three-equation model break down. In that case, there is no boundary layer at the ocean/ice interface. The melting expression simplifies to
> 
> Step 1, find the local melting point: 
>       T_f = c_0 + b_0 p
> 
> Step 2, calculate the heat flux to the ice
>       <w’T’> = \gamma_T (T_o - T_f)
> 
> Step 3, calculate the freshwater flux from melting:
> Case a: no heat conduction through the ice
>       \rho_w c_{\pw}  <w’T’> = -Lq 
> 
> Case b: heat conduction through the ice
>       \rho_w c_{\pw}  <w’T’> + \rho_i c_{pi}  \kappa_i (T_i - Tf_f) D_i = -Lq 
> 
> q is the ice mass flux [kg s-1 m-2], which can be expressed in terms of velocity of water meltwater coming into the ocean (w_b) as
> q = \rho_w_b
> 
> But yea, it’s hard to imagine the case where the model comes up with a solution of S_b = 0 in the original three equation model.  
> It can only happen if the heat transfer coefficient is infinite, if the salinity diffusion is zero, or the ocean salinity is zero. 
> If your ocean salinity is going to zero, it suggestions a different problem. Maybe a numerical artifact (undershoot/overshoot).
> 
> I attached my three-equation model notes.  From page 3 on the variable names (the epsilons) are from the shellfire code (or at least some older version of it)
> 
> Ian
> 
> 
> 
> From: MITgcm-support <mitgcm-support-bounces at mitgcm.org> on behalf of Martin Losch <martin.losch at awi.de>
> Date: Thursday, January 22, 2026 at 10:43 PM
> To: mitgcm-support at mitgcm.org <mitgcm-support at mitgcm.org>
> Subject: [EXTERNAL] Re: [MITgcm-support] shelfice_thermodynamics.F crash
> 
> I don’t know. If both solutions give the same results, I’d go with SHELFICE_USE_HEATBALANCE_FOR_FRESHWATERFLUX defined, as this is in compact form what is implemented in pkg/steep_icecavity. I have no experience with this version. The other option is a hack that just avoids the division by zero, this is only physically correct if sLoc is really zero at the same time.
> 
> M.
> 
> On 22. Jan 2026, at 21:05, Dimitris Menemenlis <dimitris.menemenlis at sjsu.edu> wrote:
> 
> Done.  I left a review.  Both options work.
> Which one do you recommend that Kayhan and I use?
> with or without SHELFICE_USE_HEATBALANCE_FOR_FRESHWATERFLUX defined?
> 
> Thanks a million for super-quick turnaround on this request.
> 
> D.
> 
> 
> On Jan 22, 2026, at 4:58 AM, Martin Losch <Martin.Losch at awi.de> wrote:
> 
> Please have a look at PR #968
> https://github.com/MITgcm/MITgcm/pull/968 <https://urldefense.us/v3/__https://github.com/MITgcm/MITgcm/pull/968__;!!PvBDto6Hs4WbVuu7!NJMtDwoC-xNdirXVV-Z-UZfpysl7_c7oQrii5iuN41ypBQpoyGEGyCxwEQZW9w2OcmeUGO-LgP7acoh3NpYJVPbeiTE$>
> 
> @Dimitry, it would be great, if you could try this code in your little test, and also review it in github.
> 
> M
> 
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