Numerical divergence in 3D tracer transport simulation during late-thaw period

[chen ding]

Hi all,

I am modeling tracer transport under freeze-thaw processes in a small catchment. 

Setup: Initial tracer load of 1,000 mol in the 0–30 cm streambed layer during the freezing phase. 

The Problem: The concentration evolution is consistent during the early thaw, but explodes into unphysical values (dozens of orders of magnitude too high) during the late-thaw stage. 

Comparison: Interestingly, running the exact same input files on a 2D grid produces reasonable results without any divergence.

Has anyone encountered similar issues where 3D simulations fail during the phase transition while 2D simulations remain stable?

Any suggestions are appreciated. 

Thanks, 

Chen

[Bo Gao]

Hi Chen,

What's your "liquid water content key" in your subsurface transport pk? The code uses total water content by default, but under freezing conditions, it should be unfrozen water. So you need to explicitly define a "unfrozen water content" in the state and use it as the value of "liquid water content key" in your subsurface transport pk. Likewise, a "surface unfrozen water content" should be defined and added to the surface transport pk. 

Best,

Bo Gao

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[Bo Gao]

Neel helped us make an xml example for permafrost transport, but I am not sure whether Ethan has merged that example to ats demos. Neel, can you please direct Chen to your PR? Thanks.

[chen ding]

Hi Bo,

Thank you very much for the insightful suggestion. I reviewed my input file, but I couldn't find a parameter explicitly named  "liquid water content key" within my subsurface transport PK.

Could you please clarify if you were referring to the "saturation liquid key" ? 

 <Parameter name="saturation liquid key" type="string" value="saturation_liquid" />

If it is not the  "saturation liquid key"  , do I need to manually register and add a new   "liquid water content key"  to the transport PK to ensure it specifically uses the unfrozen/liquid water content for the transport calculations?

Best regards,

Chen

[Neelarun Mukherjee]

Hi Chen, 

Please check https://github.com/neelarunmukherjee/ats-demos/blob/permafrost-transport-1.6/09_arctic_hydrology_transport/permafrost_column_transport.xml

This is not yet merget to the main demos. 

Here, I defined the "liquid water content key" as "liquid_water_content", which is defined in evaluator as a product of molar_density_liquid * cell_volume * porosity * saturation_liquid. (see here). Similarly for surface-liquid_water_content" is defined as surface-molar_density_liquid *  surface-cell_volume * surface-ponded_depth * surface-unfrozen_fraction. 

-Neel.

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[chen ding]

Hi Neel,

Thanks for the clarification and the link! This is very helpful. I will try this setup in my simulation.

Best, 

Chen

[Coon, Ethan]

Whups, I hadn’t seen this PR — will get it in ASAP.  Thanks Neel!

Ethan

From: ats-...@googlegroups.com <ats-...@googlegroups.com> on behalf of Neelarun Mukherjee <neel...@gmail.com>
Date: Tuesday, March 10, 2026 at 3:56 AM
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Subject: [EXTERNAL] Re: Numerical divergence in 3D tracer transport simulation during late-thaw period

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