ctsm5.4.014

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Tag name: ctsm5.4.014
Originator(s): mvdebolskiy
Date: Thu Jan 29 03:28:03 PM MST 2026
One-line Summary: Fix for xm2 and revise logic for excess ice melt

Purpose and description of changes
----------------------------------

A bug fix in excess ice melt calculation. Previously, we have set
incorrectly xm2=xm-h2osoi_ice after h2osoi_ice has already been updated.
This resulted in extra heat being available to spend on melting excess_ice
since when all h2osoi_ice has been melted (xm2=xm). This fixed now, in
addition Phasechange_beta subroutine in SoilTemperatureMod has been
renamed to Phasechange by swensosc's suggestion.

The changes are generally small and only to a few points, so NOT marking it as a
significant change in answers.

Significant changes to scientifically-supported configurations
--------------------------------------------------------------

Does this tag change answers significantly for any of the following physics configurations?
(Details of any changes will be given in the "Answer changes" section below.)

[Put an [X] in the box for any configuration with significant answer changes.]

[ ] clm6_0

[ ] clm5_0

[ ] ctsm5_0-nwp

[ ] clm4_5


Bugs fixed
----------

Fixes #3678 -- Logic error in excess ice melt calculation

Notes of particular relevance for users
---------------------------------------

Caveats for users (e.g., need to interpolate initial conditions):
The changes in short simulations are negligible, but a few points can show larger differences if run long enough. The following lists the min and max differences for a 40 year simulation.
https://github.com/ESCOMP/CTSM/pull/3718#issuecomment-3810263334

Notes of particular relevance for developers:
---------------------------------------------

Testing summary:
----------------

[PASS means all tests PASS; OK means tests PASS other than expected fails.]

regular tests (aux_clm: https://github.com/ESCOMP/CTSM/wiki/System-Testing-Guide#pre-merge-system-testing):

derecho ----- OK
izumi ------- OK

If the tag used for baseline comparisons was NOT the previous tag, note that here:


Answer changes
--------------

Changes answers relative to baseline: Yes! when use_excess_ice is TRUE

Summarize any changes to answers, i.e.,
- any configuration with use_excess_ice=.true.
- larger than roundoff changes in the soil thermal and
moisture state in gridcells where excess ice is present
in the soil at the start of the simulations. Most of the
most of the gridcells will have little differences (1e-6),
however, the difference will grow for longer (>10y) runs.

If this tag changes climate list the run(s) done to evaluate the new
climate (from https://github.com/NCAR/LMWG_dev)
- issue number(s):
- NCAR/LMWG_dev#140

Other details
-------------

Pull Requests that document the changes (include PR ids):

https://github.com/ESCOMP/CTSM/pull/3718

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