ctsm5.4.009

[CTSM Gatekeeper]

*** RESPONSES TO THIS EMAIL WILL NOT BE READ ***
===============================================================
Tag name: ctsm5.4.009
Originator(s): olyson (Keith Oleson)
Date: Mon Jan 19 12:53:18 PM MST 2026
One-line Summary: Dewpoint Temperature check for bare ground

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

Implement dewpoint temperature check for bare ground to fix RRTMG error

Cecile Hannay reported an RRTMGP crash in this 4xCO2 experiment at year 37:

NCAR/cesm_dev#246

Output from placing write statements in the CAM radiation code show that it's a baresoil patch in
a grid cell in the Arabian Penninsula. The bare soil patch occupies 99.28% of the gridcell.
Solar and longwave forcing and absorbed solar don't appear to be unreasonable, but t_grnd is 358K.

This bug fix eliminates dew flux (sets evaporation conductance to zero) x if t_grnd > forc_dewpoint, where forc_dewpoint is the dewpoint temperature at the forcing height.
This prevents large spurious negative latent heat fluxes that increase heat flux into the soil and signficantly increase surface soil temperature (t_grnd) such that soil temperature > 355K, triggering a RRTMGP error.

Contributors:
-------------
@swensosc , @wwieder , @slevis-lmwg , @dlawrenncar, ekluzek


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
----------

List of CTSM issues fixed (include CTSM Issue # and description) [one per line]:
Fixes #3589 355+ K land temperature triggers RRTMGP error

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

Caveats for users (e.g., need to interpolate initial conditions):
This is currently ONLY done for bare-soil, probably should be done for other landunits.

Changes to documentation: None, but needs to be added

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

Caveats for developers (e.g., code that is duplicated that requires double maintenance):
https://github.com/ESCOMP/CTSM/issues/3694
Documents making this a function added to QSat
This should especially be done when this is added for other landunit types

Testing summary: regular
----------------
[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! Almost ALL cases if run long enough

Summarize any changes to answers, i.e.,
- what code configurations: All
- what platforms/compilers: All
- nature of change: same climate

climate (from https://github.com/NCAR/LMWG_dev)
- issue number(s): 135 and 136

URL for LDF diagnostics output used to validate new climate:

- ILAMB: https://webext.cgd.ucar.edu/I2000/Clm60SP_ctsmdew_1deg_Dew_2000/lnd/BUGFIX/_build_DEW/
- LDF: https://webext.cgd.ucar.edu/I2000/Clm60SP_ctsmdew_1deg_Dew_2000/lnd/BUGFIX/Clm60SP_ctsmdew_1deg_Dew_2000_2005_2019-Clm60SP_ctsmdew_1deg_Control_2000_2005_2019/setsIndex.html

Standard diagnostics results are here
Other details
-------------

Pull Requests that document the changes (include PR ids):
(https://github.com/ESCOMP/ctsm/pull)
#3688 -- Implement dewpoint temperature check for bare ground to fix RRTMGP error


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