Cryptic soil layer indexing bug#1459
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… that only occurs when the code is modified to run with more than four soil layers. The logic in mpas_atmphys_driver_lsm_noahmp.F can be simplified to a single loop that will always work. Likewise, Registry_noahmp.xml does not need to have the combined number of soil plus snow levels (nzSoilLevels) hardcoded, it can be written as the sum of the levels so it always works.
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This PR mirrors the one I submitted to MONAN. I found this bug as I am working to allow the total number of soil layers (and their thickness) more flexible in MONAN. Happy to point to the subsequent PR if interested in implementing it in MPAS too. |
cenlinhe
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May 28, 2026
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This bug fix looks good to me.
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This fixes a rather cryptic bug, that occurs when the number of soil layers is not the default (4 layers) for Noah-MP, which results in the model not conserving water.
1. Soil layer indexing bug. Noah-MP uses positive indices for soil layers, and negative indices for snowpack layers. In sub-routine
lsm_noahmp_toMPAS, the code copies the depths from Noah-MP to a local structure (zsnsoxy) that assumes indices going from 1 to the number of combined snowpack and soil layers. However, the original code had one error in the conversion logic, which coincidentally worked for the specific case in which the number of soil layers is exactly the number of snowpack ayers plus 1. The proposed change follows the logic used elsewhere in the code and ensures the conversion always works.2. Hardcoded number of soil plus snow levels. File
Registry_noahmp.xmloriginally defines dimensionnzSnowLevels(total number of soil plus snowpack layers) as 7. This works when using the default number of soil layers, but causes segmentation violations otherwise.