CTSM glaciers documentation#4062
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Looks like there are conflicts to be resolved first, presumably desirable before my review. |
olyson
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Jun 3, 2026
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| Overview | ||
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| +---------------+---------------+---------------+---------------+ |
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Are the Elevation classes in the right order? I see this in the lnd_in namelist:
glacier_region_behavior = 'single_at_atm_topo','UNSET','virtual','multiple'
Is virtual and multiple switched?
The other categories seem to be consistently order, e.g.,
glacier_region_melt_behavior = 'remains_in_place','UNSET','replaced_by_ice','replaced_by_ice'
| #. Surface mass balance (SMB) - the net annual accumulation and ablation of mass at the upper surface (section :numref:`Computation of the surface mass balance`) | ||
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| #. Treatment of glacial melt water: | ||
| #. Ground surface temperature, which serves as an upper boundary condition for CISM's temperature calculation. Ice sheet models are typically run at much higher spatial resolution than CLM (for example, :math:`\sim\ 5km` versus :math:`\sim\ 100km`). To improve the downscaling of atmospheric forcing from the CLM grid to the ice sheet grid, the glaciated portion of each CLM grid cell is divided into multiple elevation classes (section :numref:`Multiple elevation class scheme`). The CESM coupler then performs horizontal and vertical interpolation to generate high-resolution fields for CISM. |
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What would you think about closing the gap between the "about" symbol and number, e.g., :math:\sim\5km versus :math:\sim\100km.
| The glacier land unit contains multiple columns based on surface elevation. These are known as elevation classes, and the land unit is referred to as *glacier\_mec*. As described in section :numref:`Glacier regions`, some regions have only a single elevation class, but they are still referred to as *glacier\_mec* land units. The default is to have 10 elevation classes whose lower limits are 0, 200, 400, 700, 1000, 1300, 1600, 2000, 2500, and 3000 m. Each column is characterized by a fractional area and surface elevation that are read in during model initialization, and then possibly overridden by CISM as the run progresses. Each *glacier\_mec* column within a grid cell has distinct ice and snow temperatures, snow water content, surface fluxes, and SMB. | ||
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| The combination of the ``Glacial melt = Replaced by ice`` and the ``Ice runoff = Melted`` behaviors results in particularly non-physical behavior: During periods of glacial melt, a negative ice runoff is generated (due to the ``Glacial melt = Replaced by ice`` behavior); this negative ice runoff is converted to a negative liquid runoff plus a positive sensible heat flux (due to the ``Ice runoff = Melted`` behavior). The net result is zero runoff but a positive sensible heat flux generated from glacial melt. Because of how physically unrealistic this is, CLM does not allow this combination of behaviors. | ||
| The atmospheric surface temperature, potential temperature, specific humidity, density, and pressure are downscaled from the atmosphere's mean grid cell elevation to the *glacier\_mec* column elevation using a specified lapse rate (typically 6.0 deg/km) and an assumption of uniform relative humidity. Longwave radiation is downscaled by assuming a linear decrease in downwelling longwave radiation with increasing elevation (0.032 W m\ :sup:`-2` m\ :sup:`-1`, limited to 0.5 - 1.5 times the gridcell mean value, then normalized to conserve gridcell total energy) :ref:`(Van Tricht et al., 2016)<VanTrichtetal2016>`. Total precipitation is partitioned into rain vs. snow as described in Chapter :numref:`rst_Surface Characterization, Vertical Discretization, and Model Input Requirements`. The partitioning of precipitation is based on the downscaled temperature, allowing rain to fall at lower elevations while snow falls at higher elevations. |
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Documentation style guide (https://escomp.github.io/CTSM//users_guide/working-with-documentation/docs-style-guide.html) has a method for putting in a degree symbol, e.g., to replace "6.0 deg/km". I assume this is degC or degK, not degF?
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| Multiple elevation class scheme | ||
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| In contrast to most CLM subgrid units, *glacier\_mec* columns can be active (i.e., have model calculations run there) even if their area is zero. These are known as "virtual" columns. This is done because the ice sheet model may require a SMB for some grid cells where CLM has zero glacier area in that elevation range. Virtual columns also facilitate glacial advance and retreat in the two-way coupled case. Virtual columns do not affect energy exchange between the land and the atmosphere. |
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I believe we can have virtual urban and lake landunits/columns now. E.g., see the discussion here: #3918.
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| CLM computes SMB using a surface energy balance (SEB) approach, in which melt depends on the combined radiative, turbulent, and conductive energy fluxes at the surface. In glaciology, SMB is typically defined as the net balance of both snow and ice accumulation and ablation. However, the SMB flux passed from CLM to CISM represents the mass balance of the underlying ice only, excluding transient changes in snow storage. Conceptually, CLM can be viewed as owning the snowpack, while CISM owns the underlying glacier ice. As a result, fluctuations in snow depth between 0 and 10 m water equivalent are not reflected in the SMB passed to CISM. In transient simulations, this treatment can delay the onset of accumulation or ablation signals in a glacier column by several decades. | ||
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| SMB is computed and sent to the CESM coupler regardless of whether and where CISM is operating. However, the effect of SMB terms on runoff fluxes differs depending on whether and where CISM is evolving in two-way-coupled mode. This is described by the variable *glc\_dyn\_runoff\_routing* (this is real-valued in the code to handle the edge case where a CLM grid cell partially overlaps with the CISM grid, but we describe it as a logical variable here for simplicity.) In typical cases where CISM is not evolving, *glc\_dyn\_runoff\_routing* will be false everywhere; in these cases, CISM's mass is not considered to be part of the coupled system. In cases where CISM is evolving and sending its own calving flux to the coupler, *glc\_dyn\_runoff\_routing* will be true over the CISM domain and false elsewhere. |
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Is glc_dyn_runoff_routing a namelist variable in CLM or CISM? Should we tell the user where it can be found?
| #. Continental glaciers, i.e., the Greenland and Antarctic Ice Sheets. | ||
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| Compared with CLM4.5 (:ref:`Oleson et al. 2013 <Olesonetal2013>`), CLM5.0 contains substantial improvements in its capabilities for land-ice science. This section summarizes these improvements, and the following sections provide more details. | ||
| #. Mountain (or alpine) glaciers, represented in the Randolph Glacier Inventory (including glaciers on the peripheries of the ice sheets). |
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I didn't see a reference for "Randolph Glacier Inventory", but maybe I missed it.
| _build*/ | ||
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| doc/.coverage | ||
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Unless intentional for some reason, this change to .gitignore should be removed.
| _publish*/ | ||
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| *.ipynb_checkpoints/ |
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Unless intentional for some reason, this change to .gitignore should be removed.
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| This treatment is primarily intended to avoid unrealistic runoff fluxes in regions where coarse atmospheric topography produces climates that are too warm to sustain glaciers realistically. Such issues are particularly important for mountain glaciers, where strong local elevation gradients are poorly resolved at typical climate-model resolutions. | ||
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| Ongoing development efforts are focused on extending CLM's mountain glacier capabilities. In particular, the hillslope hydrology configuration (`Chapter Hillslope_Hydrology <https://escomp.github.io/CTSM/tech_note/Hillslope_Hydrology/CLM50_Tech_Note_Hillslope_Hydrology.html>`_) is being adapted to support SMB calculations for mountain glaciers . These developments are intended to enable coupling between CLM and CISM for mountain glacier applications in a manner similar to the current treatment of ice sheets, leveraging recent CISM capabilities for simulating mountain glacier dynamics :ref:`(Minallah and Lipscomb et al., 2025) <Minallah2025>`. |
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Should this be using the ":numref" format for this Hillslope Hydrology reference, or is this consistent with how we refer to chapters? Just asking...
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Description of changes
Updates to CTSM documentation chapter on Glaciers
Specific notes
Contributors other than yourself, if any:
CTSM issues resolved or otherwise addressed, if any:
If answers are expected to change, describe (delete this line otherwise):
Any user interface changes (namelist or namelist defaults changes)?
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Requirements before merge: