## 2.13.3. Glacier regions and their behaviors[¶](#glacier-regions-and-their-behaviors "Permalink to this headline")
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The world’s glaciers and ice sheets are broken down into a number of different regions (four by default) that differ in three respects:

1.  Whether the gridcell’s glacier land unit contains:
    
    1.  Multiple elevation classes (section [2.13.4](#multiple-elevation-class-scheme))
        
    2.  Multiple elevation classes plus virtual elevation classes
        
    3.  Just a single elevation class whose elevation matches the atmosphere’s topographic height (so there is no adjustment in atmospheric forcings due to downscaling).
        
2.  Treatment of glacial melt water:
    
    1.  Glacial melt water runs off and is replaced by ice, thus keeping the column always frozen. In the absence of a dynamic ice sheet model, this behavior implicitly assumes an infinite store of glacial ice that can be melted (with appropriate adjustments made to ensure mass and energy conservation). This behavior is discussed in more detail in section [2.13.5](#computation-of-the-surface-mass-balance).
        
    2.  Glacial melt water remains in place until it refreezes - possibly remaining in place indefinitely if the glacier column is in a warm climate. With this behavior, ice melt does not result in any runoff. Regions with this behavior cannot compute SMB, because negative SMB would be meaningless (due to the liquid water on top of the ice column). This behavior produces less realistic glacier physics. However, it avoids the negative ice runoff that is needed for the “replaced by ice” behavior to conserve mass and energy (as described in section [2.13.5](#computation-of-the-surface-mass-balance)). Thus, in regions where CLM has glaciers but the atmospheric forcings are too warm to sustain those glaciers, this behavior avoids persistent negative ice runoff. This situation can often occur for mountain glaciers, where topographic smoothing in the atmosphere results in a too-warm climate. There, avoiding persistent negative ice runoff can be more important than getting the right glacier ice physics.
        
3.  Treatment of ice runoff from snow capping (as described in section [2.7.6](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Hydrology/CLM50_Tech_Note_Hydrology.html#runoff-from-glaciers-and-snow-capped-surfaces)). Note that this is irrelevant in regions with an evolving, two-way-coupled ice sheet (where the snow capping term is sent to CISM rather than running off):
    
    1.  Ice runoff from snow capping remains ice. This is a crude parameterization of iceberg calving, and so is appropriate in regions where there is substantial iceberg calving in reality.
        
    2.  Ice runoff from snow capping is melted (generating a negative sensible heat flux) and runs off as liquid. This matches the behavior for non-glacier columns. This is appropriate in regions that have little iceberg calving in reality. This can be important to avoid unrealistic cooling of the ocean and consequent runaway sea ice growth.
        

The default behaviors for the world’s glacier and ice sheet regions are described in [Table 2.13.1](#table-glacier-region-behaviors). Note that the standard CISM grid covers Greenland plus enough surrounding area to allow for ice sheet growth and to have a regular rectangular grid. We need to have the “replaced by ice” melt behavior within the CISM domain in order to compute SMB there, and we need virtual elevation classes in that domain in order to compute SMB for all elevation classes and to facilitate glacial advance and retreat in the two-way-coupled case. However, this domain is split into Greenland itself and areas outside Greenland so that ice runoff in the Canadian archipelago (which is inside the CISM domain) is melted before reaching the ocean, to avoid runaway sea ice growth in that region.

Table 2.13.1 Glacier region behaviors[¶](#id3 "Permalink to this table")    
| Region
 | Elevation classes

 | Glacial melt

 | Ice runoff

 |
| --- | --- | --- | --- |
| Greenland

 | Virtual

 | Replaced by ice

 | Remains ice

 |
| Inside standard CISM grid but outside Greenland itself

 | Virtual

 | Replaced by ice

 | Melted

 |
| Antarctica

 | Multiple

 | Replaced by ice

 | Remains ice

 |
| All others

 | Single

 | Remains in place

 | Melted

 |

Note

In regions that have both the `Glacial melt = Replaced by ice` and the `Ice runoff = Melted` behaviors (by default, this is just the region inside the standard CISM grid but outside Greenland itself): 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). We recommend that you limit the portion of the globe with both of these behaviors combined, in order to avoid having too large of an impact of this non-physical behavior.

