## 2.13.4. Multiple elevation class scheme[¶](#multiple-elevation-class-scheme "Permalink to this headline")
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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 [2.13.3](#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.

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\-2 m\-1, limited to 0.5 - 1.5 times the gridcell mean value, then normalized to conserve gridcell total energy) [(Van Tricht et al., 2016)](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#vantrichtetal2016). Total precipitation is partitioned into rain vs. snow as described in Chapter [2.2](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Ecosystem/CLM50_Tech_Note_Ecosystem.html#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.

This downscaling allows lower-elevation columns to undergo surface melting while columns at higher elevations remain frozen. This gives a more accurate simulation of summer melting, which is a highly nonlinear function of air temperature.

Within the CISM domain, this same downscaling procedure is also applied to all non-urban land units. The elevation of non-glacier land units is taken from the mean elevation of ice-free grid cells in CISM. This is done in order to keep the glaciated and non-glaciated portions of the CISM domain as consistent as possible.

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.

