## 2.13.2. Overview[¶](#overview "Permalink to this headline")
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CLM is responsible for computing two quantities that are passed to the ice sheet model:

1.  Surface mass balance (SMB) - the net annual accumulation/ablation of mass at the upper surface (section [2.13.5](#computation-of-the-surface-mass-balance))
    
2.  Ground surface temperature, which serves as an upper boundary condition for CISM’s temperature calculation The ice sheet model is typically run at much higher resolution than CLM (e.g., \\(\\sim\\)5 km rather than \\(\\sim\\)100 km). To improve the downscaling from CLM’s grid to the ice sheet grid, the glaciated portion of each grid cell is divided into multiple elevation classes (section [2.13.4](#multiple-elevation-class-scheme)). The above quantities are computed separately in each elevation class. The CESM coupler then computes high-resolution quantities via horizontal and vertical interpolation, and passes these high-resolution quantities to CISM.
    

There are several reasons for computing the SMB in CLM rather than in CISM:

1.  It is much cheaper to compute the SMB in CLM for \\(\\sim\\)10 elevation classes than in CISM. For example, suppose we are running CLM at a resolution of \\(\\sim\\)50 km and CISM at \\(\\sim\\)5 km. Greenland has dimensions of about 1000 x 2000 km. For CLM we would have 20 x 40 x 10 = 8,000 columns, whereas for CISM we would have 200 x 400 = 80,000 columns.
    
2.  We can use the sophisticated snow physics parameterization already in CLM instead of implementing a separate scheme for CISM. Any improvements to CLM are applied to ice sheets automatically.
    
3.  The atmosphere model can respond during runtime to ice-sheet surface changes (even in the absence of two-way feedbacks with CISM). As shown by [Pritchard et al. (2008)](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#pritchardetal2008), runtime albedo feedback from the ice sheet is critical for simulating ice-sheet retreat on paleoclimate time scales. Without this feedback the atmosphere warms much less, and the retreat is delayed.
    
4.  The improved SMB is potentially available in CLM for all glaciated grid cells (e.g., in the Alps, Rockies, Andes, and Himalayas), not just those which are part of ice sheets.
    

In typical runs, CISM is not evolving; CLM computes the SMB and sends it to CISM, but CISM’s ice sheet geometry remains fixed over the course of the run. In these runs, CISM serves two roles in the system:

1.  Over the CISM domain (typically Greenland in CESM2), CISM dictates glacier areas and topographic elevations, overriding the values on CLM’s surface dataset. CISM also dictates the elevation of non-glacier land units in its domain, and only in this domain are atmospheric fields downscaled to non-glacier land units. (So if you run with a stub glacier model - SGLC - then glacier areas and elevations will be taken entirely from CLM’s surface dataset, and no downscaling will be done over non-glacier land units.)
    
2.  CISM provides the grid onto which SMB is downscaled. (If you run with SGLC then SMB will still be computed in CLM, but it won’t be downscaled to a high-resolution ice sheet grid.)
    

It is also possible to run CESM with an evolving ice sheet. In this case, CLM responds to CISM’s evolution by adjusting the areas of the glacier land unit and each elevation class within this land unit, as well as the mean topographic heights of each elevation class. Thus, CLM’s glacier areas and elevations remain in sync with CISM’s. Conservation of mass and energy is done as for other landcover change (see Chapter [2.27](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Transient_Landcover/CLM50_Tech_Note_Transient_Landcover.html#rst-transient-landcover-change)).

