Summary:

## Overview

The Community Land Model (CLM) is responsible for computing two key quantities that are passed to the ice sheet model (CISM):

1. Surface mass balance (SMB) - the net annual accumulation/ablation of mass at the upper surface.
2. Ground surface temperature, which serves as an upper boundary condition for CISM's temperature calculation.

The ice sheet model is typically run at a much higher resolution than CLM. 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.

## Reasons for Computing SMB in CLM

1. It is much cheaper to compute the SMB in CLM for multiple elevation classes than in CISM, especially at coarser resolutions.
2. CLM's sophisticated snow physics parameterization can be used instead of implementing a separate scheme for CISM.
3. The atmosphere model can respond during runtime to ice-sheet surface changes, which is critical for simulating ice-sheet retreat on paleoclimate time scales.
4. The improved SMB is potentially available in CLM for all glaciated grid cells, not just those which are part of ice sheets.

## CISM's Role in the System

1. CISM dictates glacier areas and topographic elevations, overriding the values on CLM's surface dataset, and downscales atmospheric fields to non-glacier land units within its domain.
2. CISM provides the grid onto which SMB is downscaled.

In runs with an evolving ice sheet, CLM responds to CISM's evolution by adjusting the areas of the glacier land unit and each elevation class, as well as the mean topographic heights of each elevation class, to maintain conservation of mass and energy.