Summary of the Article:

**Surface Heterogeneity and Data Structure**

The article discusses how spatial land surface heterogeneity is represented in the Community Land Model (CLM) through a nested subgrid hierarchy. This hierarchy consists of the following levels:

1. Grid Cell: The grid cell can contain multiple land units.
2. Land Unit: The land units include glacier, lake, urban, vegetated, and crop (when the crop model is active). Each land unit can have a different number of columns.
3. Column: The column level captures variability in soil and snow state variables within a single land unit. Each column can have multiple plant functional types (PFTs).
4. Patch: The patch level represents PFTs or bare ground on the vegetated land unit, and crop functional types (CFTs) on the crop land unit.

The article explains that the subgrid levels are designed to capture different aspects of land surface heterogeneity. The column level is where the state variables and fluxes for water and energy in the soil and snow are defined. Fluxes at the PFT level are aggregated to the column level for processes related to soil and snow.

The article provides a detailed illustration of the standard configuration of the model subgrid hierarchy, showing the relationships between the different levels and the potential PFTs and CFTs that can be represented.