Impacts of topography-based subgrids with downscaling of atmospheric forcing and land surface heterogeneity on land surface processes

Topography has major control on land surface processes; however, current Earth System Models (ESMs) use spatial scales too coarse to capture the impacts of topographic heterogeneity on critical land surface processes such as runoff generation. Recently, a new topography-based subgrid structure has been implemented within the hierarchical subgrid spatial structure of the Energy Exascale Earth System Model (E3SM) Land Model (ELM) combined with downscaling of atmospheric forcing from grid cell mean to the corresponding subgrids to improve representation of land surface processes. The new subgrid structure discretizes grids into variable numbers of subgrid units depending on their topographic heterogeneity to improve the ability to capture spatial heterogeneity of atmospheric forcing and land cover influenced by topography with minimal increase in computational demand. This study focuses on evaluation of the impacts of the topography-based subgrid structure and downscaling methods on the simulations of land surface processes. For this purpose, ELM simulations with and without subgrid topographic heterogeneity and with atmospheric forcing based on grid cell mean versus atmospheric forcing downscaled following the subgrid topography are analyzed. Comparison of the simulations will aim at isolating the impact of atmospheric forcing and the impact of representing subgrid heterogeneity based on the topography-based subgrid structure in different regions of complex terrain (e.g., in regions with streamflow dominated by rain-fed vs. snow-fed runoff). Furthermore, the simulations will be compared against observations in topographically complex regions.