Capturing Finer-Scale Topographic Differences Improves Model Capability to Reproduce Observations
Topography-influenced land surface differences can cause significant climatic variability across the landscape. (Image courtesy of Aleks Dahlberg |Unsplash.com)
Downscaling of atmospheric forcing to topographic subgrid units has more pronounced impacts in high-elevation regions with major precipitation occurring during cool seasons.
The Science
Earth system models (ESMs), used in climate simulations and projections, typically use grids of 50–200 km resolution. These are considered relatively coarse with limited ability to resolve land surface heterogeneity. To enhance the capability of ESMs to simulate the impacts of small-scale land surface differences, this study introduced a new subgrid structure. Researchers also used methods to downscale atmospheric variables, such as precipitation and temperature, from the atmosphere grid to subgrid topographic units established in their earlier studies for the Energy Exascale Earth System Model (E3SM) Land Model (ELM). Analysis of ELM simulations with and without the new land surface subgrid and downscaled atmospheric variables revealed their significant impacts on snowfall, snow water equivalent, and runoff. These results were particularly evident in regions dominated by mountainous landscapes and where maximum precipitation occurred during the cool seasons. The new developments noticeably enhance the capability of ELM to reproduce measured observations for snow water equivalent (i.e., the amount of liquid water held in snow) at the Snow Telemetry (SNOTEL) sites in the western United States.
The Impact
Topographic variations have substantial impacts on surface hydrologic processes and are major drivers of the spatial variability in observed surface temperatures and precipitation. This study showed that the newly developed topography-based subgrid scheme and downscaling of atmospheric variables to finer resolution have pronounced impacts in high-elevation regions that receive their major precipitation during the cool seasons. By improving the capability of ELM to reproduce the observed snow water equivalent at 83% of the SNOTEL sites (Fig. 1) across the western United States, this study motivates the incorporation of the new model features in E3SM for understanding the regional and global water cycles and their future changes. This can in turn provide the basis for more effective water management plans and action.
Figure 1. Root-mean-square-error (RMSE) of snow water equivalent simulated by ELM featuring a topography-based subgrid scheme with (D) and without (NoD) downscaling of atmospheric forcing. ELM with D better reproduces the observed snow water equivalent at 83% of the SNOTEL sites, with greater improvement occurring over regions that receive their major precipitation during cool seasons (SON, DJF) and in areas of high topographic differences indicated by the color scheme.
Summary
This study introduced a new subgrid structure and methods to downscale atmospheric variables such as precipitation and temperature. The resolution for the latter was adjusted from the atmosphere grid to the land subgrid units in the Energy Exascale Earth System Model (E3SM) Land Model (ELM). These adjustments sought to improve ELM’s ability to represent the effects of topography-induced subgrid surface variability on land surface processes. Effects of the new developments on simulations of land surface processes were evaluated using the conterminous (i.e., contiguous) United States (CONUS) as a case study. ELM simulations with the new developments generally yielded higher amounts of snowfall, snow water equivalent, and runoff over CONUS. Effects of the new developments were found to be greater over regions dominated by higher-elevation landscapes and regions with maximum precipitation during cool seasons. The new developments also improved ELM’s ability to reproduce observed snow water equivalent at the SNOTEL sites, contributing overall enhancement of the prediction accuracy of the model. The results of this study have important implications for modeling streamflow and water resources management, both of which are strongly influenced by hydrologic processes in mountainous regions.
Publication
- Tesfa, T. K., Leung, L. R., Thornton, P. E., Brunke, M. A., & Duan, Z. Impacts of topography‐based subgrid scheme and downscaling of atmospheric forcing on modeling land surface processes in the conterminous US. Journal of Advances in Modeling Earth Systems, 16, e2023MS004064 (2024). [DOI:10.1029/ 2023MS004064]
Funding
- This work was supported by the Earth System Model Development program area of the Department of Energy, Office of Science, Biological and Environmental Research program.
Contact
- Guang Zhang, Scripps Institution of Oceanography
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