PNNL

Abstract

Sub-grid topographic heterogeneity has large impacts on surface energy balance, surface boundary conditions and land-atmosphere interactions. However, the impacts of representing sub-grid topographic effects in land surface models (LSMs) on the surface energy balance and surface boundary conditions remain unclear. Recently, a topography-based sub-grid structure and a method to account for the sub-grid topographic effects on solar radiation have been incorporated in the Energy Exascale Earth System Model (E3SM) land model (ELM). This study analyzed and evaluated the impacts of sub-grid topographic representations on surface energy balance and turbulent heat flux and scalar (co-)variances in ELM. Three sub-grid topographic representations in ELM were compared: (1) the default sub-grid structure (denoted as D) at 0.5° resolution that does not account for the sub-grid topographic variability, (2) the recently developed sub-grid topographic structure (denoted as T) at 0.5° resolution that parsimoniously resolves the sub-grid topographic variability, and (3) high spatial resolution (denoted as 1KM) to explicitly resolve the fine-scale topography at 1 km resolution. Additionally, two different solar radiation schemes in ELM were compared: (1) the default plane-parallel radiative transfer scheme (PP) that neglects the sub-grid topographic effects and (2) the parameterization scheme (TOP) that accounts for sub-grid topographic effects on solar radiation. A series of simulations with the three grid structures (i.e. D, T and 1KM) and the two different treatments of solar radiation (i.e., TOP and PP) were carried out using the Sierra Nevada, California, as case study, which is representative of a region with complex terrain. The results show significant differences between TOP and PP in the 1 km simulated surface energy balance, but the differences in the mean values and standard deviations become small when aggregated to the grid-scale (i.e., 0.5°). The T configuration can better mimic the 1KM simulations than the D configuration, and can better capture the sub-grid topographic effects on the mean values and standard deviations of surface energy balance as well as the surface boundary conditions for turbulent heat fluxes and scalar (co-)variances. These results underline the importance of representing sub-grid topographic heterogeneities in LSMs and motivate future research to understand the sub-grid topographic effects on land-atmosphere interactions over mountain areas.