Simulating surface energy fluxes using the variable-resolution Community Earth System Model (VR-CESM)

Recent advances in variable-resolution (VR) global models provide the tools necessary to investigate local and global impacts of land cover by embedding a high-resolution grid over areas of interest in a seamless and computationally efficient manner. We used two eddy covariance tower clusters in the Eastern USA to evaluate surface energy fluxes (latent heat, λ E ; sensible heat, H ; net radiation, R n ; and ground heat, G ) and surface properties (aerodynamic resistance to heat transfer, r aero ; Bowen ratio, β ; and albedo, α ) by uncoupled point simulations of the land-only Community Land Model (PTCLM4.5) and two coupled land–atmosphere Community Earth System Model (CESM1.3) simulations. The CESM simulations included a 1° uniform grid global simulation and global 1° simulation with a 0.25° refined VR grid over the Eastern USA. Tower clusters included the following plant functional types—broadleaf deciduous temperate (hardwood) forest, C3 non-Arctic grass (grass), a cropland, and needleleaf evergreen temperate (pine) forest. During the growing season, diurnal cycles of λ E and H for grass and the cropland were simulated well by PTCLM4.5 and VR-CESM1.3; however, λ E ( H ) was biased low (high) at the hardwood and pine forested sites, contributing to biases in β. Growing season R n was generally well simulated by CLM4.5 and VR-CESM1.3; however, modeled elevated albedo (indicative of snow cover) persisted longer in winter and spring leading to large biases in R n and α . The introduction of a VR grid does not adversely impact surface energy fluxes compared to 1° uniform grids and highlights the usefulness of this approach for future efforts to predict land–atmosphere fluxes across heterogeneous landscapes.