Assessing the Atmospheric Response to Subgrid Surface Heterogeneity in the Single-Column Community Earth System Model, Version 2 (CESM2)

Land-atmosphere interactions are central to the evolution of the atmospheric boundary layer and the subsequent formation of clouds and precipitation. Existing global climate models represent these connections with bulk approximations on coarse spatial scales, but observations suggest that small-scale variations in surface characteristics and co-located turbulent and momentum fluxes can significantly impact the atmosphere. Recent model development efforts have attempted to capture this phenomenon by coupling existing representations of subgrid-scale (SGS) heterogeneity between land and atmosphere models. Such approaches are in their infancy and it is not yet clear if they can produce a realistic atmospheric response to surface heterogeneity. Here, we implement a parameterization to capture the effects of SGS heterogeneity in the Community Earth System Model (CESM2), and compare single-column simulations against high-resolution Weather Research and Forecasting (WRF) large-eddy simulations (LESs), which we use as a proxy for observations. The CESM2 experiments increase the temperature and humidity variances in the lowest atmospheric levels, but the response is weaker than in WRF-LES. In part, this is attributed to an underestimate of surface heterogeneity in the land model due to a lack of SGS meteorology, a separation between deep and shallow convection schemes in the atmosphere, and a lack of explicitly represented mesoscale secondary circulations. These results highlight the complex processes involved in capturing the effects of SGS heterogeneity and suggest the need for parameterizations that communicate their influence not only at the surface but also vertically. Land surface temperature and soil moisture are known to influence the daily evolution of the overlying atmosphere and the formation of clouds and rainfall. While global climate models represent these interactions on relatively coarse spatial scales (i.e., 100 km or greater), smaller scale differences in surface characteristics are increasingly recognized for their ability to impact the atmosphere. Here, we implement a new feature in a climate model that communicates information on small-scale surface differences from the land to the atmospheric model. We compare the results of this addition against a high-resolution model that has previously been used to isolate the impacts of surface flux gradients, with the latter serving as a proxy for observations. Though there are some encouraging signs of the implemented approach to drive an atmospheric response to surface variability, we find that a missing representation of large-scale circulations between warm/cool surfaces likely limits model agreement. A new method of conveying information on subgrid-scale surface heterogeneity to the atmosphere is introduced to Community Earth System Model, version 2 A comparison with large-eddy simulations suggests that the atmospheric response to heterogeneity in CESM2 is too vertically constrained Future model development efforts should focus on heterogeneity representations that account for secondary circulations more realistically