Assessing the Sensitivity of Atmospheric Convective Updrafts to Subgrid Land Surface Heterogeneity in CESM2

Land surface temperature and moisture gradients on scales of 10s to 100s of km can lead to the formation of mesoscale secondary circulations between warm/dry and wet/cool surfaces. These circulations play an important role in the local formation and organization of clouds and precipitation. Earth System Models (ESMs), however, often operate on relatively coarse spatial resolutions that can make capturing these phenomena a challenge. To enable more realistic land-atmosphere coupling and represent their interactions at a sub-grid scale, we introduce a new approach within the Community Earth System Model (CESM2) that links existing subgrid-scale parameterizations in current versions of the Community Land Model (CLM5) and the Community Atmosphere Model (CAM6). Tile-level fluxes and states from CLM5 are used directly within a newly developed atmospheric turbulence parameterization, CLUBB-MF. CLUBB-MF combines the high order moist turbulence scheme of CAM6, Cloud Layers Unified by Binormals (CLUBB), with a multi-plume mass flux (MF) scheme. The latter portion captures stronger and more organized updraft plumes compared to the eddy diffusivity represented by CLUBB.

To assess the impact of surface heterogeneity information on the development of the near-surface atmosphere, we conduct a series of single-column atmosphere model (SCAM) experiments driven by forcing data at the DOE Atmospheric Radiation Measurement (ARM) Southern Great Plains site. In the control case, the atmosphere assumes a homogeneous surface, with each of the updraft plumes in CLUBB-MF receiving the same grid-mean fluxes and states from which they are initialized at the surface. This is compared against a heterogeneous case, in which the plumes are initiated over individual sub-grid surfaces, each with its own unique fluxes. A comparison between the two cases over summer (June-August) days in 2015 suggests that the addition of heterogeneity can significantly alter the characteristics of updraft plumes that, in some cases, can result in lower 2m temperature and humidity and reduce the formation of low clouds. Research to better understand the atmospheric sensitivity under various environmental conditions is underway, and is expected to help inform further model developments.