Coupling Subgrid-Scale Surface Heterogeneity to the Convective Boundary Layer in the GFDL Global Model (AM4.0-LM4.0): Parameterization Development and Climate Impacts

Atmospheric boundary layer (ABL) flows over spatially heterogeneous surfaces give rise to internal boundary layers and secondary circulations that profoundly affect turbulent transport. Such processes occur at subgrid scales (SGS) in climate models, and while most land models employ parameterizations for surface heterogeneity, ABL schemes typically assume horizontal homogeneity. These schemes therefore overlook potential influences of land heterogeneity on atmospheric transport and subsequent climate impacts. This paper presents and evaluates a new parameterization that couples an eddy-diffusivity mass-flux (EDMF) ABL scheme to SGS land heterogeneity in the Geophysical Fluid Dynamics Laboratory (GFDL) global atmosphere-land model AM4.0-LM4.0. The parameterization, EDMF-HET, combines a heterogeneity-induced mixing length formulation derived from dimensional analysis with a statistical approach that couples convective updrafts to surface patchiness. EDMF-HET is evaluated against the baseline EDMF and a previous AM4.0 scheme (AM4-Lock) in global Atmospheric Model Intercomparison Project simulations and against observational data sets. Results indicate that land heterogeneity enhances continental cloud cover over most regions—an effect driven by increased entrainment and cooling near the ABL top. This increased cloudiness in EDMF-HET slightly exacerbates AM4's global-mean bias in shortwave cloud radiative effect at the top of the atmosphere, but the associated reduction in downwelling shortwave radiation alleviates some of AM4's regional warm bias, particularly in central North America. EDMF-HET also reduces tropical land precipitation, mitigating AM4's excessive rainfall in these regions, and shifting the surface energy partitioning toward higher sensible heat fluxes while lowering runoff. Despite these effects, EDMF-HET had negligible impact on the global-mean climate, suggesting that land heterogeneity mainly redistributes mass and energy regionally.

Duke Scholars

Author Nathaniel Chaney Civil and Environmental Engineering