The spatiotemporal evolution of atmospheric boundary layers over a thermally heterogeneous landscape

Abstract. We study the diurnal variability in the atmospheric boundary layer (ABL) across spatial scales (between ∼100 m and ∼10 km) of irrigation-driven surface heterogeneity in the semi-arid landscape of the 2021 Land surface Interactions with the Atmosphere over the Iberian Semi-arid Environment (LIAISE) experiment on the northeastern Iberian Peninsula. We combine observational analysis with explicit simulation of the ABL using observationally driven large-eddy simulation (LES) to better understand the physical mechanisms controlling ABL dynamics in heterogeneous regions. Our choice of spatial scales represents current and future single grid cells of global models, demonstrating how the sources and magnitude of subgrid-scale heterogeneity vary with model resolution. There is an observed positive buoyancy flux over the irrigated fields driven primarily by moisture fluxes, whereas, over the non-irrigated fields, there is a classical buoyancy profile driven by the surface sensible heat flux. The surface heterogeneity is felt most strongly near the surface; however, at approximately 1000 m above the surface, there appears to be a blending zone of mean scalars (i.e., potential temperature and specific humidity), indicating that the heterogeneity mixes into a new mean state of the atmosphere. There is a stable internal boundary layer (IBL; as defined as the first stable layer in individual radiosonde potential temperature profiles) up to approximately 500 m over the irrigated area. Taking advantage of the spatiotemporal extent of LES results, we perform spectral analyses to find that the ABL height had an integral length scale of ∼800 m matching that of the imposed surface fluxes. Between the irrigated and non-irrigated areas, there is an adjustment of the ABL as it crosses the boundary up to 500 m upwind of the boundary. We observe a variable-dependent blending zone between scales in the middle of the ABL, but it is limited by the entrainment zone effectively introducing another source of heterogeneity driven by upper-atmosphere conditions.