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

Abstract. We study the diurnal variability of 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 2021 LIAISE experiment. We combine observational analysis with the 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 represent current and future single grid cells of global models, which demonstrates how the sources and strength of sub-grid scale heterogeneity vary with model resolution.

From observations, there is a positive buoyancy flux over the irrigated fields driven primarily by moisture fluxes, whereas over the non-irrigated fields, there is a linearly decreasing buoyancy flux profile. The surface heterogeneity is felt most strongly near the surface; however, near 1000 m, there appears to be blending zone of mean scalars indicating that the heterogeneity mixes into a new mean state of the atmosphere. There is an stable internal boundary layer of ~500 m over the irrigated area. Taking advantage of the three-dimensionality of the LES results, we perform spectral analyses to find that the ABL height had an integral length scale of ~800 m matching that of the surface fluxes. Between irrigated and non-irrigated areas, there is an adjustment of the ABL characteristics 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.