Mesoscale structure of the atmospheric boundary layer across a natural roughness transition at White Sands National Park

Roughness is ubiquitous to many atmospheric boundary layer (ABL) flows, and often ABLs encounter roughness transitions. This step-change introduces a new flow scale: the internal boundary layer (IBL) which modifies transport of momentum, heat, and particles/pollutants. Many well-known models for predicting the growth of the IBL exist, but not for the flow structure within. We deploy wall-modeled large eddy simulation to simulate the natural roughness change of the dune field at White Sands National Park in New Mexico, with flow conditions at Reτ ∼ O(10⁶). We validate our simulations against available field observation data and show evolution of mean velocities and boundary stress, τ_b, after the step-change. We also provide a prediction of IBL thickness, δ_IBL, and compare a power-law fit to existing models. Lastly, we investigate the mesoscale interaction of large- and small-scales within the IBL via amplitude modulation analysis. Our results support the existence of δ_IBL, but show existing models built on δ_IBL fail to predict τ_b, as they do not address streamwise roughness heterogeneity in their current form, and our results suggest models for sediment flux would fail as well.