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A wind tunnel study of neutral and stable atmospheric boundary layer evolution over a canopy patch


Landscape heterogeneity, complex topographic features, and atmospheric stability present significant challenges for predicting fluxes of momentum, heat, moisture, and climate-controlling constituents in the atmospheric boundary layer (ABL) because similarity solutions break down. Canopies have been shown to enhance land-atmosphere turbulent flux, but few studies address how canopy heterogeneity affects the ABL due to the difficulties with finding ideal field cases and uncertainty in numerical approaches to accurately simulate high Reynolds number separated flows within the ABL. Understanding the impact of atmospheric stability on complex boundary layer flows is also of great importance as stability presents significant challenges for subgrid-scale models in large-eddy simulations. Alternatively wind tunnel experiments provide an ideal environment to simulate a stationary stable boundary layer and test how the ABL adjusts over canopy heterogeneity. We conducted experiments in the St. Anthony Falls Laboratory stratified atmospheric boundary layer wind tunnel to determine the effects of atmospheric stability on the boundary layer evolution from a rough, flat surface over a homogeneous (2h x 1v) canopy patch. We compare results from PIV and custom x-wire/cold-wire anemometry for stable and neutral conditions. We find marked differences in the morphology of the flow as well as in turbulence quantities. The results have important applications to deforested lands, areas with shelterbelts/windbreaks, lake sheltering, and biogeochemical flux measurements in complex landscapes.