Spatial structures in atmospheric boundary-layer flow – wind tunnel modeling

Coherent structures play a dominant role in momentum and mass transfer in turbulent boundary layers. It is very difficult to directly monitor those structures in an atmospheric boundary layer because their detection from point measurements is very uncertain and continuous spatial atmospheric measurements are seldom. Wind tunnel modeling of ABL flows offers a well-controlled environment and allows the utilization of state-of-the-art experimental methods such as Particle Image Velocimetry with a high temporal and spatial resolution. We have analyzed flow measurements from the meteorological observatory Kopisty (Czech Republic) where an 80m tall meteorological tower equipped with 3D sonic anemometers at four levels is installed. We have built a wind-tunnel scaled model of the observatory and its vicinity to study spatial coherent structures and their characteristics in neutrally stratified ABL flows. The wind-tunnel measurement revealed large organized structures as low- and high-momentum regions or clusters of correlated longitudinal and vertical velocity fluctuations (e.g., sweep and ejection events). Those structures are not eddies, they are in the shape of irregular waves. They can be a few hundred meters long and up to a hundred meters wide. The structures’ size is increasing with the height above ground and in the lowest 100 m, the majority of them are attached to the ground. Both measurements and model results showed the dependence of the flow structure on the upwind terrain conditions. The coherent structures are smaller and stronger in the flow coming from the hilly forested terrain (southwest wind direction) compared to the flow coming from the lakeside (northeast wind direction).