Experiments in the Stable Atmosphere: A Novel Boundary Layer Wind Tunnel Design

Stable atmospheric boundary layers are characterized by intermittent turbulence and strong buoyancy effects. Reynolds numbers defined by the friction velocity frequently exceed one million. The gradient Richardson number (a ratio of buoyancy and mechanical shear) is positive in stable conditions. When the Richardson number exceeds the critical threshold (~0.25), yet remains below unity, turbulence may persist sporadically. This behavior limits the applicability of lower order closure schemes which assume a continuously turbulent region. Due to the challenges of turbulence measurements over large spatial regions of the atmosphere, wind tunnel experiments are a feasible analog to field experiments. Variable-density wind tunnels provide a viable avenue to generate atmospheric flows in experimental conditions at moderate flow speeds. The Supertank is a novel experimental facility that maintains an atmospherically relevant Reynolds number and captures buoyancy effects. Supertank experiments will reach farther into the stable atmospheric regime than previously possible in wind tunnels, advancing the study of turbulence closure in the atmospheric boundary layer.