Wind-tunnel study of atmospheric stable boundary layers over a rough surface

Wind-tunnel simulations of the atmospheric stable boundary layer (SBL) developed over a rough surface were conducted by using a thermally stratified wind tunnel at the Research Institute for Applied Mechanics (RIAM), Kyushu University. The present experiment is a continuation of the work carried out in a wind tunnel at Colorado State University (CSU), where the SBL flows were developed over a smooth surface. Stably stratified flows were created by heating the wind-tunnel airflow to a temperature of about 40-50°C and by cooling the test-section floor to a temperature of about 10°C. To simulate the rough surface, a chain roughness was placed over the test-section floor. We have investigated the buoyancy effect on the turbulent boundary layer developed over this rough surface for a wide range of stability, particularly focusing on the turbulence structure and transport process in the very stable boundary layer. The present experimental results broadly confirm the results obtained in the CSU experiment with the smooth surface, and emphasizes the following features: the vertical profiles of turbulence statistics exhibit different behaviour in two distinct stability regimes with weak and strong stability, corresponding to the difference in the vertical profiles of the local Richardson number. The two regimes are separated by the critical Richardson number. The magnitudes in turbulence intensities and turbulent fluxes for the weak stability regime are much greater than those of the CSU experiments because of the greater surface roughness. For the very stable boundary layer, the turbulent fluxes of momentum and heat tend to vanish and wave-like motions due to the Kelvin-Helmholtz instability and the rolling up and breaking of those waves can be observed. Furthermore, the appearance of internal gravity waves is suggested from cross-spectrum analyses.