Laboratory and numerical studies of the atmospheric stable boundary layers

We have investigated the turbulence phenomena of SBLs for a wide range of stability, particularly focusing on the effects of strong stratification on turbulent boundary layers, using a thermally stratified wind tunnel. In parallel with wind tunnel experiments, to understand the turbulence features and fluid dynamics in detail, we have also performed numerical simulations of SBLs under the boundary conditions similar to those in the wind tunnel experiments. The numerical studies based on a finite-difference method (FDM) are direct Navier-Stokes simulations without any turbulence model (DNS). Under the Boussinesq approximation, the governing equations consist of the Navier-Stokes, continuity and energy equations for 3D incompressible stratified flows. Stable stratification rapidly suppresses the fluctuations of streamwise velocity and temperature as well as the vertical velocity fluctuation. Momentum and heat fluxes are also significantly decreased with increasing stability and become nearly zero over the whole boundary depth of the boundary layer with very strong stability. From the flow visualization in both wind tunnel experiment and DNS, wave-like motions driven by buoyancy and waves due to the Kelvin-Helmholtz instability can be observed locally and intermittently in a SBL flow with strong stability.