Turbulence structure of stable boundary layers with a near-linear temperature profile

By using a thermally stratified wind tunnel, we have successfully simulated stably stratified boundary layers (SBL), in which the mean temperature increases upward almost linearly. We have investigated the flow structure and the effects of near-linear stable stratification on the transfer of momentum and heat. The vertical profiles of turbulence quantities exhibit different behaviour in two distinct stability regimes of the SBL flows with weak and strong stability. For weak stability cases, the turbulent transfer of momentum and heat is basically similar to that for neutral turbulent boundary layers, although it is weakened with increasing stability. For strong stability cases, on the other hand, the time-mean transfer is almost zero over the whole boundary-layer depth. However, the instantaneous turbulent transfer frequently occurs in both gradient and counter-gradient directions in the lower part of the boundary layer. This is due to the Kelvin-Helmholtz (K-H) shear instability and the rolling up and breaking of K-H waves. Moreover, the internal gravity waves are observed in the middle and upper parts of all stable boundary layers.