Transition from attached to detached eddies in stably stratified boundary layers

Research on canonical turbulent boundary layers (TBLs) in recent decades has provided growing support for A. A. Townsend's hypothesis regarding attached eddies. The height-dependent size of these eddies is realized in physical space in part by the behavior of uniform momentum zones. Yet, canonical conditions are rarely realized in practical applications such as the atmospheric boundary layer (ABL) where buoyancy effects are typically relevant. This presentation uses large-eddy simulations of the ABL to investigate the influence of stable stratification on the properties of the attached eddies. The organization of eddies in canonical TBLs is similarly observed for the stable ABL: the turbulent flow field preferentially organizes into zones of uniform velocity and temperature separated by thin layers with enhanced gradients. The "jump" in velocity and temperature across the gradient layers remains proportional to the surface shear stress and heat flux regardless of stratification. However, the zones become smaller and lose their dependence on the wall position as the stratification increases. These trends in the turbulence organization can be used to explain logarithmic mean profile deviations in stratified conditions that are traditionally modelled using empirical similarity relations.