Geophysical Fluid Dynamics: Stratified Flows

A long-lived Ekman boundary layer is distinguished from its nocturnal counterpart by the presence of ambient stratification that is independent of surface cooling. We perform direct numerical simulations at a Reynolds number of 900, based on the laminar Ekman layer depth, to investigate the turbulent structure of the stable Ekman boundary layer (SABL) under three distinct stratification regimes: surface cooling-dominated, ambient stratification-dominated, and a balanced regime with comparable contributions from both mechanisms. Our results show that dominant ambient stratification induces a thermal inversion that primarily affects the temperature field, while turbulent kinetic energy remains only weakly impacted. Analysis of the turbulent potential energy budget reveals an additional production term associated with ambient stratification, whereas the term linked to the mean temperature gradient transitions from a source to a sink, behaving more like a transport term. We further identify a three-tiered interaction structure governing energy transport: (1) between the mean flow and second-order statistics, (2) between second-order statistics and the turbulent potential energy budget, and (3) between turbulent kinetic and potential energies via turbulent heat flux, facilitating conservation of total turbulent energy. Finally, we validate the dimensionless gradients of momentum and scalars using an extended similarity theory (Zilitinkevich and Esau, 2007) and assess limitations in turbulence closure models arising from modifications to the turbulent potential energy budget due to ambient stratification.

Funding acknowledgement

NSF (award #2203610)