Coherent Structures in Weakly Stable Boundary Layers: Results from Fine Mesh LES

Coherent structures spanning a range of spatial scales, e.g., thermal plumes, shear-convective rolls, and hairpin vortices, are known to populate the convective and neutral atmospheric boundary layers. These structures promote turbulent transport and are key agents for generating turbulent fluxes of momentum and scalars at the surface and the entrainment zone. In the present work, we identify coherent structures in the less-studied weakly stable boundary layer using databases generated by fine mesh large-eddy simulation (LES). The boundary-layer studied is similar to the GABLS-1 weakly stable Arctic boundary layer described by Beare et al (2006), but with stronger homogeneous surface cooling. The LES are performed with 512^3 gridpoints in a 400^3 meter domain (spatial resolution of 0.78 m) over a time period of nearly 10 hours (equivalent to more than 420,000 time steps). We use a suite of eduction techniques, linear-stochastic estimation, two-point spatial correlations and flow visualization, to identify the dominate structures in these stable boundary layers. Similar to neutral boundary layers, we find coherent vortical structures emanate from the surface and propagate upward filling the boundary layer. The vertical tilt of the structures however depends on the background stratification and changes sign in the region of the low-level jet, i.e., the structures tilt upward from the surface below the jet and downward above the jet. Increasing stratification appears to reduce the upward tilt of the structures near the surface. Flow visualization also shows a strong correlation between the instantaneous vertical temperature gradient and these coherent structures. The implications of these structures on modeling the relationship between surface stress and the near surface winds is discussed.