Turbulent Winds and Temperature Fronts in Large-Eddy Simulations of the Stable Atmospheric Boundary Layer

The nighttime high-latitude stably stratified atmospheric boundary layer (SBL) is computationally simulated using high-Reynolds number large-eddy simulation on meshes varying from 200(3) to 1024(3) over 9 physical hours for surface cooling rates C-r = [0.25, 1] Kh(-1). Continuous weakly stratified turbulence is maintained for this range of cooling, and the SBL splits into two regions depending on the location of the low-level jet (LLJ) and C-r. Above the LLJ, turbulence is very weak and the gradient Richardson number is nearly constant: Ri similar to 0.25. Below the LLJ, small scales are dynamically important as the shear and buoyancy frequencies vary with mesh resolution. The heights of the SBL and Ri noticeably decrease as the mesh is varied from 200(3) to 1024(3). Vertical profiles of the Ozmidov scale L-o show its rapid decrease with increasing C-r, with L-o < 2 m over a large fraction of the SBL for high cooling. Flow visualization identifies ubiquitous warm-cool temperature fronts populating the SBL. The fronts span a large vertical extent, tilt forward more so as the surface cooling increases, and propagate coherently. In a height-time reference frame, an instantaneous vertical profile of temperature appears intermittent, exhibiting a staircase pattern with increasing distance from the surface. Observations from CASES-99 also display these features. Conditional sampling based on linear stochastic estimation is used to identify coherent structures. Vortical structures are found upstream and downstream of a temperature front, similar to those in neutrally stratified boundary layers, and their dynamics are central to the front formation.