Probing the role of buoyancy in the stable atmospheric boundary layer: direct and large-eddy simulations

Understanding and parameterizing turbulent fluxes in statically-stable atmospheric boundary layers (SABLs) remains a challenging yet very important problem in geophysical fluid dynamics. The complexities of these flows are further exacerbated by the increased sensitivity to unsteadiness and surface variability. To address the role of these exacerbating factors, direct numerical simulations and large eddy simulations are performed. Under the highest stabilities, global intermittency (the almost compete decay of turbulence and then its regeneration) is observed. The intermittent bursts are important to study under these conditions since they become the main agent of vertical transport in the SABL. Under more moderate stabilities, continuous turbulence is maintained, but it is significantly damped compared to neutral flows. This reduction of the TKE under stable conditions is very well known; however, in this prsentation, we show that it is mainly triggered by reduced mechanical production associated with reduced transport of Reynolds stresses from aloft toward the surface, rather than by direct destruction of TKE by buoyancy. Variability of surface temperature is shown to result in unexpected flow patterns: TKE is potentially higher under the more stable patches due to advection, and the subsidence and lofting of air over the different patches can counteract the effect of spatial TKE variability on the vertical fluxes.