Structures, temperature fronts, and intermittent behavior in stable boundary layers

Past observational and computational evidence clearly demonstrates that coherent structures are key agents for generating turbulent fluxes of momentum and scalars in geophysical boundary layers. In the present work, we seek to identify coherent structures in the less-studied weakly stable boundary layer using fine mesh databases generated by large-eddy simulation (LES). The canonical boundary-layer studied is the GABLS-1 weakly stable Arctic boundary layer described by Beare et al. (2006). The LES are performed with (200^3, 512^3, 1024^3) meshes in a 400^3 meter domain with spatial resolution varying from (2, 0.78, 0.39) m over a time period of nearly 10 hours; this equates to almost 900,000 time steps for the finest resolution simulation. We find that the boundary layer statistics change markedly when the resolution jumps from 2 m to 0.78 m, but less so when the resolution decreases from 0.78 m to 0.39 m. The bulk Richardson number is a sensitive indicator of the simulation resolution and decreases with finer resolution primarily as a consequence of changing mean shear. As part of the analysis, we use a suite of eduction techniques, namely 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, tilt forward, and fill the boundary layer. As a consequence of the coherent structures and background stable stratification large-scale forward-leaning (tilted) temperature fronts emerge. We emphasize that the temperature fronts are spawned internally in the boundary layer, ie with no heterogenous forcing. Instantaneous vertical temperature transects at fixed x-y locations, which cut across the temperature fronts, then appear spatially intermittent. Flow visualization confirms a strong correlation between the instantaneous local vertical temperature gradient and the temperature fronts induced by the coherent structures. We speculate that these stable boundary-layer dynamics are generic features that can be used to partially interpret the temperature intermittency observed from kites and tethered balloons (eg., Balsley et al 2003).