Three-Dimensional Buoyancy- and Shear-Induced Local Structure of the Atmospheric Boundary Layer

Abstract

Three-dimensional visualization together with statistical measures are used to describe the instantaneous local structure of the atmospheric boundary layer (ABL) under various stability states using large-eddy simulation (LES) data. To explore the relative roles of buoyancy and shear in ABL structure, a wide range of ?zi/L ABL states, from 0.44 to 730, is analyzed. It is known that buoyancy-induced updrafts and downdrafts are primarily responsible for the upward flux of momentum, heat, and passive scalar, and strongly influence near-ground horizontal motions. These buoyancy-induced features of the convective boundary layer (CBL) are presented here in clearly observable 3D visual images of vertical velocity and temperature, showing large turbulent cell-like structure several zi in horizontal extent. The horizontal length scales of the temperature field near the ground are found to be of the order of the horizontal velocity length scales. It is noted by comparing visual structure with spectra that the disparity in the near-ground horizontal scale between temperature and vertical velocity reflects the structure of more localized thermals within the large-scale cells. By contrast, the structure of the near-neutral atmospheric boundary layer is quite different. Recent LES studies have shown that, like the flat-plate boundary layer, the dominant energy-containing motions in a near-neutral atmospheric boundary layer are near-ground shear-induced regions of high- and low-speed flow. Several features of the low-speed streaks are examined. Most importantly, there exists an influence by zi-scale outer eddies on the structure of near-ground streaks, which, it is argued, strengthens at higher ?zi/L. Warm fluid accumulates in these low-speed streaks, localizing buoyancy forces there that, at sufficiently high ?zi/L, drive the warm fluid vertically within sheets aligned with the mean wind. These coherent sheetlike updrafts turn at the capping inversion to form the often-observed large-scale streamwise roll vortices. In this way, it is argued, shear-induced near-ground structure of the surface layer directly influences the global structure of the moderately convective ABL. It follows, therefore, that inadequacies in subgrid-scale parameterization near the ground can influence the structure of the entire ABL. In particular, the well-known overprediction of mean shear near the ground by standard Smagorinsky closures increases the streamwise coherence of the shear-induced low-speed streaks, thereby increasing the overall streamwise coherence of the vertical velocity field.