3-DIMENSIONAL BUOYANCY-INDUCED AND SHEAR-INDUCED LOCAL-STRUCTURE OF THE ATMOSPHERIC BOUNDARY-LAYER

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 s hear in ABL structure, a wide range of -z(i)/L ABL states, from 0.44 t o 730, is analyzed. It is known that buoyancy-induced updrafts and dow ndrafts are primarily responsible for the upward flux of momentum, hea t, and passive scalar, and strongly influence near-ground horizontal m otions. These buoyancy-induced features of the convective boundary lay er (CBL) are presented here in clearly observable 3D visual images of vertical velocity and temperature, showing large turbulent cell-like s tructure several z(i) in horizontal extent. The horizontal length scal es of the temperature field near the ground are found to be of the ord er of the horizontal velocity length scales. It is noted by comparing visual structure with spectra that the disparity in the near-ground ho rizontal 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 nea r-neutral atmospheric boundary layer are near-ground shear-induced reg ions of high-and low-speed flow. Several features of the low-speed str eaks are examined. Most importantly, there exists an influence by z(i) -scale outer eddies on the structure of near-ground streaks, which, it is argued, strengthens at higher -z(i)/L. Warm fluid accumulates in t hese low-speed streaks, localizing buoyancy forces there that, at suff iciently high -z(i)/L, drive the warm fluid vertically within sheets a ligned with the mean wind. These coherent sheetlike updrafts turn at t he capping inversion to form the often-observed large-scale streamwise roll vortices. In this way, it is argued, shear-induced near-ground s tructure of the surface layer directly influences the global structure of the moderately convective ABL. It follows, therefore, that inadequ acies in subgrid-scale parameterization near the ground can influence the structure of the entire ABL. In particular, the well-known overpre diction of mean shear near the ground by standard Smagorinsky closures increases the streamwise coherence of the shear-induced low-speed str eaks, thereby increasing the overall streamwise coherence of the verti cal velocity field.