Composite coherent structures in a convective planetary boundary layer flow

Three-dimensional flow structures in a strongly convective atmospheric boundary layer with -zi/L~17 are studied through a conditional sampling technique. The data were generated by a large-eddy simulation code. We conditionally sampled fifteen quantities, including fluctuating velocity components, fluctuating temperature, fluctuating pressure, heat flux, momentum fluxes, vertical transport of horizontal kinetic energy, pressure transport, fluctuating vorticity magnitude and total vorticity vector field. As expected, the conditionally sampled vertical fluctuating velocity, fluctuating temperature, and fluctuating pressure are strongly correlated. The conditionally sampled pressure transport and vertical transport of horizontal turbulent kinetic energy are physically connected as well, consistent with those observed in instantaneous fields. We also found that the updraft motions are elongated in the along-wind direction. The updraft core which has the strongest vertical motions positions at about the center of the elongated updraft. The whole elongated structure draws ambient air into its base, forming a large-scale, finite-length, butterfly-shaped circulation. It resembles convective rolls observed in the slightly convective boundary layer. Analysis of the conditionally sampled vorticity vector field suggests that vortical structures may be formed at four distinct regions with respective to the updraft thermals: large-scale horseshoe-shaped vorticity lines are wrapped around the strong updraft; small-scale arch-shaped vorticity lines drag behind the strong updraft; helical vorticity lines originate from the core of the strong updraft; converging vorticity lines are found aloft the neck of the large-scale horseshoe-shaped vorticity lines. Conditionally sampled negative momentum fluxes physically correspond to the regions where vortical structures may be present, viz., the flanks and the upwind site of the updraft core. The isosurface of strong momentum flux in the vicinity of the updraft core appears to a mix of helical vorticity and horseshoe-shaped vorticity. In other words, near the surface, strong momentum flux joins the helical vorticity at the center of the strong updraft but with increasing height it connects to the leg and neck of the horseshoe-shaped vorticity. To assess the reliability of the conditionally sampled fields, the conventional flow statistics (i.e., the vertical distribution of average statistics) are reconstructed using the conditional data. The results are in excellent agreement with the statistics obtained from the conventional approach. Finally, a deterministic, conceptual model is proposed to illustrate coherent motions in strongly convective atmospheric boundary layers. The interplay of turbulent eddies of different forms and scales, such as miniature rolls, thermal plumes, vortical structures, and ejection and sweep eddies, is discussed.