Thermals, vortices and fluxes in a strongly convective planetary boundary layer flow

Coherent structures in a strongly convective planetary boundary layer flow are studied. The overall updraft motions are elongated in the mean wind direction, forming a miniature roll circulation. 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 are 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. We also connected the conditional fields with the conventional flow statistics by reconstructing the conventional flow statistics from the conditional fields. The results show that the reconstructed statistics agree quite well with the statistics obtained from the conventional approach.