Local pressure-transport structure in a convective atmospheric boundary layer

Abstract

Local pressure-transport structure in a convective atmospheric boundary layer is studied through large-eddy simulation and a conditional sampling technique. Two cases are simulated: A free-convection boundary layer and a sheared convective boundary layer with −zi/L≈17, where zi is the boundary layer height and L is the Monin–Obukhov length. Results show that pressure-transport flux tends to increase turbulent kinetic energy in the lower part of the sheared convective boundary layer. Furthermore, the root-mean-square resolved pressure fluctuation and the resolved negative pressure fluctuation due to −u1,2ru2,1r become much stronger in the sheared case. Flow visualization demonstrates that strong pressure transport is physically correlated with vortical structure embedded within large-scale updrafts. A conditional sampling technique is applied to study statistical characteristics of resolved fields surrounding strong pressure transport events. The conditional field reveals a boundary-layer-scale roll circulation with a large-scale thermal located at its center and characterized by a negative pressure minimum. Conditional pressure transport is a gain in the lower part of the pressure minimum and a loss in the upper part. The conditional vorticity lines converge to four distinct regions relative to the thermal: Large-scale horseshoe-shaped vorticity lines are wrapped around the thermal; small-scale arch-shaped vorticity lines drag behind the thermal; helical vorticity lines originate in the thermal core; and converging vorticity lines are found above the neck of the large-scale horseshoe-shaped vorticity lines. These regions roughly coincide with conditional negative momentum fluxes. We thus conclude that local pressure-transport structures are spatially associated with localized low pressure regions and strong vertical vorticity fluctuations, being embedded within thermals and advected along with large-scale convective rolls.