Local pressure-transport structure in a convective atmospheric boundary layer

Local pressure-transport structure in a convective atmospheric boundary lay er is studied through large-eddy simulation and a conditional sampling tech nique. Two cases are simulated: A free-convection boundary layer and a shea red convective boundary layer with -z(i)/L approximate to 17, where z(i) 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 r oot-mean-square resolved pressure fluctuation and the resolved negative pre ssure fluctuation due to -u(1,2)(r)u(2,1)(r) become much stronger in the sh eared case. Flow visualization demonstrates that strong pressure transport is physically correlated with vortical structure embedded within large-scal e updrafts. A conditional sampling technique is applied to study statistica l characteristics of resolved fields surrounding strong pressure transport events. The conditional field reveals a boundary-layer-scale roll circulati on 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 condit ional vorticity lines converge to four distinct regions relative to the the rmal: Large-scale horseshoe-shaped vorticity lines are wrapped around the t hermal; small-scale arch-shaped vorticity lines drag behind the thermal; he lical vorticity lines originate in the thermal core; and converging vortici ty lines are found above the neck of the large-scale horseshoe-shaped vorti city lines. These regions roughly coincide with conditional negative moment um fluxes. We thus conclude that local pressure-transport structures are sp atially associated with localized low pressure regions and strong vertical vorticity fluctuations, being embedded within thermals and advected along w ith large-scale convective rolls. (C) 2000 American Institute of Physics. [ S1070-6631(00)02405-3].