STRUCTURE OF THE ENTRAINMENT ZONE CAPPING THE CONVECTIVE ATMOSPHERIC BOUNDARY-LAYER

The authors use large-eddy simulation (LES) to investigate entrainment and structure of the inversion layer of a clear convectively driven p lanetary boundary layer (PBL) over a range of bulk Richardson numbers, Ri. The LES code uses a nested grid technique to achieve fine resolut ion in all three directions in the inversion layer. Extensive flow vis ualization is used to examine the structure of the inversion layer and to illustrate the temporal and spatial interaction of a thermal plume and the overlying inversion. It is found that coherent structures in the convective PBL, that is, thermal plumes, are primary instigators o f entrainment in the Ri range 13.6 less than or equal to Ri less than or equal to 43.8. At Ri = 13.6, strong horizontal and downward velocit ies are generated near the inversion layer because of the plume-interf ace interaction. This leads to folding of the interface and hence entr ainment of warm inversion air at the plume's edge. At Ri = 34.5, the i nversion's strong stability prevents folding of the interface but stro ng horizontal and downward motions near the plume's edge pull down poc kets of warm air below the nominal inversion height. These pockets of warm air are then scoured off by turbulent motions and entrained into the PBL. The structure of the inversion interface from LES is in good visual agreement with lidar measurements in the PBL obtained during th e Lidars in Flat Terrain field experiment. A quadrant analysis of the buoyancy flux shows that net entrainment Bur (or average minimum buoya ncy flux <(w theta)over bar>(min)) is identified with quadrant IV <(w( -)theta(+))over bar> < 0 motions, that is, warm air moving downward. P lumes generate both large negative quadrant II <(w(+)theta(-))over bar > < 0 and positive quadrant III <(w(-)theta(-))over bar> > 0 buoyancy fluxes that tend to cancel. The maximum vertical gradient in potential temperature at every (x, y) grid point is used to define a local PBL height, z(i)(x, y). A statistical analysis of z(i) shows that skewness of z(i) depends on the inversion strength. Spectra of z(i) exhibit a sensitivity to grid resolution. The normalized entrainment rate w(c)/w (), where w(c) and w(*) are entrainment and convective velocities, va ries as ARi(-1) with A approximate to 0.2 in the range 13.6 less than or equal to Ri less than or equal to 43.8 and is in good agreement wit h convection tank measurements. For a clear convective PBL, the author s found that the finite thickness of the inversion layer needs to be c onsidered in an entrainment rate parameterization derived from a jump condition.