Vertical-velocity skewness in the buoyancy-driven boundary layer

A number of puzzling features of the skewness of the vertical velocity field, S_w(z), are found in observations and large-eddy simulations (LES) of the buoyancy-driven planetary boundary layer (PBL). For example, observations of S_w(z) in cases where the air is heated from below indicate that S_w(z)>0 and remains relatively constant for z≥0.3z_i, whereas all large-eddy simulations of these cases show a continuing increase of S_w(z) with height. In cases where the air is both heated from below and cooled from above, as in some of the stratus-topped PBL cases S_w is positive in the upper layer and negative in the lower layer. In the present paper we examine the skewness distribution from direct numerical simulations of several simple archetypes of buoyancy-driven turbulent flow. -from Authors