The vertical-velocity skewness in the atmospheric boundary layer without buoyancy and Coriolis effects

One of the main features of near-neutral atmospheric boundary layer (ABL) turbulence is the positive vertical velocity skewness S k w above the roughness sublayer or the buffer region in smooth-walls. The S k w variations are receiving renewed interest in many climate-related parameterizations of the ABL given their significance to cloud formation and to testing sub-grid schemes for Large Eddy Simulations (LES). The vertical variations of S k w are explored here using wind tunnel and flume experiments collected above smooth, rough, and permeable-walls in the absence of buoyancy and Coriolis effects. These laboratory experiments form a necessary starting point to probe the canonical structure of S k w as they deal with a key limiting case (i.e., near-neutral conditions). Diagnostic models based on cumulant expansions, realizability constraints, and constant mass flux approach routinely employed in the convective boundary layer as well as prognostic models based on third-order budgets are used to explain variations in S k w for the idealized laboratory conditions. The failure of flux-gradient relations to model S k w from the gradients of the vertical velocity variance σ w 2 are explained and corrections based on models of energy transport offered. Novel links between the diagnostic and prognostic models are also featured, especially for the inertial term in the third-order budget of the vertical velocity fluctuation. The co-spectral properties of w ′ / σ w vs w ′ 2 / σ w 2 are also presented for the first time to assess the dominant scales governing S k w in the inner and outer layers, where w ′ is the fluctuating vertical velocity and σ w is the vertical velocity standard deviation.