The vertical-velocity skewness in the inertial sublayer of turbulent wall flows

We provide empirical evidence that within the inertial sub layer of adiabatic turbulent flows over smooth walls, the skewness of the vertical velocity component $S{k}_w$ displays universal behaviour, being constant and constrained within the range $S{k}_w\approx 0.1-0.16$, regardless of flow configuration and Reynolds number. A theoretical model is proposed to explain the observed behaviour, including the observed range of variations of $S{k}_w$. The model clarifies why $S{k}_w$ cannot be predicted from down-gradient closure approximations routinely employed in meteorological and climate models whereby $S{k}_w$ impacts cloud formation and dispersion processes. The model also offers an alternative and implementable approach.