The share of the mean turbulent kinetic energy in the near-neutral surface layer for high- and low-wind conditions

We examine the dependence of the share of mean turbulent kinetic energy (TKE) among the three velocity components on wind speed in the almost-neutral surface layer. To contrast the general behaviour and the local effects, four datasets are considered, corresponding to different surface and
environmental conditions. It results that, for high-wind (i.e., wind speed 10 m s-1), the shares are well dened and about the same for all sites. As wind weakens (i.e.,  1 m s-1 or less), large case-to-case variability occurs for the horizontal components giving, on median, an almost isotropic state. Through spectral analysis, we relate this behaviour to low-frequency, submeso motions and the lack of conditions required by Reynolds averaging. The implications on closures and modelling are also discussed.