We examine the dependence on wind speed of the share of the mean turbulent kinetic energy among the three velocity components in the near-neutral surface layer. To contrast the general behaviour and the local effects, four datasets are considered, corresponding to different surfaces and environmental conditions. For high wind speeds (i.e., wind speed ≈ 10 ms −1 ), the shares are well-defined and about the same for all sites. As wind speed decreases (becoming ≈ 1 ms −1 ), large record-to-record variability occurs giving, on average, an almost isotropic state for the horizontal velocity components. Through spectral analysis, we relate this behaviour to the low-frequency, submeso motions and to the lack of conditions required by Reynolds averaging. The implications for modelling are also discussed, showing that the wind speed, or a related quantity, must be accounted for, besides stability, in second-order closures.

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The effect of submeso motions on small-scale turbulence is studied considering the budget of the vertical flux of stream-wise momentum, uw , in the atmospheric stable boundary layer (SBL). A parameter expressing the strength of the submeso effect is defined, and the budget is evaluated from observations both for small and large submeso effect. It results that submeso motions affect the efficiency of the vertical transport by small-scale turbulence, having implications on the terms composing the momentum flux budget and on its corresponding closures.

This paper presents a new modification of the second order turbulence closure that removes the critical gradient Richardson number limitation typically found in Mellor-Yamada style models. The mean wind speed and potential temperature profiles are derived for the newly modified model in terms of similarity and structure functions depending on the gradient Richardson number. The derivation is based on a second order boundary layer approximation in neutral to very stable stratification conditions. Some recent closure assumptions for pressure-temperature and heat flux are considered. Variances and covariances of the turbulent fluctuations are also investigated with respect to the gradient Richardson number. The new model predictions are confronted with some well known models.