The Share of the Mean Turbulent Kinetic Energy in the Near-Neutral Surface Layer for High and Low Wind Speeds

Abstract

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.

Motions on Second-Order Moment Budgets in the Stable Atmospheric Boundary Layer

“…2b), in agreement with the literature [32]. Thus, taking c = 1 [9], κ = 0.35, and ( −uw / w 2 ) 1/2 = 0.8, we obtain c uw = 0.3.…”

“…Concerning unstable conditions, it is well recognized that horizontal velocity statistics do not follow MOST because of the direct effect of large-scale, convective eddies, which span the entire boundary-layer depth [8]. But critical conditions for MOST applicability occur also for near-neutral conditions when wind speed is low [9].…”

Interaction Between Waves and Turbulence Within the Nocturnal Boundary Layer

“…For n < 10 −2 , the spectral levels are larger than expected, suggesting the presence of energetic nonturbulence wave motions. As oftentimes observed in the literature (e.g., Schiavon et al 2019), the vertical velocity component is marginally modified by the wave activity. The division between low and high frequencies is emphasized by a spectral gap dividing wave and smallscale contributions between f = 6 × 10 −3 s −1 and f = 2 × 10 −2 s −1 (corresponding to periods in the range 50 ÷ 167 s), with a characteristic cutting frequency at 1 ÷ 2×10 −2 s −1 (50 ÷ 100 s).…”

On the parametrizations for the dissipation rate of the mean turbulence kinetic energy in stable conditions

“…In particular, A u = 102, A v = 17, B u = 33, and B v = 9.5 are taken from the Kansas spectra (Kaimal and Finnigan 1994), whilst A w = 4.3 and B w = 4.2 from previous analysis (Schiavon et al 2019) (because a model different from…”