engleski

Spectral turbulence characteristics of the stable boundary layer over non-homogeneous terrain

The turbulence characteristics of flows over heterogeneous and patchy vegetation still present an ongoing issue, mainly due to the lack of experimental results. In this study we focus on turbulence spectral characteristics of the wintertime, nighttime boundary layer over heterogeneous terrain. Measurements were performed on a tall mast which was situated in a small area of walnut forest. Measurements at five levels above the canopy height (approximately h=18 m) were influenced by the upwind conditions which represent different types of surfaces, namely, agricultural land, forested hills and urban surfaces. We performed an analysis of the spectral structure, local isotropy and dissipation rates of turbulent kinetic energy. Results showed that horizontal and vertical velocity spectra have the predicted -5/3 slope in the high-frequency range indicating the existence of the inertial subrange at the upper four levels (z>1.5h), whereas the lowermost level (just above the canopy) is clearly influenced by the roughness elements showing that on average there is no well-defined inertial subrange. Testing the local isotropy hypothesis more thoroughly resulted in a ratio of the horizontal spectral densities (Sv/Su) approaching the 4/3, while the ratio of the vertical to the longitudinal spectral density (Sw/Su) was less than 1 for all levels indicating an anisotropic turbulence above the canopy. Spectral peaks of wind components showed clear height dependence with an apparent shift toward higher frequencies moving from the lowest to the highest level. Moreover, spectra of all velocity components exhibited strong stability dependence with a tendency for a peak shift to higher frequencies as stratification becomes more stable. Spectra normalized by the local friction velocity showed a good collapse in the inertial subrange for the three highest levels, suggesting that far from the canopy height the flow has reached a local equilibrium. The distinct features that we observed in our results might be attributed to different dynamic forcing caused by surfaces of different roughness that influenced the flow.

Turbulence spectra; Heterogeneous terrain; Local isotropy; Dissipation rate of turbulent kinetic energy

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