engleski

Low-level turbulence characteristics over inhomogeneous surface during wintertime

In this dissertation, the local similarity scaling approach was examined based on the multi-level measurements of atmospheric turbulence in the wintertime (December 2008 - February 2009) stable atmospheric boundary layer (SBL) established over a heterogeneous surface influenced by mixed agricultural, industrial and forest surfaces. The 62 m tower (levels 20, 32, 40, 55 and 62 m above ground) was situated in the middle of some 120 m - 480 m area of 18 m high walnut trees. The heterogeneity of the surface was characterized by spatial variability of both roughness and topography. In a first step local similarity theory in terms of flux-variance and flux-gradient relationships was investigated. Local similarity scaling was found to be promising even at forested site with highly inhomogeneous fetch conditions. Flux-variance and flux-gradient relationships responded differently to non-homogeneous surface characteristics. The flux-variance relationships obey local scaling but the corresponding non-dimensional functions do not exhibit the same parameter values as over HHF terrain. The roughness sublayer (RSL) influence appears to be larger than the distortion due to inhomogeneous surface conditions. Flux-gradient relationships, on the other hand, seem to be less influenced by surface inhomogeneity: they exhibit the same distinction into sub- and supercritical turbulence regimes as over horizontally homogeneous and flat (HHF) terrain and the data followed quite closely the respective stability functions from the literature. Analysis of spectral turbulence characteristics indicated that canopy scaling was more successful compared to the traditional surface layer scaling. The fact that canopy scaling can be successfully applied even within the transition layer suggests that large coherent eddies, which are generated at the canopy top, are dominating the turbulence structure up to a height which is more than three times the canopy height. For the present complex site local isotropy was not found. Namely, the reduced spectral power in the vertical velocity component was consequently reflected in the smaller values of the turbulent kinetic energy (TKE) dissipation rate estimated from the vertical component spectra compared to the estimates from horizontal components. This result has implications on the corresponding budgets of variances, as well as TKE budget, and for obtaining good correspondence with the commonly applied Kansas spectral models. Finally, analyzing the budget terms of the TKE equation, non-equilibrium conditions were found. The non-local dynamics are considered to be the main reason for the observed imbalance of TKE in the transition layer as well as for the observed breakdown of z-less regime in the strongly stable conditions. In the RSL, the turbulent transport of TKE is considered to be the main cause of the observed TKE imbalance in the neutral conditions.

Stable boundary layer; Local scaling; Forest canopy; Roughness sublayer; Turbulent kinetic energy; Spectral models; TKE budget

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