An analysis of the turbulence structure in a perturbed boundary layer and in low-wind regimes is presented. The study is based on 15 months of continuous wind and turbulence measurements gathered, within the framework of the Urban Turbulence Project, at three levels (5, 9 and 25 m) on a mast located in the outskirts of the city of Turin (Italy). The aim of the work is to investigate low-frequency processes in a perturbed boundary-layer. In fact, the urban canopy and the heat island, together with frequent low-wind conditions, interact with and modify the turbulence structure. In order to investigate this modification, the velocity Eulerian autocorrelation functions together with both the Eulerian and Lagrangian timescales are shown and compared with the classical theory. The comparisons show that in low-wind cases the velocity autocorrelation functions are not simply exponential but present an oscillating behaviour. A method of normalization is proposed together with an analysis on the applicability of this function. The estimated Lagrangian timescales are compared with two widely used parametrizations. It is found that the presence of the urban fabric influences the turbulence time-scales and suggests the development of new parametrizations. Finally, higher-order statistics are evaluated and the relationship between higher-order and lower-order moments are analysed, pointing out the effects due to the urban environment.

Abstract.Low wind conditions in a suburban area were investigated through the analysis of sonic anemometer observations gathered at three vertical levels, 5 m, 9 m and 25 m, during an experimental campaign carried out in the city of Turin in Northern Italy. The data analysis allowed assessing how the surface-layer variables are affected by the specific heterogeneous conditions characterized by calm regime. The applicability of the known theories for the surface layer and their limitations in such environment were investigated. The roughness and the stability parameters, the friction velocity and the wind velocity standard deviations were analyzed accounting for the variability of the wind speed and direction and of the urban structure. Formulations for some of these quantities, to be used in air pollution modelling, were considered, discussed and verified with the observations. The low-wind regime proved to be more effective in influencing the characteristics of the surface layer structure and of the turbulence than the urban geometry itself, this last mainly affecting the surface roughness parameters.

No abstract