Experimental investigation of surface-layer parameters in low wind-speed conditions in a suburban area

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.