The turbulent velocity is a key variable determining the dispersion of pollutant in the atmosphere, therefore a large interest is expressed by scientists in estimating this quantity at all scales, in particular in the urban context . The flow and turbulence structure of the atmospheric boundary layer above urban areas is significantly perturbed by the density and distribution of buildings and other obstacles, by the thermal effect of the ‘urban heat island’ and by the presence of topographical inhomogeneities. In the framework of the UTP Project, a thorough investigation of the characteristics of the flow and turbulence in urban canopy was pursued both with an experimental approach, carrying out an intensive observational field campaign in the city of Turin (north-west of Italy), and analysing the observed data. In this work we analyse the observed standard deviations of the wind velocity fluctuations gathered for 15 months during the UTP campaign at a 25 m mast, equipped with three sonic anemometers at 5 m, 9 m, 25 m height. Turin is characterised by a complex urban fabric and by a very frequent low wind regime. We studied the distribution of the wind-velocity fluctuation standard deviations as a function of the atmospheric stability and we drew an analytical function to describe these variables, on the basis of a non-linear best fit on the measured data. Then, we compared the observed quantities to correspondent values as calculated by two literature turbulence parameterizations, used in advanced dispersion models for the turbulent velocity fluctuations, Hanna (1982) and Degrazia et al. (2000). From the analysis performed, we inferred that, in UTP case, the low-wind conditions might be more critical than the urban geometry for the correct parameterization of the boundary layer turbulence. We found that the two parameterizations are efficient in unstable and neutral conditions, while in the stable case they do not represent the actual turbulence of the boundary layer in the UTP experiment. The formulation derived by the best-fit of the observations may therefore be considered as a solution to parameterize the standard deviations of the wind velocity fluctuations in low-wind regime and stable conditions.
Analysis of urban boundary-layer turbulence on thebasis of an experimental campaign in Turin city - UTP Project
RICHIARDONE, Renzo
2011-01-01
Abstract
The turbulent velocity is a key variable determining the dispersion of pollutant in the atmosphere, therefore a large interest is expressed by scientists in estimating this quantity at all scales, in particular in the urban context . The flow and turbulence structure of the atmospheric boundary layer above urban areas is significantly perturbed by the density and distribution of buildings and other obstacles, by the thermal effect of the ‘urban heat island’ and by the presence of topographical inhomogeneities. In the framework of the UTP Project, a thorough investigation of the characteristics of the flow and turbulence in urban canopy was pursued both with an experimental approach, carrying out an intensive observational field campaign in the city of Turin (north-west of Italy), and analysing the observed data. In this work we analyse the observed standard deviations of the wind velocity fluctuations gathered for 15 months during the UTP campaign at a 25 m mast, equipped with three sonic anemometers at 5 m, 9 m, 25 m height. Turin is characterised by a complex urban fabric and by a very frequent low wind regime. We studied the distribution of the wind-velocity fluctuation standard deviations as a function of the atmospheric stability and we drew an analytical function to describe these variables, on the basis of a non-linear best fit on the measured data. Then, we compared the observed quantities to correspondent values as calculated by two literature turbulence parameterizations, used in advanced dispersion models for the turbulent velocity fluctuations, Hanna (1982) and Degrazia et al. (2000). From the analysis performed, we inferred that, in UTP case, the low-wind conditions might be more critical than the urban geometry for the correct parameterization of the boundary layer turbulence. We found that the two parameterizations are efficient in unstable and neutral conditions, while in the stable case they do not represent the actual turbulence of the boundary layer in the UTP experiment. The formulation derived by the best-fit of the observations may therefore be considered as a solution to parameterize the standard deviations of the wind velocity fluctuations in low-wind regime and stable conditions.
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