Stable and convective boundary-layer flows in an urban array

In this paper non-neutral approaching flows were employed in a meteorological wind tunnel on a regular urban-like array of rectangular buildings. As far as stable stratification is concerned, results on the flow above and inside the canopy show a clear reduction of the Reynolds stresses and an increment of the Monin-Obukhov length up to 80%. The roughness length and displacement height were also affected, with a reduction up to 35% for the former and an increment up to 12% for the latter. A clear reduction of the turbulence within the canopy was observed. In the convective stratification cases, the friction velocity appears increased by both the effect of roughness and unstable stratification. The increased roughness causes a reduction in the surface stratification, reflected in an increase of the Monin-Obukhov length, which is double over the array compared to the approaching flow. The effect on the aerodynamic roughness length and displacement height are specular to the SBL case, an increase up to 50% of the former and a reduction of the same amount for the latter.Comment: 11 figures; accepted version; to appear in the Journal of Wind Engineering and Industrial Aerodynamic

Urban boundary layers over dense and tall canopies

Wind tunnel experiments were carried out on four urban morphologies: two tall canopies with uniform-height and two super-tall canopies with a large variation in element heights (where the maximum element height is more than double the average canopy height, $h_{max}$=2.5 $h_{avg}$). {The average canopy height and packing density were fixed across the surfaces to $h_{avg} = 80$ mm, and $\lambda_{p} = 0.44$, respectively.} A combination of laser doppler anemometry and direct drag measurements were used to calculate and scale the mean velocity profiles {within the boundary layer depth, $\delta$}. In the uniform-height experiment, the high packing density resulted in a `skimming flow' regime with very little flow penetration into the canopy. This led to a surprisingly shallow roughness sublayer ($z\approx1.15h_{avg}$), and a well-defined inertial sublayer above it. {In the heterogeneous-height canopies, despite the same packing density and average height, the flow features were significantly different.} {The height heterogeneity enhanced mixing thus encouraging deep flow penetration into the canopy. A deeper roughness sublayer was found to exist and extend up to just above the tallest element height (corresponding to $z/h_{avg} = 2.85$)}, which was found to be the dominant lengthscale controlling the flow behaviour. {Results points toward the existence of an inertial sublayer for all surfaces considered herein despite the severity of the surface roughness ($\delta/h_{avg} = 3 - 6.25$)}. This contrasts with previous literature.Comment: 25 pages, 12 figures. Revised submission to Boundary-Layer Meteorolog

INTERCOMPARISON, SENSITIVITY AND UNCERTAINTY ANALYSIS BETWEEN DIFFERENT URBAN DISPERSION MODELS APPLIED TO AN AIR QUALITY ACTION PLAN IN TUSCANY, ITALY

The Tuscan Regional Administration funded project MoDiVaSET-2 (MOdellistica DIffusionale per la VAlutazione di Scenari Emissivi in Toscana 2) was established in order to develop a decision support modelling system for implementing the Air Quality Action Plan for the metropolitan area of Florence, Prato and Pistoia. The objective of the work is to build an integrated meteorological and dispersion models for simulating and evaluating different future emission scenarios of PM10, NOx and NO2 in the study area. With this purpose, the project included several 1-year long dispersion modelling applications and a detailed evaluation study, including sensitivity, validation and uncertainty analysis. Several dispersion models (ADMS-Urban, CALPUFF, CALINE4, SAFE AIR II and CALGRID) were applied and evaluated against monitoring data; the intercomparison between different models is crucial in order to develop reliable modelling techniques. The obtained results point out the importance of including the following critical factors: smaller scale effects (monitoring stations are often located in complex environments; this implies a decrease in the effectiveness of validation studies) and secondary pollution (primary PM10 levels are only a small part of the total PM10 concentrations; much of the urban PM10 is actually produced by chemical transformations and other physical mechanisms, for example, resuspension). In order to understand the weight of these issues, further modelling options (full chemistry and street canyon simulations) were investigated by using CAMx and smaller scale nested models. All the factors listed above affected the evaluation work. However, this does not alter the validity of the scenario analysis, because it is based on the differences between calculated primary pollutants concentrations