Third International Workshop on "Geographical Analysis, Urban Modeling, Spatial Statistics"

1. Maria Ioannilli and Enrico Rocchi University of Rome “Tor Vergata” “ Urban Roughness Parameters Calculation in the City of Rome by Applying Analytical and Simplified Formulations: Comparison of Results” The International Conference on Computational Science and Applications Perugia 2008 Geographical Analysis, Urban Modelling, Spatial Statistics

3. Urban Canopy Parametrization (UCP) URBAN CANOPY PARAMETERIZATION The geometric and morphological characteristics of the urban installation are represented through the set of parameters below Urban Canopy Parametrization (UCP) Mean vegetation height Vegetation plan area density Vegetation top area density Vegetation frontal area density Mean orientation of streets Plan area fraction surface covers Percent directly connected impervious area Building material fraction Mean building height Standard deviation of building height Building height histograms Building wall-to-plan area ratio Building height-to-width ratio Building plan area density Building rooftop area density Building frontal area density Mean canopy height Canopy plan area density Canopy top area density Canopy frontal area density Roughness length Displacement height Sky view factor Vegetation, Other UCPs Building UCPs Canopy UCPs Mean vegetation height Vegetation plan area density Vegetation top area density Vegetation frontal area density Mean orientation of streets Plan area fraction surface covers Percent directly connected impervious area Building material fraction Mean building height Standard deviation of building height Building height histograms Building wall-to-plan area ratio Building height-to-width ratio Building plan area density Building rooftop area density Building frontal area density Mean canopy height Canopy plan area density Canopy top area density Canopy frontal area density Roughness length Displacement height Sky view factor Vegetation, Other UCPs Building UCPs Canopy UCPs

4. These two parameters express the change of the aerodynamic characteristics of the territory caused by the city The city, in fact, is characterized by a so called urban roughness, which is the cause of the wind velocity decrease, and of turbulence and energy’s exchanges increase. The following pictures show three examples of different degrees of urban roughness : high (a), middle (b) and low (c) (a) (b) (c) Urban Canopy Parametrization (UCP) - Roughness length (Z0) and Displacement height (Zd) Urban Canopy Parametrization (UCP) For the aim of the work, the roughness length and the displacement height have been selected among the range of the UCP parameters.

8. Raupach’s model (1994) has been developed for random building arrangements. Raupach introduces these two equations that tie the aerodynamic parameters to the middle height of the buildings and to the frontal area index Raupach’s model can be used for frontal area densities lower than 0.1–0.2 because the model cannot describe overlapping of sheltering in dense arrays. Increasing the involved frontal area, the effect of mutual sheltering becomes more meaningfulI and the evaluation of Zo can be overestimated Raupach’s model (1994) Urban Canopy Parametrization (UCP) Roughness length (Z0) and Displacement height (Zd) : calculation models

10. They are assumed to have triangular x – z cross section, and their width is assumed to be equal to the buildings width w. In this way, the recirculation volume is approximated as: RECIRCULATION ZONE where L R and L F are the frontal and leeward recirculation zone lengths L R and L F are estimated from numerical and experimental results. They are approximated as L R + L F = 4L g where L g is the geometrical influence scale: Urban Canopy Parametrization (UCP) Roughness length (Z0) and Displacement height (Zd) : calculation models Mutual sheltering in regular arrangements with large lateral spacings (say Sy/w >1) may not be well described by an average upward flow displacement z d . Hence, an “in-plane” zero displacement height z d,pl is used instead of z d ; for z d,pl , only the buildings rows are considered, not the streets parallel to the wind in between. In normal arrays z d and z d,pl are related by: z d = (w / dy) z d,pl The calculation of z d,pl is a function of : dimension of the recirculation zone, density and arrangement of the obstacles

11. Urban Canopy Parametrization (UCP) Roughness length (Z0) and Displacement height (Zd) : calculation models STAGGERED ARRAYS WITH OVERLAPPING ROWS (w > dy) dy / w < 1 NORMAL ARRAYS for low densities, if: Sx > 4Lg for high densities, if: STAGGERED ARRAYS for low densities, if : for high densities, if :

12. Because of measuring problems related to evaluation of urban roughness parameters, a new approach using a roughness mapping tool has been tested: evaluation of roughness length Zo and zero displacement Zd from cadastral databases which contains all buildings within a 2.7 x 2.2 km 2 area in the centre of Strasbourg The basic model of the present application, for the calculation of roughness length is Bottema’s model (1997). The main difference between this application and the basic model is the evaluation of the zero displacement height Zd. For irregular groups, a direct calculation of Zd from the volume of buildings and their recirculation zones is far too complicated. Therefore, a simple power-law approximation of regular-group-model results is used: Assumed wind direction: N or S Urban Canopy Parametrization (UCP) The ordinary application In this equation Zd is a function of plan area density Bottema-Mestayer’ application (1998)

15. (4) General assumptions for the calculation of the parameters 1 – Qualification of the block's fronts relatively to the wind direction

24. Tools : software GIS ARC/Info Implementation Because of the strong territorial character of the studied problem, the GIS software has been chosen as an effective tool of resolution, particularly in the optics to develop an automatic procedure of calculation valid for every urban context We have chosen, therefore, to use the software ArcInfo, with all its extensions We have elaborated a AML script (ARC/Info Macro Language) to implement the automatic procedure of calculation on the Municipio IX

26. STEP 1 : development of basic cover Implementation 1 2 3 4 1 - input ( polygonal volumetric units ) 2 - elaboration (urban blocks polygonal) 3 - elaboration (linear blocks topology) 4 - elaboration (single fronts identification )

27. Choice of the wind direction Selection of the involved arcs : 694 su 1431 STEP 2 : calculation of geometric parameters and development of the reference final file Implementation N 340° N 140°

28. STEP 2 : calculation of geometric parameters and development of the reference final file Implementation middle points dx dy

31. Rocchi Raupach Bottema Mestayer Results Zd – Comparison among the three methods

32. Results Z0 – Comparison among the three methods Rocchi Raupach Bottema Mestayer

33. Results Zd– 3D visualization of the comparison among the three methods Rocchi Raupach Bottema Mestayer Rocchi Raupach Bottema Mestayer

34. Results Z0– 3D visualization of the comparison among the three methods

35. Rocchi Bottema/Mestayer Raupach Rocchi Raupach Rocchi Bottema/Mestayer Bottema/Mestayer Raupach Zd COMPARISON Z0 COMPARISON Results Zd and Z0 as a function of the frontal area density after the exclusion of the “border effects”

36. The comparison has been done with a grid cell 2-D 200m x 200m Results Comparison among the obtained results with two different wind directions

37. CONCLUSIONS By comparing the results, we can observe that the three curves' trend is quite similar while a more sensible difference is detected in the z 0 and z d values. Moreover, this difference increases with the lower values of frontal and planar density values. This trend can be explained by analyzing the different approaches adopted in the previous three models. The Bottema-Mestayer' model is empirical and adopts, as principal parameters, the building's height and their planar density; so it doesn't consider the mutual distance between the building's blocks. Moreover, from many application, it seems that the model tends to return overestimated values with frontal density values greater than 0.2. The developed procedure, on the contrary, is founded on the analytical evaluation of the mutual distance between buildings and these distances influence, due the weight they have in the model formulation, the results. The analytical adopted approach allows to apply the model in whichever urban context and wind direction. Few studies are now available, concerning the application of theoretical models of urban roughness parameters estimation at existing urban contexts; so it is quite difficult to fully appreciate the goodness of the obtained results. At this time we are looking for the validation of the procedure by employing it in urban areas already tested with other methods.