Morphological Parameters for Urban Dispersion Models

Because of increasing concern about air quality in cities the dispersion of pollutants over and through urban areas is an increasingly important research area. As a result of recent advances in digital photogrammetry and remote sensing, databases of the actual 3D geometry of city centre areas are becoming more easily available at low cost. The aim of the present work is to reduce this large amount of data to a structured input for dispersion models, i.e. to extract the important flow and dispersion parameters from the urban morphology data.

Considering a pollutant plume travelling over a dense urban area, two cases are differentiated according to the relative depth of the pollutant plume (hp) with respect to the height of the buildings (H). When hp/H >>1, the relevant dispersion parameters are the aerodynamic roughness length (z0) and to a lesser extent the displacement height (d). These parameters can be calculated using the plan area ratio (lp ) and the frontal area ratio (lf) of the buildings (Grimmond and Oke, 1999).

When hp/H=< 1, the situation is far less well understood and is still unclear what the relevant parameters for dispersion models are. Indeed, the dispersion problem is very likely to depend on the category of flow type as a function of the width to height aspect ratio of the street (Oke 1978). Further, it has been shown that the length over height ratio of the street canyons is also of importance, as well as, obviously, the wind direction. We believe that the relevant geometrical parameters for dispersion models should include the along-wind and the cross-wind street canyon aspect ratios distribution for any given wind direction.

In both cases, hp/H >>1 and hp/H=< 1, the relevant parameters are obtained by analysing urban Digital Elevation Models (DEMs) which are regularly spaced grids of elevation values. The DEMs can be viewed as greyscale maps, where the level of grey is proportional to the height of the urban surface, and analysed with image processing techniques. Examples of the parameters obtained from high-resolution databases (pixel size ~1m) for London, Toulouse, Berlin and Birmingham will be presented.

The parameters calculated for Birmingham city centre are entered as input variables into a operational dispersion model (ADMS3) and numerical results are analysed and compared with measurements obtained in a recent tracer field experiment (Britter et al. 2000, Cooke et al.).

References

Britter, R. E., Caton, F., Cooke, K., Di Sabatino, S. and Simmonds, P. 2000: Dispersion of a passive tracer within and above an urban canopy: Birmingham experiment. Third Symposium on the Urban Environment, University of California, Davis.

Cooke K.M., P.G. Simmonds, G.N. Nickless, (2000) The Development of a Highly Sensitive and Selective Technique to Monitor Tracer Dispersion within an Urban Environment. Third Symposium on the Urban Environment, University of California, Davis.

Grimmond, C.S.B and Oke, T. R. 1999: Aerodynamic properties of urban areas derived from analysis of surface form.J. Appl. Meteorol., 38, 1261-1292.

Oke, T. R. 1978: Boundary Layer Climates. J