Urban morphological analysis for mesoscale meteorological and dispersion modeling applications: current issues

Accurate predictions of air quality and atmospheric dispersion at high spatial resolution rely on high fidelity predictions of mesoscale meteorological fields that govern transport and turbulence in urban areas. However, mesoscale meteorological models do not have the spatial resolution to directly simulate the fluid dynamics and thermodynamics in and around buildings and other urban structures that have been shown to modify micro- and mesoscale flow fields. Mesoscale models therefore have been adapted using numerous approaches to incorporate urban effects into the simulations. One approach is to introduce urban canopy parameterizations to approximate the drag, turbulence production, heating, and radiation attenuation induced by sub-grid scale buildings and urban surface covers. Preliminary results of mesoscale meteorological and air quality simulations for Houston demonstrated the importance of introducing urban canopy parameterizations to produce results with high spatial resolution that accentuates variability, highlights important differences, and identifies critical areas. Although urban canopy parameterizations may not be applicable to all meteorological and dispersion models, they have been successfully introduced and demonstrated in many of the current operational and research mode mesoscale models, e.g., COAMPS, HOTMAC, MM5, and RAMS.

The primary consequence of implementing an urban parameterization in a mesoscale meteorological model is the need to characterize the urban terrain in greater detail. In general, urban terrain characterization for mesoscale modeling may be described as the process of collecting datasets of urban surface cover physical properties (e.g., albedo, emissivity) and morphology (i.e., ground elevation, building and tree height and geometry characteristics) and then processing the data to compute physical cover and morphological parameters. Many of the surface cover and morphological parameters required for mesoscale meteorological models are also needed by atmospheric dispersion models. Thus, most of the discussion below is relevant to both types of modeling.

In this paper, the term urban morphological analysis will be used to define the component of urban terrain characterization concerned with the morphological parameters. Furthermore, the focus will be building morphological parameters; therefore, the term urban morphological analysis will refer exclusively to the task of inventorying, computing or estimating building morphological parameters. Several approaches to perform urban morphological analysis exist; however, all have in common three types of practice issues related to the uncertainty of (1) data, (2) parameter definitions and calculation methods, and (3) extrapolation techniques. The objective of this paper is to describe the state-of-the-practice of urban morphological analysis by reviewing the primary approaches presented in the literature and outlining and commenting on key aspects of the three types of practice issues listed above.