Implementation of an Urban Canopy Parameterization in WRF-chem. Preliminary results

The impact of a city on the planetary boundary layer structure is relevant both for the dispersion of the pollutants and for the local microclimate. Any strategy to improve urban climate (e.g. by mitigating the urban heat island intensity) based on a modification of the urban structure or urban materials, must also be evaluated in terms of its impact on pollutant dispersion (e.g. modification of mixing height and temperature which affect chemical concentrations and reactions). On the other hand, a strategy to reduce urban pollution can have an impact also on urban climate (e.g. reduction of aerosols concentration modifies radiation). In order to evaluate all these complex feed-backs a meteorological model must be able to resolve the most important atmospheric features of the urban canopy, as well as the physics and the chemistry of the pollutant in an interactive way. Within this context, an urban canopy parameterization (Martilli et al., 2002) has been implemented in the WRF-Chem modeling system (Grell et al., 2005) in order to represent effects of the urban morphology on atmospheric processes. The urban parameterization accounts for the drag induced by the buildings, the turbulent effects, the trapping of radiation and shadowing in the canyon and the exchanges of heat between the interior and exterior of the buildings. In this contribution, the technique used in the implementation will be presented by means of bi-dimensional tests, showing the capability of the improved model to reproduce the most important features of the urban boundary layer. In order to appreciate the impact of the atmospheric features induced by the urban morphology on chemical atmospheric behavior, also some results of reactive pollutants dispersion will be presented.

References: Grell, GA; Peckham, SE; Schmitz, R; McKeen, SA; Frost, G; Skamarock, WC; Eder, B. 2005, Fully coupled "online" chemistry within the WRF model, Atmospheric Environment, 39 (37): 6957-6975. Martilli, A., Clappier, A., and Rotach, M. W., 2002: An urban surface exchange parameterization for mesoscale models', Boundary-Layer Meteorol., 104, 26, 304.