SIMULATION OF METEOROLOGICAL FIELDS WITHIN AND ABOVE URBAN AND RURAL CANOPIES WITH A MESOSCALE MODEL (MM5) ⋆ SYLVAIN DUPONT ⋆⋆ , TANYA L. OTTE ⋆⋆⋆ and JASON K. S. CHING ⋆⋆⋆ NOAA Air Resources Laboratory, Atmospheric Sciences Modeling Division, Research Triangle Park, North Carolina, U.S.A. (Received in final form 18 November 2003) Abstract. Accurate simulation of air quality at neighbourhood scales (on order of 1-km horizontal grid spacing) requires detailed meteorological fields inside the roughness sub-layer (RSL). Since the assumptions of the roughness approach, used by most of the mesoscale models, are unsatisfactory at this scale, a detailed urban and rural canopy parameterisation, called DA-SM2-U, is developed inside the Penn State/NCAR Mesoscale Model (MM5) to simulate the meteorological fields within and above the urban and rural canopies. DA-SM2-U uses the drag-force approach to represent the dynamic and turbulent effects of the buildings and vegetation, and a modified version of the soil model SM2-U, called SM2- U(3D), to represent the thermodynamic effects of the canopy elements. The turbulence length scale is also modified inside the canopies. SM2-U(3D) assesses the sensible and latent heat fluxes from rural and urban surfaces in each of the computational layers inside the canopies by considering the shadowing effect, the radiative trapping by the street canyons, and the storage heat flux by the artificial surfaces. DA-SM2-U is tested during one simulated day above the city of Philadelphia, U.S.A. It is shown that DA-SM2-U is capable of simulating the important features observed in the urban and rural RSL, as seen in the vertical profiles of the shear stress, turbulent kinetic energy budget components, eddy diffusivity, potential air temperature, and specific humidity. Within the canopies, DA-SM2-U simulates the decrease of the wind speed inside the dense canopies, the skirting of the flow around the canopy blocks, warmer air inside the vegetation canopy than above open areas during the night and conversely during the day, and constantly warmer air inside the urban canopy. The comparison with measurements shows that the surface air temperature above rural and urban areas is better simulated by DA-SM2-U than by the ‘standard version’ of MM5. Keywords: Eddy diffusivity, Energy budget, Mesoscale models, Rural and urban canopies, Urban boundary layer. ⋆ Disclaimer. The information in this manuscript has been prepared under funding by the United States Environmental Protection Agency. It has been subjected to Agency review and approved for publication. Mention of trade names or commercial products does not constitute endorsement or recommendation for use. ⋆⋆ Present address: Sylvain Dupont, INRA-EPHYSE, B.P. 81, 33883 Villenauve D’Ornon, France. E-mail: sdupont@bordeaux.inra.fr ⋆⋆⋆ Gn assignment to the National Exposure Research Laboratory, U.S. Environmental Protection Agency. Boundary-Layer Meteorology 113: 111–158, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands.