Impact of coupling a microscale computational fluid dynamics model with a mesoscale model on urban scale contaminant transport and dispersion Mukul Tewari a, ⁎, Hiroyuki Kusaka b , Fei Chen a , William J. Coirier c , Sura Kim c , Andrzej A. Wyszogrodzki a , Thomas T. Warner a a National Center for Atmospheric Research, Boulder, Colorado, United States b University of Tsukuba, Tsukuba, Japan c CFD Research Corporation, Huntsville, Alabama, United States article info abstract Article history: Received 31 December 2008 Received in revised form 24 December 2009 Accepted 19 January 2010 Results are presented from a study designed to evaluate the impact upon urban area transport and dispersion (T&D) modeling accuracy by coupling a microscale computational fluid dynamics (CFD) model with a mesoscale numerical weather prediction (NWP) model. The CFD model taking part in the evaluation was the CFD-Urban model while the NWP model was the Weather Research and Forecasting (WRF) model. The following two different approaches of supplying initial and boundary conditions to drive CFD-Urban were evaluated by comparing the resulting tracer gas transport fields to field data: (i) using observation obtained from a single sounding site during the URBAN 2000 field experiment and (ii) using WRF output in quasi-steady mode. The WRF and the CFD-Urban model results were evaluated against data obtained from the Intensive Observation Period (IOP) 10 during the URBAN 2000 field experiment. It was found that the CFD-Urban T&D prediction was significantly improved when using wind fields produced by downscaling WRF output as initial and boundary conditions. One key reason for such success is that the turning of lower boundary layer wind and pressure gradient are well represented in the time-varying three-dimensional WRF fields. © 2010 Elsevier B.V. All rights reserved. Keywords: WRF model CFD model Transport and dispersion 1. Introduction The objective of this study is to explore the potential benefit of coupling a microscale transport and dispersion (T&D) model with a mesoscale numerical weather prediction model in improving T&D modeling in complex urban environments. In the past decade, much progress has been made in order to improve the prediction of airflow and its dispersion in urban regions. For instance, Chan and Leach (2007) developed a computational fluid dynamics (CFD) model called Finite Element Model in 3-Dimensions (FEM3MP) to simulate airflow and dispersion of chemical/biological agents released in urban areas, and evaluated the model with observations from the Intensive Operating Period (IOP) 3 and 9 of the JU-2003 field study conducted in Oklahoma City, Oklahoma. Warner et al. (2004) evaluated the T&D using HPAC (Hazard Prediction and Assessment Capability) model against the URBAN 2000 data for simulating a Sulfur hexafluoride (SF 6 ) release scenario in Salt Lake City, Utah. On the other hand, mesoscale models were used to study the T&D and urban processes; e.g. Chin et al. (2005). Miao et al. (2009) have used the Weather Research and Forecasting model (WRF) coupled with the Urban Canopy Model (UCM) (hereafter WRF_UCM) to study the urban heat island and its influence on the diurnal evolution of boundary layer structures over the Beijing metropolitan regions. Using the fine-scale WRF_UCM, Miao and Chen (2008) indicated that the WRF model with 500-m grid spacing is able to simulate the formation of horizontal convective cells over the Beijing areas. Atmospheric Research 96 (2010) 656–664 ⁎ Corresponding author. 3450, Mitchell Ln, Boulder, CO 80301, United States. E-mail address: mukul@ucar.edu (M. Tewari). 0169-8095/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.atmosres.2010.01.006 Contents lists available at ScienceDirect Atmospheric Research journal homepage: www.elsevier.com/locate/atmos