Impact of coupling a microscale computational fluid dynamics model with a mesoscale model on urban scale contaminant transport and dispersion

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.