Estimating urban dispersion meteorology from suburban measurements

Meteorological variables such as surface friction velocity and heat flux are critical inputs to the current generation of dispersion models such as AERMOD (the AMS/EPA Regulatory Model). Because urban measurements are usually not available, there is a need for methods that can estimate urban meteorological variables from more routinely available rural, suburban or airport measurements.

This paper used a two-dimensional internal boundary-layer (IBL) model to estimate urban variables from suburban measurements. This model was evaluated by Luhar, Venkatram and Lee (2006) using data from the Basel Urban Boundary Layer Experiment (BUBBLE) conducted in the city of Basel, Switzerland (Rotach et al., 2005). In this paper, the IBL model is used to analyze data collected in Riverside County, California in 2007.

Comparison of the suburban and urban measurements shows that the urban wind speed, friction velocity and turbulent velocities are lower than the corresponding suburban values at the measurement height of 3 m. However, the turbulent intensities are higher at the urban site. When suburban and urban conditions are stable, the best estimates of the surface friction velocity at the urban site are obtained when it is assumed that both sites are neutral. For unstable conditions, assuming that the Obukhov lengths at the upwind suburban and urban site are the same yields acceptable results. The IBL model generally overestimates the friction velocity at the Riverside urban site because the measurements are within the roughness-sublayer (RSL). The local friction velocity profile suggested by Rotach (2001) can reduce this overestimation, although this correction is sensitive to the choice of the RSL height.