An investigation of flow regimes affecting the Kansas City region

The simulation of transport and diffusion of airborne pollutants in a complex terrain environment can be very difficult. Results from the Department of Energy project Atmospheric Studies in Complex Terrain (ASCOT) clearly indicate that shallow terrain differences (10 – 20 m) can result strong horizontal variability in the windfield and significant vertical shear. In such situations, contaminants released with small differences in height or location can easily be advected along widely divergent trajectories. These studies suggest that even modest terrain features can have a significant influence on the nature of pollutant transport under stable conditions.

Influence from complex terrain was of particular concern for the second Missouri-Kansas Ozone Study (MOKAN) of 2003-2004. Previous photochemical modeling from the 2000 MOKAN study, which focused primarily upon the Kansas City metropolitan area, did not meet EPA draft model performance criteria. Diagnostic evaluation cited one area of potential concern from the previous meteorological modeling - insufficient resolution of both the horizontal and vertical grid structure. The coarser horizontal and vertical resolution from the 2000 study was reflective of the state of computing capability. However, more recent meteorological model modeling suggested that the coarser resolution resulted in the inability of the meteorological model to adequately replicate many unique features of the structure of the local meteorological fields induced by the convergence of two river valleys in the core of the metropolitan area.

In order to investigate the influence of complex terrain in the Kansas City metro area, atmospheric modeling staff from USEPA Region 7 and the Kansas Department of Health and Environment (KDHE) conducted high resolution meteorological and photochemical modeling for two 8-hour ozone episodes (August 1998 and July 2002). The Penn State/NCAR Mesoscale Meteorological Model (MM5), in conjunction with the lagrangian model FLEXPART, was used to investigate the detailed local scale flow structure of the Kansas City region for these episodes. MM5 was operated at a resolution of approximately 1 km in order to adequately capture the terrain features associated with the Kansas and Missouri River Valleys. The meteorological and lagrangian particle model results clearly indicated under stable conditions that the coupling of near-surface flow and flow above was weak and often resulted in significant directional shear when simulating sources with a hypothetical elevated term release. Under stable atmospheric conditions, contaminant transport in near-surface flows tended to follow the directional gradient of the river valleys while sources with elevated release terms were usually transported in the direction of prevailing synoptic conditions. Utilizing the high resolution wind fields, photochemical modeling simulations were made with CAMx. The photochemical modeling results also confirmed that, under certain stable conditions, pollutant transport was highly influenced by the local terrain features. The results show that both regional and synoptic scale flows influenced the meteorology within the Kansas City area over the study periods. The simulated circulations also provide a physical basis for understanding the high spatial and temporal variability of ozone concentrations observed over the Kansas City monitoring network.