A Study of Stability Conditions in an Urban Area

In simulating airflow and tracer dispersion in urban areas, it is critically important to use appropriate stability conditions. Due to the lack of relevant field measurements and other reasons (such as limitations of existing turbulence models), most modelers often assume that nearly neutral conditions are appropriate to use for the entire urban area being simulated. The main argument for such an assumption is that atmospheric stability (as seen in the Richardson number) is determined by both mechanical stresses and buoyant forcing but, for a typical urban setting with a given thermal stability or sensible heat flux, the building-induced mechanical stresses can become so dominant to drive the resulting stability toward nearly neutral conditions.

Results from our recent simulations of two Joint URBAN 2003 releases by Chan and Lundquist (2005) and Lundquist and Chan (2005), presented at the 9th Annual George Mason University Conference on Atmospheric Transport and Dispersion Modeling, Fairfax, VA, July 18-20, 2005, appear to support partially the assumption that urban areas tend toward neutral stability. Based on a model-data comparison for winds and tracer concentration in the near field and a comparison of wind and turbulence profiles in the urban wake region, our results indicate that neutral stability assumption appears to be valid for IOP 9 (a nighttime release with moderate winds) and also appears to be valid for IOP 3 (a daytime release with strong buoyant forcing) in the urban core area but is invalid in the urban wake region.

In this study, we will extend the above investigation into more of the Joint URBAN 2003 releases and use observed wind and turbulence measurements in addition to those in the wake region. By a more detailed model-data comparison of the wind and concentration fields and changes of the friction velocity field in the entire computational domain, we expect to be able to arrive at a more definitive characterization of stability conditions for the urban area of Oklahoma City.

Acknowledgement. This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48.