Plume Dispersion Anomalies in a Nocturnal Urban Boundary Layer in Complex Terrain

The URBAN 2000 experiments were conducted in the complex urban and topographical terrain in Salt Lake City, Utah, in stable nighttime conditions. Unexpected plume dispersion often arose because of the interaction of complex terrain and mountain-valley flow dynamics, drainage flows, synoptic influences, and urban canopy effects, all within a nocturnal boundary layer. It was found that plume dispersion was strongly influenced by topography, that dispersion can be significantly different than what might be expected based upon the available wind data, and that it is problematic to rely on any one urban-area wind measurement to predict or anticipate dispersion. Small-scale flows can be very important in dispersion, and their interaction with the larger-scale flow field needs to be carefully considered. Some of the anomalies observed include extremely slow dispersion, complicated recirculation dispersion patterns in which plume transport was in directions opposed to the measured winds, and flow decoupling. Some of the plume dispersion anomalies could only be attributed to small-scale winds that were not resolved by the existing meteorological monitoring network. The results shown will make clear the difficulties in modeling or planning for emergency response to toxic releases in a nocturnal urban boundary layer within complex terrain.