Some plume dispersion highlights from JU03 and URBAN 2000

Daytime and nighttime stability and turbulence conditions in the Central Business District during JU03 were similar. Nevertheless, significant day-night differences in plume dispersion were observed. Nighttime plumes were more likely to have reduced concentration fluctuation intensities, higher normalized surface concentrations, suppressed vertical mixing, a greater prevalence of data characterized by a normal distribution, and less susceptibility to high toxic loads for a given mean concentration and pollutant toxicity. Daytime plumes were more likely to have higher concentration fluctuations, lower normalized surface concentrations, more uniform vertical mixing, a greater prevalence of data characterized by a lognormal distribution, and greater susceptibility to high toxic loads. These differences are likely related to a combination of wind direction and meander, release location, stability conditions in the approach flow, and convective mixing. The potential roles of meander and convective mixing will be briefly examined.

The IOP7 experiment in URBAN 2000 demonstrated how small scale flows can play a key role in plume dispersion, at least in an urban boundary layer set in complex terrain. This can lead to the appearance of apparent “upwind dispersion” and make modeling very difficult.