Simulation of non-buoyant plume transport: how well does atmospheric stability control the short term fate of a plume?

Atmospheric transport of aerosols, pollutants or toxic agents released from a source will eventually form a plume like structure. Simulation of these containment plumes over domains larger than urban scales, is challenging as the plume transport is subjected to both local and meso-scale forcings. In our study, emphasis is given to the plume front advection as it is the leading edge of the plume and a centroid is defined as the point along the plume front, where the mass is concentrated. The schematics of the plume transport considered in this study and a 3D visualization of a developed plume are shown in the figure. We studied the dynamics and behavior of the plume front and its centroid’s vertical location. The regional Weather Research and Forecasting (WRF-ARW) model was adopted for simulating atmospheric transport of the plume by releasing passive tracers. The plume generated by the passive tracers is considered as a non-buoyant plume and the tracers used in the study have no physical properties as well as they have no effect on the environmental parameters. The influence of atmospheric conditions at the time of release and effect of release altitude are addressed for the plume transport within first four hours after the release. We used Monin-Obukhov length (L) as a measure of atmospheric stability and overall 15 cases were simulated during summer and winter months with different atmospheric stability conditions falling under Pasquill stability classes B, C and E at the time of release. Also, the effect of release altitude was studied by releasing the tracers at ground level, near ground level (21m) and 130m above ground level. The plume front centroid altitude is made non-dimensional by using Planetary Boundary Layer height (H) at that location and instance. From the results, the plume front vertical propagation is categorized into four types based on the behavior of the plume front centroid's vertical advection as shown in the figure. Among the categories proposed, the critical ones are the cases in which the plume front stays close to the surface thus retaining high concentrations. This categorization is meant to serve as a quick forecast tool in estimating the plume front centroid’s vertical height relative to the planetary boundary layer. We also observed that the short term plume characteristics are highly sensitive to the inversion layer height and the local wind speed. We identified from couple of cases whose release altitude is close to the inversion layer, that the plume front is pushed down towards the ground forming a fumigation type of plume structure. From the results, we observed that for a plume to cover a larger area, the desired conditions are neutral or stable atmosphere with moderate winds. In a stable environment due to lack of strong turbulent mixing, the plume concentrations remain high without too much diffusion and moderate winds will help carry this concentrated plume over long distances, thus covering a larger area. Similarly, if the plume is intended to last for a short amount of time, the desired release conditions are unstable environment with inversion layer much above the level of release. This will ensure that the plume is subjected to turbulent mixing because of the unstable ambience and so the plume gets diluted. From the above statements, it can be inferred that the level of concentration of a plume is dictated by the atmospheric stability while the range of plume transport is dictated by the local wind.