Simulations of dispersion in complex terrain using a higher-order closure model

A three-dimensional higher-order closure dispersion model is used to simulate dispersion from an elevated point source in an area with complex terrain in the Colorado River Valley (southwestern U.S.). The central part of the domain is characterized by the valley surrounded by ridges typically 1000 m high. The mean and turbulence values predicted by a higher-order closure fluid dynamic model are input to the dispersion model. The model uses a polar coordinate system with its origin at the point source. In the first part of the study the dispersion calculations were evaluated against surface and aircraft measurements of SO2 concentrations. The simulated SO2 concentrations agree reasonably well with both the measured surface and aircraft data. In the second part of the study the dispersion model reveals the highly complex structure and evolution of the plume in this type of topographic forcing. It is shown that the plume is quite frequently divided into two separate parts because the wind field diverges over this complex terrain. Furthermore:, the model is able to simulate the asymmetry and tilting of the plume caused by wind shear and local upslope flows.