Pollutant Transport and Diffusion in Katabatic Flows

Abstract

The characteristics of pollutant transport and diffusion of a passive contaminant in a two-dimensional katabatic flow over a simple slope are examined using a primitive equation hydrodynamic model. It is shown that pollutants released above the drainage layer can be entrained into the layer and diffused to the ground surface. For elevated release within the drainage layer, subsidence in the flow leads to relatively high surface concentrations of pollutants close to the stack. Pollutants released at ground level can spread through the entire depth of the drainage layer. This vertical diffusion is more effective for a shallow slope, resulting in higher concentrations at all heights, than for a steeper slope. These dispersion characteristics are quite different from those for stable flows over flat terrain. The differences result from increases of boundary-layer depth, wind speed, and turbulence as the katabatic flow develops downslope. The katabatic flow and dispersion model is tested by simulating the perfluorocarbon and heavy methane tracer releases for Night 4 of the 1980 ASCOT field study in Anderson Creek Valley, California. These tests show that the observed concentrations and the depth of the drainage layer in the lower region of the slope are underpredicted because the model could not simulate the convergence of drainage air (pooling) in the valley basin. The nightly average values of the observed concentrations, however, are predicted well. It is concluded that the model is applicable to nearly two-dimensional open slopes.