INFLUENCE OF CANYON-INDUCED FLOWS ON FLOW AND DISPERSION OVER ADJACENT PLAINS

The study investigates two effects that a valley or canyon opening ont o a plain can have on flow and contaminant dispersion over the downwin d plain. The first effect is the channeling of strong ambient flow by the canyon when the wind is nearly aligned with the canyon axis. Two c ases showed that these conditions produced a region of focused flow do wnwind of the canyon mouth. The second effect is the formation of cany on exit jets on nights with weaker ambient flow. In two case studies u nder these conditions strong exit jets formed that were several hundre d meters deep. The jets remained narrow and strong at least 10 km onto the plains, and in one of the cases the jet extended more than 20 km over the plains. These deep jets only lasted 2-3 h, and they had a sma ll but significant effect on surface-released tracer transport as indi cated by surface sampling. We hypothesize that the near-surface advect ion of tracer was accomplished by a thin katabatic layer of flow, and that an elevated release or elevated sampling would have indicated a g reater effect of the exit jet on tracer transport.