Plume dispersion and bifurcation in directional shear flows associated with complex terrain

Abstract

The results of three atmospheric tracer studies involving winds in complex terrain are discussed. Winds above complex terrain frequently exhibit directional shear due to the presence of relatively shallow layers of air driven by surface heating and cooling. The three tracer tests were conducted to elucidate pollutant transport and dispersion in multi-directional flows in complex terrain. During the first test, in the Lost Hills region of the San Joaquin Valley of California, the tracer (sulfur hexaflouride, SF 6) was split into two distinct plumes by wind directional shear. During the second test, conducted in the California coastal mountains near Clear Lake in northern California, under conditions approaching minimum wind directional shear, no plume bifurcation was detected. The tracer dispersion characteristics during this test could be accurately modeled using the Gaussian plume model using dispersion coefficients corresponding to “D” stability. The third test was also conducted from the Clear Lake area and during this test the SF 6 plume was again bifurcated due to the action of wind directional shear. Estimates of the amount of tracer within each of the plumes were made for each test. These estimates accounted for essentially all of the tracer and showed that significant amounts of tracer (and thus airborne pollutants) can be transported by the winds aloft. Predictions of pollutant transport based on surface wind measurements cannot normally be expected to provide good results in complex terrain. The tracer experiments also showed that destabilization of the atmosphere by complex terrain can lead to efficient vertical transport of airborne pollutants through stable layers of air such as nocturnal ground-based inversions.