Turbulent Diffusion in Complex Terrain

Observed variations of ?? and ?? in the nocturnal drainage flow in a deep valley

“…In contrast, median ad, values are insensitive to wind speed, and remain nearly constant at 0.09 to 0.13 rad during the nighttime stable conditions. Following local sunrise (after 0600MST), while the valley warmed and the flow direction began to turn up-valley, u0 and a+ increased sharply to their maximum The observed u6 values seem to have been much less affected by complex terrain though they were larger than those normally observed over flat terrain; this is in agreement with the results of Egan (1975) and Panofsky et al (1978). The vertical wind speed (and direction) fluctuations are produced by small eddies, which scale with the height above the ground and adjust rapidly to changes in terrain.…”

“…This information can be used directly to specify the dispersion parameters in puff transport models such as INPUFF- (Petersen and LavdFs, 1986), which has been modified for the simulation of tracer concentration data in the Brush Creek valley (Rae er al., 1989), as well as for the study of the temporal and spatial variations of these turbulence parameters in the nocturnal drainage flow in complex terrain. The direct measurement of turbulence (standard deviations of fluctuations of the wind directions, or of the velocity components) is the preferred approach to specify dispersion parameters in air quality models, especially for complex terrain, Egan (1975) and Panofsky et al, (1978) noted that lateral diffusion is enhanced by mesoscale complex terrain features to a greater extent than the vertical diffusions. Hanna (1981) showed that values of u0 in complex terrain during nighttime stable conditions are generally enhanced over those measured or predicted by similarity theory over flat terrain.…”

Evaluation of a Puff Dispersion Model in Complex Terrain

“…In addition, the same dispersive action of the terrain would be expected to produce a positive bias in Gaussian plume estimates. As suggested by Egan 17 and Hanna, 18 terrain induced bias can be reduced by artificially increasing the plume sigma values or adjusting the stability categories. Improvement in point by point accuracy of the simulated concentration values would be difficult, however, without a more explicit treatment of thermal and aerodynamic effects beyond the scope of Gaussian modeling.…”

“…16 These stability estimates are appropriate to the flat terrain locations of most of the meteorological measurement sites and do not reflect known enhancements of sigma-theta appropriate to the rugged terrain itself. 17 ' 18 Meteorological conditions were updated by the model at 15-minute intervals.…”

A Modification of the Gaussian Dispersion Equation to Accommodate Restricted Lateral Dispersion in Deep River Valleys

“…1 ' 2 Use of the standard Gaussian dispersion equation to represent this enhanced lateral plume spread in a narrow valley may be inappropriate since consideration should also be given to the restriction of the horizontal plume spread due to impingment against the valley walls. This technical note describes a simple modification to the Gaussian equation which was developed to accommodate this effect.…”