Model Evaluation of the Impact of Thermally Induced Valley Circulations in the Lake Powell Area on Long- Range Pollutant Transport

Observations in the Lake Powell area have indicated that a significant visibility impairment often exists in this area located in southern Utah and northern Arizona (a schematic illustration of the area is provided in Figure 1). Since several National Park areas within this region are mandated by federal legislaton as Class I areas under the Prevention of Significant Deterioration (PSD) of the federal air quality program, considerable research effort has been devoted in recent years to understand the atmospheric processes affecting visibility in that area. Studies evaluating the potential contribution of local sources in the area to the visibility impairment is reported, for example, in Bluemental et al., and Yu and Pielke. Most of the focus in recent years, however, has been directed to visibility oriented studies including evaluations of the impact of long-range transport of polluted air masses into the area during the summer. During the summer, a shallow thermal low dominates the lower atmosphere of the southwest United States (e.g., Tang and Rieter). Studies reported, for example, by Pitchford, Marias, Flucihini et al., Ashbaugh et al. suggest that southwest synoptic flow associated with that meteorological system may lead to the long-range transport of polluted air masses from southern California and southern Arizona into the Lake Powell area. Also, these studies suggested situations involved with southeast synoptic flow advecting polluted air into the Lake Powell area from New Mexico and Texas. This second common transport situation is associated with flow around the west side of a subtropical ridge which is situated to the east of the thermal heat low. All of the transport studies in this region, however, have been based on observed synoptic wind data. The temporal and spatial resolution of such data is typically insufficient to evaluate the impact of mesoscale generated circulations (i.e., mountain/valley flows with typical horizontal scales of less than several hundred kilometers) on the large-scale transport. Therefore, in those studies, the transport modification of a pollutant mass by mesoscale circulations were not considered. Also in those studies, turbulence processes were not considered. Hence, while these synoptic transport evaluations provide bulk information concerning air movement over mesoscale domains, additional evaluations are required when thermally-induced circulations exist along the path of the pollution (see, for example, Pielke et al., for a detailed evaluation and discussion of this aspect). The Lake Powell area, which is located in the Colorado River basin, is affected by valley-induced circulations. Our study is designed specifically to provide an evaluation of modifications to longrange transport by mesoscale effects in that area during the summer using the modeling tools outlined in the next section. Generally, the impact of mesoscale thermally-induced circulations on longrange transport of a polluted air mass is unaddressed in the literature. Therefore, although the present study provides a specific evaluation for the Lake Powell area, it also provides some insight as to the related impact of valley circulations in the general case.