1.3
The interaction of down-valley and canyon flows and their effect on mean vertical motions in the Salt Lake Valley
Jerome D. Fast, PNNL, Richland, WA; and L. S. Darby and R. M. Banta
A month long meteorological field campaign sponsored by the Department of Energy’s Environmental Meteorology Program was conducted in the Salt Lake Valley during October 2000 to study vertical transport and mixing (VTMX) processes. Measurements made during the 2000 VTMX field campaign focused on nocturnal stable periods and morning and evening transition periods within the urban valley. A wide range of remote sensing and in situ measurements were made including those from surface meteorological stations, temperature data loggers, sonic anemometers, radiosondes, radar wind profilers, sodars, and lidars. Thermally-driven circulations, including nocturnal down-valley and canyon flows along the Wasatch Mountains, were regularly observed when the synoptic forcing was weak. Measurements from the surface stations and NOAA/ETL’s Doppler lidar indicated complex convergence patterns in the valley atmosphere. At times, southerly down-valley flow occurred throughout most of the valley atmosphere with the strongest wind speeds in the valley center. However, at other times the down-valley flow was weak and strong canyon flows propagated over the valley. To study the mean vertical motions that arise from the interaction of the down-valley and down-slope flows, a mesoscale model was run for several of the intensive observation periods with a 550 m horizontal grid spacing. Since the model qualitatively reproduced the characteristics of the down-valley and canyon flows as measured by radiosondes, radar wind profilers, and the Dopper lidar, the simulated models vertical motions were examined. Over the northeastern end of the valley where the down-valley and Parleys Canyon flows converged, a narrow band of rising motions greater than 5 cm s-1 was simulated. Strong sinking motions were produced over the valley sidewalls as the flow from Parleys canyon descended into the valley. Over the downtown area the motions were generally sinking, which would tend to trap pollutants in that region during the night. The predicted vertical motions will be compared with measured vertical velocity at a few locations in the valley. While a 5 cm s-1 vertical velocity may seem small, it is large enough to transport an air parcel vertically 180 m in one hour. The converging flows persisted for several hours so that the contribution of the mean vertical motions to pollutant transport is likely to be significant. Pollutants may be vented out of the nocturnal boundary layer by flows converging in the valley or trapped in the layer by divergent flows in other locations.
Session 1, PBL Structure and Circulations I
Monday, 17 June 2002, 8:45 AM-10:45 AM
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