12.5
Ceilometer boundary layer measurements during the DOE VTMX/URBAN field experiment in Salt Lake City
Brent M. Bowen, LLNL, Livermore, CA, CA; and R. J. Calhoun and J. Räsänen
The DOE VTMX/URBAN field experiment was conducted during October 2000 in Salt Lake City in order to characterize nocturnal urban transport and diffusion and to evaluate simulation models. An array of wind and tracer sensors were located downtown during Intensive Operational Periods (IOP's) in order to determine small-scale wind fluctuations and tracer transport and dispersion, respectively. In addition, an enhanced lidar ceilometer was operated during the experiment in order to provide continuous and detailed boundary layer information. The ceilometer operated for about one half of the month, including during 7 of the IOP's. It also was operated at two other sites in order to compare and contrast with measurements taken downtown. Finally, two nighttime transects were conducted starting from a site near the foothills east of the city to 3 sites progressively farther west toward the valley center, and then returning to the starting point site by site. The ceilometer operated for at least 30 to 40 minutes at each site in order to capture regional boundary layer similarities and differences.
Results indicate that aerosol backscatter quickly develops after sunrise and extends up to one kilometer AGL during the late morning as the convective boundary layer develops on several sunny days. On nights with clear skies and katabatic winds, the aeorosol backscatter suggests a nocturnal boundary layer (NBL) between 200 and 300 m or so, with strongest returns within the lowest 50 m. However, elevated layers of strong backscatter are detected up to 500 m or so on several nights, especially at the foothill and downtown (IOP) sites. The two transects show a relatively continuous NBL 200 to 300 m deep with elevated layers of strong backscatter in the city and stronger near-surface returns reaching up to 50 to 100 m AGL near the valley center. One IOP is examined when intense backscatter up to 200 to 300 m AGL suddenly collapses to the ground and rebuilds and separates in the lowest 300 m. Simultaneous measurements from a weather station on top of a nearby building and a profiler indicate a surge of northerly winds followed by a general rise and three sharp oscillations in dew point temperature. Finally, the ceilometer was able to detect other phenomena such as cloud bases, virga, rainfall, blowing dust from strong winds, and even dust from a street sweeper.
Session 12, Urban field projects: URBAN-VTMX
Thursday, 23 May 2002, 1:30 PM-3:44 PM
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