Project Sagebrush: Revisiting Short-Range Dispersion Using Modern Instrumentation

Most of the open-terrain tracer studies that researchers use for evaluating dispersion models at short ranges were conducted in the 1950s and 1960s. Perhaps the best known of these classical tracer studies is Project Prairie Grass, which took place in Nebraska in 1956. Prairie Grass and other early studies were well designed and backed by substantial resources, but they still had many limitations. For example, Prairie Grass was restricted to a summer dry period in July and August 1956 and thus did not consider possible seasonal variability. Moreover, measurements of vertical dispersion were limited to a single set of towers 100 m downwind with a maximum concentration measurement height of 17.5 m. Direct surface flux measurements were not available at the time, so stability and fluxes were inferred from mean wind and temperature profiles.

Both tracer technology and turbulence instrumentation have advanced significantly since the early studies. The Field Research Division (FRD) of the NOAA Air Resources Laboratory is planning a new series of tracer releases to revisit short-range dispersion in open terrain. These studies, called Project Sagebrush in a nod to Project Prairie Grass, will take place at the Idaho National Laboratory in Southeast Idaho using modern turbulence instrumentation and tracer technology. This project is planned to be a multi-year effort, with Phase 1 taking place in October 2013.

The primary tracer to be used in Project Sagebrush is SF6, with three different perfluorocarbons also available. FRD has approximately 150 bag samplers for measuring time-average concentrations together with ten trace-gas analyzers capable of measuring concentration fluctuations at 1 Hz. A 34-tower meteorological network, 915 MHz radar profiler with RASS, sodars, and sonic anemometers are available for measuring boundary-layer characteristics including turbulence. Collaborators are bringing additional instrumentation.

Phase 1 of Project Sagebrush will take place in October of this year. The focus of this phase will be dispersion in unstable to neutral atmospheric conditions. Five tracer releases are planned for the 4 Pasquill-Gifford stability categories A through D. An aircraft from the University of Tennessee Space Institute will be used for real-time vertical tracer sampling.

This presentation will compare and contrast Projects Prairie Grass and Sagebrush and provide preliminary results from Project Sagebrush Phase 1.