16.6 Comparison of model and airborne measurement of AgI plumes from ground-based generators

Thursday, 23 August 2012: 5:00 PM
Priest Creek C (The Steamboat Grand)
Lulin Xue, NCAR, Boulder, CO; and X. Chu and B. Geerts

The ability of the WRF model to simulate plumes of tracers released from point sites on the ground is tested by comparing the model output with airborne measurements over complex terrain. Target weather is wintertime, strong-flow, low-stability conditions. In this case the tracer is AgI particles, typically 0.1-0.4 micron in diameter, released pyrotechnically in high concentrations from select point sources upwind of a target mountain. Airborne measurements of AgI generated ice nuclei (IN) plumes. The sampling aircraft used an updated NCAR acoustic IN counter over the Medicine Bow and Sierra Madre mountains in Wyoming. One of the nine cases which show strong plumes has been simulated in WRF including 500 m grid spacing runs along with a 100-m grid spacing Large Eddy Simulation (LES). The result suggest that WRF can reasonably simulate the dispersion and the diffusion of AgI particles in the atmosphere. Although the model under predicted the absolute magnitude of AgI concentration because of the imperfect representation of wind speed and direction in the model causing discrepancies in turbulent properties in the LES simulation, the corresponding high concentration of AgI particles was successfully simulated by the LES. Based on the results of this study, a simulation of finer resolution better agrees with observations. Comparisons between the model and observed upwind sounding show that the surface wind and humidity are not simulated very accurately. Another initial and boundary data (RTFDDA forecast data) which match the sounding better will be tested in the future instead of NARR data.
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