Rorig, Miriam; Solomon, Robert; Krull, Candace; Peterson, Janice; Ruthford, Julia; Potter, Brian. 2013. Analysis of meteorological conditions for the Yakima Smoke Intrusion Case Study, 28 September 2009. Res. Pap. PNWRP-597. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 30 p. On 28 September 2009, the Naches Ranger District on the Okanogan-Wenatchee National Forest in south-central Washington state ignited an 800-ha prescribed fire. Later that afternoon, elevated PM2.5 concentrations and visible smoke were reported in Yakima, Washington, about 40 km east of the burn unit. The U.S. National Weather Service forecast for the day had predicted good dispersion conditions and winds that would carry the smoke to the less populated area north of Yakima. We undertook a case study of this event to determine whether conditions leading to the intrusion of the smoke plume into Yakima could have been predicted before the burn was ignited, either from forecasts and model output available on the day of the burn or from higher resolution model output made available only after the event. We evaluated three different meteorological model predictions: (1) 4-km resolution hourly weather predictions from the Weather Research and Forecasting (WRF) model that were available to forecasters on the day of the burn; (2) 4-km resolution WRF predictions at 10-minute intervals; and (3) 1.33-km resolution WRF predictions at 10-minute intervals. We found that predicted winds from the 4and 1.33-km model resolutions compared well with each other, whereas there were some differences in the predicted planetary boundary layer height over Yakima. We also used the high-resolution 1.33-km WRF output to generate smoke dispersion predictions using the BlueSky Smoke Modeling Framework. Results showed that forecasters and regulators using either the model output available on the day of the burn or the higher-resolution model output generated afterward, would not have anticipated the meteorological conditions that resulted in the smoke intrusion that day.

Submitted for the DFD10 Meeting of The American Physical Society Wildfire simulation using a chemically-reacting plume in a crossflow ROBERT BREIDENTHAL, University of Washington, TRAVIS ALVARADO1, University of Texas, BRIAN POTTER, Pacific Wildland Fire Sciences Laboratory — Water tunnel experiments reveal the flame length of a chemically-reacting plume in a crossflow. Salt water containing a pH indicator and a base is slowly injected from above into the test section of a water tunnel containing an acidic solution. The flame length is measured optically as a function of the buoyancy flux, crossflow speed, and volume equivalence ratio of the chemical reaction. Based on earlier work of Broadwell with the transverse jet, a simple dilution model predicts the flame length of the transverse plume. The plume observations are in accord with the model. As with the jet, there is a minimum in the flame length of the plume at a transition between two self-similar regimes, corresponding to the formation of a pair of counter-rotating vortices at a certain crossflow speed. At the transition, there is a maximum in the entrainment and mixing rates. In an actual wildfire with variable winds, this transition may correspond to a dangerous condition for firefighters.

When a gap forms in a forest canopy, the first and most immediate effect on the exposed area is an increase in radiative exchange near the ground. More sunlight reaches the ground during the daytime, and at nighttime the ground is more exposed to longwave radiation influences from the sky. These changes in radiation lead directly to a different near-ground temperature climate than what existed previously. Furthermore, spatial gradients in radiation and temperature now exist within the gap region that did not exist before the gap formed.

1 ABSTRACT.—Observational data and an atmospheric radiation model were combined to examine nocturnal cooling in a northern hardwood forest. The model provided a mechanism for examining which properties of the forest and atmosphere play a significant role in preventing radiative freeze or frost events. The results of the simulations suggest that site basal area exerts some control over heat loss, with higher basal areas providing more heat retention than lower basal areas. This information is extended in the form of maps for Minnesota, Michigan, and Wisconsin indicating the amount of basal area needed to protect against the magnitudes of late spring freezes historically experienced in different locations.