Abstract
The Yarnell Hill Fire, triggered by dry lightning on 28 June 2013, was initiated by hot and dry westerly winds, which rapidly shifted to north-northeast by convective-induced outflows. This sudden wind shift led to the demise of 19 firefighters. This study focuses on the environment and its predictive potential in terms of erratically changing the fire spread. Three numerical sensitivity tests are performed investigating the evolving synoptic-meso-β scale environmental wind flow: (1) deactivating the evaporative cooling, (2) deactivating surface-driven diurnal heating/cooling, and (3) removing the mountain. Results show the strong north-northeasterly wind induced by the density current(s) and the diurnal surface sensible heating played the most significant roles in enhancing the mesoscale environment conducive to the rapid change in the fire spread direction. While the mountain played a less significant role in weakening the magnitude of the airflow affecting the fire, it still had an impact. Additionally, the Hot-Dry-Windy (HDW) index is calculated to determine its predictor role with respect to the atmosphere affecting the fire. The focus is not on feedback from explicit fire heating on the larger environment but rather the role of the environmental physical processes in causing the convectively induced rapid wind shifts.
Highlights
On Friday, 28 June 2013, a lightning strike from nearby convection triggered a small bush fire in west central Yavapai County, Arizona, just to the west of Yarnell, in a region known as the Yarnell Hills
A comparison will be performed between the simulated wind magnitude, wind direction, and the simulated relative humidity for the CNTL case, and the observed surface wind magnitude, wind direction, and relative humidity at Cherry, Peeples Valley, and Stanton, AZ, USA
This study has investigated factors that contributed to the generation of the density current and its wind shifts that suddenly redirected the Yarnell Hill Fire near Yarnell, Arizona, during the late afternoon of 30 June 2013
Summary
On Friday, 28 June 2013, a lightning strike from nearby convection triggered a small bush fire in west central Yavapai County, Arizona, just to the west of Yarnell, in a region known as the Yarnell Hills. Non-standardized practices led to poor performance with two commercial weather stations These studies have indicated that the unpredictability and poor understanding of fundamental dynamical processes at the complex terrain scale of local wind strength and direction makes the numerical prediction of these winds and wind surges challenging, especially on small time scales as in the Yarnell Hill Fire. In the Yarnell case, the density current moved from the northeast to the southwest into an environment with significant vertical wind direction shear Near the fire, this created a sudden shift in wind direction and change in intensity (Figure 2).
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