Effects of Density Current, Diurnal Heating, and Local Terrain on the Mesoscale Environment Conducive to the Yarnell Hill Fire

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.

The Multi-Scale Dynamics Organizing a Favorable Environment for Convective Density Currents That Redirected the Yarnell Hill Fire

“…The nested grid numerical simulation employed Weather Research and Forecasting-ARW Version 4 (WRF-ARW) [32], as specified in [13]. Their simulations were employed in this manuscript to focus on the mesoscale jet adjustments and organizing circulations for convection and density current initiation and motion across the Black Hills, Bradshaw Mountains, and Weaver Mountains northeast of Yarnell.…”

“…This microburst is consistent with the drier sub-cloud layer, deeper mixed layer, larger DCAPE, and weaker In (h), red isoline is the Eastern Weaver Mountain perimeter, the solid line is the cross-section in Figure 3b, a dark black x is Yarnell, a light black x at 34.22 • N, 112.76 • W is the fire fighters' location, and the black square is the incident command post. From [13].…”

“…This type of study only deals with seasonal dynamical processes not individual case study multiscale processes. Finally, the third category, which has been quite rare, is that focusing on a single intense convective event [12,13]. McCollum et al [12] analyzed the "surprise" 23 to 24 July 1990 Southwest Area Monsoon Project (SWAMP) extreme rainfall event in Phoenix, where few, if any, downscale dynamical linkages from the synoptic to mesoscales were unambiguously identified, given the lack of upper air observations.…”

“…The surface and radar data were substantial and analyzed in depth but not high-resolution rawinsondes or numerical simulations. In another manuscript recently submitted on the Yarnell Hill Fire, Ising et al [13] demonstrated how the general evolution of the key density current features from the MR to Yarnell were dependent on evaporational cooling, surface sensible heating, and the unique terrain structure near Yarnell. We will show some of these results in this manuscript as well, in so far as they support the density current evolution.…”

“…Simulated Meso-β/γ-Scale Low-Level WindsFigure22a-g indicates the longevity and intensity of the convectively driven surface density current and its southwestward propagation against the surface flow from the MR to Yarnell. These figures from the WRF 777-m simulation are located along the vertical cross-section depicted in Figure3a, while the WRF 777-m-simulated Figure22his centered near Yarnell[13].…”