Convective Density Current Circulations That Modulated Meso-γ Surface Winds Near the Yarnell Hill Fire

On 30 June 2013, 19 members of the Granite Mountain Hotshots firefighters were killed fighting a wildfire near Yarnell in the mountains of Central Arizona. They succumbed when the wildfire driven by erratic winds blocked their escape route and subsequently overran their location. In this manuscript we extend previous analyses of the environment and downscale organization of convective circulations that redirected the wildfire. That analysis started at the scale of Rossby Wave Breaking over western North America and extended to a local convective gust front that redirected the wildfire which trapped the firefighters. In this manuscript we delve deeper into the organization of circulations triggered by that (primary) gust front close to the location of the tragedy. We do so by employing a Large Eddy Simulation down to near canyon scale focused on five convectively-driven features: 1) the initial deep and prolonged gust front and its dynamic hydraulic head, 2) a front-rear jet and it’s coupled hydraulic head’s ascending motions that organized high-based convection, 3) a high-based microburst-induced downdraft organized initially by ascending flow in 2) that eventually transported mass and entropy to the surface causing a 4) meso-high pressure center and confluence zone to encompass the firefighters’ location which established a favorable environment for 5) near canyon scale circulations surrounding the fire. These complex meso-γ scale circulations were the result of terrain-modified convective forcing in an environment with progressively increasing large scale Downdraft Convective Available Potential Energy and decreasing large-scale vertical wind shears. The atmosphere thus transitioned from supporting a very deep and long-lived convective density current to an isolated elevated convective microburst with substantial mass and wind outflow into a canyon redirecting the ongoing wildfire.