The High Park fire: Coupled weather‐wildland fire model simulation of a windstorm‐driven wildfire in Colorado's Front Range

Abstract

Weather affects wildland fires at scales from multiseasonal precipitation patterns and anomalies, through synoptic and mesoscale weather patterns, to convective scale motions including fire-induced winds. This work analyzed the first day's growth of the 2012 High Park fire, which occurred in Colorado's Front Range during widespread drought and an unseasonal June windstorm, assessing to what extent the Coupled Atmosphere-Wildland Fire Environment coupled numerical weather prediction-wildland fire behavior model could reproduce the event, burn severity patterns, and how the drought impact on fuel moisture impacted the event. Simulated mountaintop wind speeds reaching 47 m s À1 and gravity wave overturning created strong, gusty surface winds. During the first 9 h, the simulated fire grew underneath the gravity wave's crest and downdraft, sheltered from the windstorm. The simulated fire then climbed a ridge, was exposed to the windstorm, and rapidly traveled east, covering 15 km in 12.3 h. Burning routed up or down drainages caused finger-like streaks in maximum fire intensity. Reference fire mapping information supported the simulated early growth toward the north, splitting around topographic features, while the simulation's underestimate of extent accrued to 2 km over 21.3 h. While the control simulation employed horizontal grid spacing of 123 m, a simulation refined to 370 m captured some wave motions and overall direction but further underestimated extent and lacked details such as turns in direction, splitting, or fingering at the leading edge. Compared to a simulation with moderately dry fuel conditions, a range of drought-like fuel moisture conditions produced fires that extended 0-39% farther.

Some Requirements for Simulating Wildland Fire Behavior Using Insight from Coupled Weather—Wildland Fire Models

“…This discrepancy has been underemphasized in the literature and its causes not discussed. Other work [56] using the convective-scale model CAWFE reproduced gravity wave overturning, a phenomenon underlying previous Front Range windstorm events, during the High Park Fire. They noted that in other studies, other models failed to reproduce the overturning waves, characterized by fine-scale temperature and velocity gradients, creating only some vertical displacement and surface wind acceleration in the lee, and typically underestimated peak wind speeds.…”

“…Physics fidelity demands horizontal grid spacing on the order of one hundred meters [56,57] to capture fine-scale atmospheric circulations amid complex terrain and delineate fire-induced circulations, the effects of which are noticeably diluted at grid spacing of approximately 1 km or greater [58]. For comparison, this is more than an order of magnitude finer than current U.S. operational numerical weather prediction models.…”

“…Numerous studies (e.g., [15,17,[56][57][58][59]63,64] and others) have applied coupled weather-fire models to simulate large wildland fire events. The standard approach, taken from NWP, is to begin with a regional model domain with the approximate grid spacing of a larger-scale forecast or analysis product, either of which may provide initial and boundary conditions (10-12 km).…”

“…In Colorado, wind event-driven fires are primarily associated with unseasonal windstorms, driven by breaking atmospheric gravity waves (e.g., the 2012 High Park Fire [56]. Southern California windstorm-driven fires are associated with Santa Anas-seasonal pressure-driven offshore flows-and sundowners.…”

“…They noted that in other studies, other models failed to reproduce the overturning waves, characterized by fine-scale temperature and velocity gradients, creating only some vertical displacement and surface wind acceleration in the lee, and typically underestimated peak wind speeds. These discrepancies appear to arise from the factors cited in Sections 4 and 5, notably the resolution (as breaking waves only appeared using horizontal grid spacing under 300 m [56] while [74] used 667 m and [59] used 500 m); excessive dissipation at fine scales in WRF; and flow distortion when along-terrain gradients are tilted into the vertical.…”