Abstract
Massive wildfires and extreme fire behavior are becoming more frequent across the western United States, creating a need to better understand how megafire behavior will evolve in our warming world. Here, the fire spread model Prometheus is used to simulate the initial explosive growth of the 2020 August Complex, which occurred in northern California (CA) mixed conifer forests. High temperatures, low relative humidity, and daytime southerly winds were all highly correlated with extreme rates of modeled spread. Fine fuels reached very dry levels, which accelerated simulation growth and heightened fire heat release (HR). Model sensitivity tests indicate that fire growth and HR are most sensitive to aridity and fuel moisture content. Despite the impressive early observed growth of the fire, shifting the simulation ignition to a very dry September 2020 heatwave predicted a >50% increase in growth and HR, as well as increased nighttime fire activity. Detailed model analyses of how extreme fire behavior develops can help fire personnel prepare for problematic ignitions.
Highlights
In 2020, wildland fires burned over 4 million acres in California (CA), the most recorded in a single year (Figure 1) [1]
The calibrated control run resulted in a four-day burned area of 52,245 ha, which is comparable to the observed Doe Fire size on 22 August: 52,281 ha (Figure 2)
The simulated fire stopped spreading for 10–12 h each night and early morning because one or more of the burning condition thresholds were not met during that hourly time step, i.e., the wind speed/initial spread index/fire weather index was too low or the RH was too high
Summary
In 2020, wildland fires burned over 4 million acres in California (CA), the most recorded in a single year (Figure 1) [1]. A large portion of the burned area resulted from an anomalous dry lightning storm that struck in the middle of an August 2020 heatwave [2]. Fires ignited all over the central and northern portion of the state, later joining into a number of massive burns, referred to as complexes. Low winter precipitation causes more flammable, rapidly drying fuels, which was evident in July 2020 by below average live fuel green-up and dead fuel moisture content. Moisture advected from tropical storm Fausto, off the coast of Baja California, streamed northward, causing over 15,000 lightning strikes throughout central and northern CA [1]
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