Extreme wildfire events in the Sierra Nevada Mountains

In recent years, numerous large, high impact fires have occurred in the Sierra Nevada Mountains and elsewhere causing loss of natural resources, high severity damage to the soils comprising important watersheds, and poor air quality. Such fires have been widely attributed to fuel accumulation due to fire suppression and long-term drought, while statistical correlations of acres burned to climatological factors and vegetation state derived from remote sensing have been interpreted to suggest the trend toward increasing fire activity will continue in the future. Other studies assert that day-to-day weather rather than longer-term climatological patterns has more impact in shaping their extent. These differing perspectives both imply that such extreme fires are associated with unusual conditions that can be diagnosed and perhaps, anticipated, from weather, climatological, or other environmental data.

Other approaches show the limitations in using such approaches to explain particular events. From a fire behavior scale perspective, the factors leading to rapid spread rates and intense fires are well known. Newer coupled weather-fire models such as CAWFE^® have shown that by accurately modeling fine-scale circulations (grid spacing on the order of a few hundreds of meters) in complex terrain such as the Sierras and fire-induced winds, that is, winds created by heat release from the fire itself, the distinctive growth and sudden accelerations in a wildfire event can be captured. Previous CAWFE simulations of the 2014 King fire in the Sierras showed that weather circulations at scales beneath that resolved by surface station networks and fire-induced winds alone can produce extreme fire events even in weak to moderate weather and fuel conditions and that drought contributed little to the rapid growth. In this work, we used CAWFE simulations of real and hypothetical wildfires to elucidate the mechanisms through which extreme fires arose in the complex weather, fuel, and terrain environment of the Sierra Nevada Mountains and the degree to which they were shaped by environmental factors from the microscale to the climatological.

We found that a thorough understanding of the multi-scale dynamics leading to an extreme wildfire event provided a better basis from which to generalize about their occurrence. Previous work showed that while broad climatological factors such as drought and land management consequences such as fuel accumulation are limited in their impact to certain locations such as sloped terrain, in those locations, they compounded each other. In this work, we show that combinations of fine-scale factors can determine whether events on the order of recent megafires in the area occur or not. It is not necessarily disheartening that climatological factors alone may not provide specific guidance and that they apply unevenly across a fire or that fine-scale factors beneath current weather station density could make or break a megafire. Rather, we use this understanding to inform how limited fuel mitigation resources can be applied most effectively to lessen wildfire impacts under a range of potential future conditions.