Abstract
On 8–9 October 2017, fourteen wildfires developed rapidly during a strong Diablo wind event in northern California including the Tubbs Fire, which travelled over 19 km in 3.25 h. Here, we applied the CAWFE® coupled numerical weather prediction-fire modeling system to investigate the airflow regime and extreme wind peaks underlying the extreme fire behavior using simulations that refine from a 10 km to a 185 m horizontal grid spacing. We found that as Diablo winds travelled south down the Sacramento Valley and fanned out southwestward over the Wine Country, their strength waxed and waned and their direction wavered, creating varying locations near fire origins where wind overrunning topography reached 30–40 m/s, along with streaks and bursts of strong winds in the lee of some topographic features and stagnation downstream of others. Despite a statically stable layer in the lowest 1.5 km, the high Froude number flow sometimes resembled a hydraulic jump. Elsewhere, the flow behaved similarly to neutrally-stratified flow over small hills, creating wind extrema that exceeded 40 m/s at the crest of some lesser hills including near the Tubbs fire ignition, but which shed bursts of high speed winds that travel downstream at approximately 5–7-min intervals. Nonetheless, simulated fire growth lagged satellite detection of fire arrival in Santa Rosa by up to 1 h, although whether the data detect fire line or spotting is ambiguous. A forecast simulation with a 370 m horizontal grid spacing produced an on-time fire line arrival in Santa Rosa, with calculations executed 4 times faster than real time on a single computer processor.
Highlights
On 8–9 October 2017, more than 170 wildfires ignited in the Wine Country, northern coastal ranges, and Butte and Nevada Counties to the west, north, and east of California’s northern SacramentoValley (Figure 1)
10 km wide along a ridge) where wind overrunning topography reached 30–40 m/s, steady streaks or km wide along a ridge) where wind overrunning topography reached 30–40 m/s, steady streaks or transient eddies of high speed air formed in the lee of some topographic features, and terrain created transient eddies of high speed air formed in the lee of some topographic features, and terrain created sheltered, low-speed areas downstream of others (Figure 8 and Video S1)
Because surface weather station data was sparse and largely missed extreme winds that anecdotal reports suggest occurred, the large-scale synoptic environment was changing throughout the event, and the three-dimensional topography was too complex for analytical solutions, numerical simulations refining from mesoscale to microscale were used to examine the flow regime
Summary
On 8–9 October 2017, more than 170 wildfires ignited in the Wine Country, northern coastal ranges, and Butte and Nevada Counties to the west, north, and east of California’s northern SacramentoValley (Figure 1). Fourteen large fires grew rapidly, some joining into multi-fire complexes. Many fires appeared and rapidly spread during local peaks of an unusually strong downslope wind event. Such multi-day events, termed Diablo winds, are a recognized meteorological feature of the region [3,4] and have been linked to erratic wildfire behavior, fatalities, and past destructive wildland–urban interface fires such as the 1991 Oakland Hills Fire [5] and the similar 1964 Wine Country fires [6]. In 2017’s event, the widespread outbreak of fires and their extremely rapid spread raise questions about
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