The Generation and Forecast of Extreme Winds during the Origin and Progression of the 2017 Tubbs Fire

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.Atmosphere 2018, 9, 462 2 of 22 the airflow regime that occurred and the underlying mechanisms leading to the extreme winds.The events hint at exceptional wind extrema, yet these lie between surface weather station network data locations. While regional elevated winds are captured in operational simulations, if there are extrema, they could be under-resolved in operational forecasts. We examined these issues using two approaches. First, we performed a retrospective research simulation with the CAWFE ® (derived from Coupled Atmosphere-Wildland Fire Environment) coupled numerical weather prediction-wildland fire behavior modeling system. Second, we configured CAWFE without a priori knowledge of how the fire unfolded in order to predict the flow regime and subsequent Tubbs Fire growth, examining the predictability of the winds including their extrema and fire behavior.Atmosphere 2018, 9, x FOR PEER REVIEW 2 of 22 2017's event, the widespread outbreak of fires and their extremely rapid spread raise questions about the airflow regime that occurred and the underlying mechanisms leading to the extreme winds. The events hint at exceptional wind extrema, yet these lie between surface weather station network data locations. While regional elevated winds are captured in operational simulations, if there are extrema, they could be under-resolved in operational forecasts. We examined these issues using two approa...

Computational modeling of extreme wildland fire events: A synthesis of scientific understanding with applications to forecasting, land management, and firefighter safety

“…Several studies have applied CAWFE to these wind driven events to investigate the development of unrecognized extreme microscale wind phenomena and how these made fire events unfold, and the difficulty of using observations at the time to evaluate them. These reproduced key features and timing, and showed that strong winds in the Tubbs Fire [23] (Fig. 5a) occurred on secondary hilltops, rather than tall ridges, and produced pulses of strong winds exceeding 30-40 m s -1 with 4-13 minute pulses that appeared in high temporal resolution wind speed data.…”

“…Studies have shown this has led to underestimates in peak wind extrema in wind-driven fire events [23], smoothed gradients across interfaces precluding WRF from producing gravity wave overturning (discussed in Coen 2018) -a primary mechanism for producing strong downslope winds driving fire events such as the 2012 High Park Fire [24] and shear instabilities [25], and smoothing gradients underlying simulation of the full development of fire phenomena such as fire whirls [20,26]. [41] and demonstrated over wildfires [42] though their broader use is still restricted.…”

“…Embers complicate validation, as it may be difficult to identify the leading edge of the fire and satellite active fire detection data, which is commonly used for validation, may be difficult to interpret [23], as detections may be airborne embers or smoke plumes rather than surface fire.…”

“…CAWFE has also been configured as a deterministic forecast, in which it is fast enough to produce a forecast for a large fire (innermost domain 26 km x 26 km), for example, the Tubbs Fire, four times faster than real time on a single processor [23]. California respectively, to see if it is possible to more precisely identify extreme wind locations [62] and refine PSPSs.…”

Large Outdoor Fire Modeling (LOFM) workshop summary report

“…A recent, large loss wildfire event should be selected, all available data collected, and the event reconstructed in as much detail as possible using a computational fluid dynamics (CFD) model. An excellent example of reconstruction at the landscape scale for the 2017 Tubbs fire in California is given by Coen et al [67]. The goal of this grand challenge is to use meter or sub-meter scale resolution with a physics-based combustion model and a long-range spotting model.…”