The 2017 Thomas Wildfire: Observations and Coupled Weather–Wildland Fire Modeling of Both Wind-Driven and Plume-Driven Fire Behavior during an Extended Santa Ana Event

The Thomas wildfire was a massive wildfire that devastated Ventura and Santa Barbara counties during a strong, extended Santa Ana event, burning over 114,000 ha as the wind event waxed and waned for just over two weeks; the fire itself persisted for over a month. Although the unusually strong Santa Ana winds were apparently the primary factor in the ignition and spread of the fire, the vast 50 km x 30 km area of the fire, variable terrain and airflow regimes, and evolving meteorological environment create a complex fire environment not easily typecast as a wind-driven event. We use airborne infrared observations using the FireMapper instrument, satellite active fire detection data from the Visible and Infrared Imaging Radiometer Suite (VIIRS), and coupled weather - wildland fire simulations using the CAWFE^®coupled weather - wildland fire modeling system to investigate the interplay of weather, terrain, fire-atmosphere interactions, and fire activity and the exchange between wind and plume-driven fire behavior across the fire and throughout the fire's first week. Results show that early growth was characterized by expected rapid wind-driven spread southwest down the Santa Clara River valley to the Pacific Ocean, driven by a shallow river of stable air. However, subsequent days' growth was characterized by growth normal to the flanks of the fire and to ambient winds, extending the fire to the north and south, generating deep plumes and active fire spread driven by fire-induced winds up drainages. Notably, this second type of growth occurred whether or not the Santa Anas were currently blowing strongly in other areas of the fire (as may be indicted in area surface weather stations), as drainages may be sheltered at low levels by steep terrain. Subsequently, as the fire reached areas exposed to Santa Anas, rapid wind-driven spread resumed, creating a second day of over 24,000 ha wind-driven growth. This second type of growth - growth driven by fire-induced winds during a wind event -, the coexistence of which has rarely been noted in Santa Ana-driven fires and is underemphasized in wildland fire science and firefighter safety materials, may have played a role in a firefighter fatality later in a less active area of the fire. This work casts light on the false dichotomy present in wildland fire tradition between solely wind- vs. plume-driven fires.