UC Santa Barbara

Coastal Santa Barbara County, California is characterized by extreme downslope wind events on the southern side of the Santa Ynez Mountains known as Sundowner winds (or Sundowners) that can rapidly spread wildfires toward highly populated regions. This research investigates these strong winds and the risk of wildfire spread in this region. The second chapter of this research explores the seasonal and diurnal patterns of wind, temperature, dew point, and a fire weather index, as well as their spatiotemporal variability, from an observational perspective. Utilizing station and buoy data, distinct differences were found between stations on the mountain slopes and stations on the foothills and on the coastal plain. Additionally, correlations revealed relationships between stations and buoys, and analysis of a vertical wind profiler at the Santa Barbara airport provided evidence of differences during Sundowners in the east and west parts of coastal Santa Barbara. Due to the dependence on winds in its calculation, fire weather indices were highest at stations on the mountain slopes, though all stations had recorded at least one time with critical fire weather. In the third chapter, the ability of an operationally-used uncoupled wildfire model called FARSITE (Fire Area Simulator) in reconstructing two wildfires that rapidly spread by Sundowner winds was tested. One wildfire was simulated relatively well, though some differences in spread direction were evident. The perimeter and burn area of the second wildfire was underestimated in the simulations due to limitations built into the spotting algorithm. It was determined that the elevation of the launch site of an ember must be lower than the landing site. In the simulations for the Santa Barbara case studies, fire spread was predominantly downslope, thus the launch sites were commonly higher in elevation and spotting did not occur. This finding has ramifications for wildfires that are rapidly spread by spotting ahead of the fire front. In the fourth chapter, I created wildfire risk maps for this high-risk region. Three ignition modeling methods were employed using distances from previous ignitions and roads and trails, and these were input into the wildfire model with three varying wind scenarios: climatological winds, a composite of Sundowners, and a case study of an extreme Sundowner event. Simulations commonly hit areas on the mountain slopes overlooking downtown Santa Barbara and Montecito, and also near a major north-south running evacuation route, in part due to clustering of ignitions in this region. The simulations using climatological winds spread generally outward from the ignition locations and in contrast simulations run using extreme winds spread further south into the wildland-urban interface and toward more urban areas. These three chapters advance knowledge of typical versus extreme winds in this region, demonstrate how simulations align with previous influential wildfires, provide insight into wildfire model limitations, and identify potentially high-risk regions for wildfires, increasing wildfire resilience and preparedness.