Mountain Waves and Terrain Induced Windstorms in the Slopes of the Santa Ynez Mountains in Santa Barbara, California

Santa Barbara County (SB) in Southern California exhibits one of the worst wildfire hazard scenarios with impacts to populated areas in the west coast of the U.S. The Santa Ynez Mountains (SYM), oriented east-west for about 100km, rises abruptly from a narrow coastal plain with elevations generally increasing from west to east, where they exceed 1200m. Gusty downslope winds known as Sundowner winds (or Sundowners) are frequently observed in the lee of the SYM. These winds typically peak from sunset to mid-morning, are accompanied by rapid warming and decreased relative humidity, and are considered the most significant fire weather regime affecting coastal SB. Sundowners have been observed under a variety of synoptic-scale conditions and have enhanced the severity of all major wildfires that affected the region (17 since 1955). Additionally, windstorms in coastal SB represent hazardous conditions for aviation and navigation. This study examines hindcast simulations of Sundowner winds observed under distinct synoptic forcings. The Weather Research & Forecast (WRF) model at 1km resolution (55 vertical levels) is utilized to investigate mountain waves associated with significant Sundowner events. Simulations are validated with station data available in the region. These simulations reveal intriguing aspects relevant to understanding downslope wind storms in coastal ranges. We show that there is large geographic variability in the onset and intensity of the winds. This variability is largely attributable to a complex combination of mountain elevation, boundary layer structure at the crest of the SYM mountain range, boundary layer structure and evolution upslope and downslope of the SYM, and coastal boundary layer erosion and displacement. We show that mountain waves can be present in some situations and these waves are more likely to occur east of the SYM range where higher elevations and interactions with the San Rafael Mountains significantly influence mountain flow during the evening hours. The presence of critical layers (wind reversal, self-induced critical layers) appear important for the development of mountain waves. Understanding these mechanisms significantly enhance the capability of forecasting Sundowner winds and can contribute to understanding and predicting coastal windstorms in regions with similar topography and climatic characteristics.