No abstract

Sundowner winds are downslope gusty winds often observed on the southern slopes of the Santa Ynez Mountains (SYM) in coastal Santa Barbara (SB), California. They typically peak near sunset and exhibit characteristics of downslope windstorms through the evening. They are SB’s most critical fire weather in all seasons and represent a major hazard for aviation. The Sundowner Winds Experiment Pilot Study was designed to evaluate vertical profiles of winds, temperature, humidity, and stability leeward of the SYM during a Sundowner event. This was accomplished by launching 3-hourly radiosondes during a significant Sundowner event on 28–29 April 2018. This study showed that winds in the lee of the SYM exhibit complex spatial and temporal patterns. Vertical profiles showed a transition from humid onshore winds from morning to midafternoon to very pronounced offshore winds during the evening after sunset. These winds accompanied mountain waves and a northerly nocturnal lee jet with variable temporal behavior. Around sunset, the jet was characterized by strong wind speeds enhanced by mountain-wave breaking. Winds weakened considerably at 2300 PDT 29 April but enhanced dramatically at 0200 PDT 29 April at much lower elevations. These transitions were accompanied by changes in stability profiles and in the Richardson number. A simulation with the Weather Research and Forecasting (WRF) Model at 1-km grid spacing was examined to evaluate the skill of the model in capturing the observed winds and stability profiles and to assess mesoscale processes associated with this event. These results advanced understanding on Sundowner’s spatiotemporal characteristics and driving mechanisms.

Extreme, downslope mountain winds often generate dangerous wildfire conditions. We used the wildfire spread model Fire Area Simulator (FARSITE) to simulate two wildfires influenced by strong wind events in Santa Barbara, CA. High spatial-resolution imagery for fuel maps and hourly wind downscaled to 100 m were used as model inputs, and sensitivity tests were performed to evaluate the effects of ignition timing and location on fire spread. Additionally, burn area rasters from FARSITE simulations were compared to minimum travel time rasters from FlamMap simulations, a wildfire model similar to FARSITE that holds environmental variables constant. Utilization of two case studies during strong winds revealed that FARSITE was able to successfully reconstruct the spread rate and size of wildfires when spotting was minimal. However, in situations when spotting was an important factor in rapid downslope wildfire spread, both FARSITE and FlamMap were unable to simulate realistic fire perimeters. We show that this is due to inherent limitations in the models themselves, related to the slope-orientation relative to the simulated fire spread, and the dependence of ember launch and land locations. This finding has widespread implications, given the role of spotting in fire progression during extreme wind events.

Downslope windstorms occur all over the world when air flow interacts with a sufficiently large mountain barrier (Durran, 1990). Consistent with the variety of mountain barriers around the world, characteristics of downslope windstorms differ in onset, duration, strength, and related adiabatic processes (Jack-

Coastal Santa Barbara (SB) County in Southern California, characterized by a Mediterranean climate and complex topography, is a region prone to downslope windstorms that create critical fire weather conditions and rapidly spread wildfires. The Santa Ynez Mountains, oriented from east to west, rise abruptly from the coast, separating air masses from the ocean and the Santa Ynez Valley. The juxtaposition of these geographic features generates spatiotemporally variable wind regimes. This study analyzes diurnal-to-seasonal wind cycles and extremes in this region using hourly data from eight weather stations and four buoys for the period 1998-2019. Data from a vertical wind profiler at the Santa Barbara airport in Goleta, CA was extracted from August 2016 to September 2020. Air temperature, dew point temperature, and the Fosberg fire weather index are examined at land stations. We show that cycles in wind speed vary spatiotemporally; mountain (valley and coastal) stations exhibit a pronounced semiannual (annual) cycle, and wind maxima is observed during the evening (afternoon) at mountain (valley and coastal) stations. Differences in wind speed percentiles were evident among stations, particularly at and above the 75th percentile. Strong winds recorded at buoys were significantly correlated (between r = 0.3-0.5) to land stations. However, cross-correlational analysis did not reveal any temporal lags between mountain stations and buoys. Distributions of temperature and dew point during extreme winds differed between east and west mountain stations. Significant fire weather conditions were most frequent at mountain stations in Refugio and Montecito, with 5% occurrence in the spring and over 3% occurrence in fall. Weaker summertime winds lowered fire weather conditions at Montecito in the summer.extreme winds, fire weather, Mediterranean climate, mountain winds, Southern California, Sundowner winds, thermally driven circulations | INTRODUCTIONCoastal Southern California is characterized by a Mediterranean climate, with dry summers and wet winters. Nonetheless, regional differences in climate are attributed to the complex terrain spanning the U.S. West Coast. Santa Barbara (SB) County, situated in Southern California, provides an example of a region where