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Abstract In wildfire-prone coastal Santa Barbara, California, downslope winds observed on the southern slopes of the east-west oriented Santa Ynez Mountains are known as Sundowner winds (or Sundowners). One important feature of Sundowners is the remarkable spatial and temporal variability in lee slope jet characteristics. Besides the intensity of the flow approaching the mountain range, the acceleration of the lee slope jet can be influenced by reflected gravity waves associated with one or more of the following mechanisms: a self-induced critical level, an inversion close to mountaintop, and the presence of a mean-state critical level (MSCL). The relative contribution of these mechanisms to the enhancement of Sundowners is yet unknown. This study uses 32-yr simulations (hourly, 1-km grid spacing) complemented with observations collected during the Sundowner Winds Experiment (SWEX) to better quantify the relative contribution of these mechanisms and to quantify the importance of MSCLs. We show that when an MSCL is present below 5 km, less atmospheric forcing is necessary to attain similar lee-slope jet strengths compared to when MSCLs are absent or above 5 km. This was evidenced from simulations and verified with observations. Although MSCLs during Sundowners occur year-round, their relative frequency increases in summer, when temperatures are high and fuels are dry, enhancing wildfire risk. Properly identifying these processes contributes to improved understanding and predictability of Sundowners and many other hazardous downslope windstorms in coastal environments. 

Surface winds over California can compound fire risk during autumn, yet their long-term trends in the face of decadal warming are less clear compared to other climate variables like temperature, drought, and snowmelt. To determine where and how surface winds are changing most, this article uses multiple reanalyses and Remote Automated Weather Stations (RAWS) to calculate autumn 10 m wind speed trends during 1979–2020. Reanalysis trends show statistically significant increases in autumn night-time easterlies on the western slopes of the Sierra Nevada. Although downslope windstorms are frequent to this region, trends instead appear to result from elevated gradients in warming between California and the interior continent. The result is a sharper horizontal temperature gradient over the Sierra crest and adjacent free atmosphere above the foothills, strengthening the climatological nocturnal katabatic wind. While RAWS records show broad agreement, their trend is likely influenced by year-to-year changes in the number of observations.