The Relevance of Mean-State Critical Levels for the Intensification of Downslope Winds in a Coastal Mountainous Environment

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. Significance Statement Wildfires in coastal Santa Barbara, Southern California, are among the most disruptive hazards and are influenced by strong winds that accelerate down the southern slopes of the Santa Ynez Mountain range. These winds are locally known as Sundowner winds, due to their appearance around sunset. Our goal is to understand how different atmospheric situations influence the surface winds at the south-facing slopes of the mountain range. We find that when the atmosphere north of this mountain range contains a wind reversal (i.e., northerly winds turning southerly aloft), weaker winds below this reversal will induce winds on the south-facing slope that are stronger than with similar upstream winds without a reversal. Our findings are relevant to improve forecasting of Sundowner winds.