Characteristics of Sundowner winds near Santa Barbara, CA from a dynamically downscaled climatology. Part I: Environment and effects near the surface

Sundowners are downslope windstorms that occur over the southern slopes of the east-west-trending Santa Ynez range in Santa Barbara County, California. In the past, many extreme fires in the area, including the Painted Cave, Montecito Tea, Jesusita, and Sherpa fires, have occurred during sundowner events. A high-resolution 11-yr dynamically downscaled climatology was produced using a numerical weather prediction model in order to elucidate the general dynamical characteristics of sundowners. The downscaled climatology is validated with observations during the 2016 Sherpa fire. A sundowner index (SI) is computed from the climatology that quantifies the magnitude of adiabatic warming and northerly (downslope) wind component during sundowner events. The SI allows for the classification of historical events into categories of various strengths. The primary characteristics of strong sundowners from this classification include 1) internal gravity wave breaking over the Santa Ynez range, 2) initiation in the western Santa Ynez range with eastward progression over the course of a day, 3) a maximum likelihood of occurrence in April and May near 2000 Pacific standard time, and 4) a limited downstream extent for most events, such that the long-term historical weather station, Santa Barbara airport, often does not experience moderate events. Analysis of an operational forecast rubric composed of the surface pressure difference from Bakersfield to Santa Barbara indicates that this rubric is not skillful. However, offshore pressure gradients are skillful and are related to the strong northwesterly alongshore jet. The findings presented herein can be used to provide guidance for fire weather forecasts and support resource allocation during fire suppression efforts.