UC Santa Barbara

The residents of Santa Barbara County know Sundowners as the extreme northerlydownslope windstorms that enhance after sunset on the southern slopes of the Santa Ynez Mountains (SYM). Sundowners are notoriously known to dramatically and unpredictably influence wildfire spread in the region. The SYM reside north of the east-west coastline and reach over 1200 meters, creating a barrier between the cool ocean and the warm inland environments. Further north, the V-shaped Santa Ynez Valley (SYV) separates the SYM from the San Rafael Mountains (SRM), which exceed 2000 meters and influences the regions circulations.

The goal of this work is to examine the importance of mountain waves in relation toSundowner spatial variability and the influence of regional topography on mountain wave and Sundowner behavior. This work investigates “Eastern Sundowners”, or Sundowners with NE direction that are particularly strong over eastern SYM (Montecito region). For this purpose, we use the existing simulated 30-year climatology at 1 km grid resolution performed with the Weather Research and Forecasting Model (WRF). The present work developed a mountain wave intensity metric based on the maximum downward vertical velocity observed on the southern lee slope of the SYM and SRM. Using the mountain wave metric, we demonstrate that there is a linear and positive correlation between modeled surface wind speed and modeled mountain wave intensity in both the SYM and SRM mountain ranges. Additional correlation analysis with the mountain wave metric confirms that mountain wave activity in the region is linearly and positively correlated with conditions known to be important to mountain wave activity such as the Froude Number, mountain top stability, cross mountain wind speeds, and stable layer height.

This work uses the mountain wave metric to investigate the regions mountain wavediurnal cycles and the upstream influence of the SRM on Sundowners in the SYM. The findings of the latter suggest that the relative elevation of both mountain ranges, combined with the valley width, are two important mechanisms explaining the great spatial variability in high intensity Sundowner winds. This study also provides important elements to explain factors associated with Eastern Sundowners. These results help in explaining the great spatial variability seen in Sundowner high intensity winds and to why Eastern Sundowner events have a hot spot of activity in the Montecito region. These findings are relevant to improve understanding and predictability of Sundowner winds, contributing to better fire weather forecasts and red-flag warnings, thus increasing resiliency of the Santa Barbara County to wildfire disasters.