Downslope Wind Storms in Coastal Santa Barbara

The frequent gusty downslope winds accompanied by rapid warming and decreased relative humidity are considered among the most significant fire weather regimes affecting southern California coastal areas in the vicinity of Santa Barbara (SB). These events, locally known as “Sundowner” winds, have affected 15 major wildfires in the region since 1955. The Santa Barbara (SB) region of California is characterized by unique topography. The Santa Ynez Mountains, spanning a length of about 100km and oriented approximately east-west with elevations greater than 1200m, rises abruptly from a narrow coastal plain. About 200,000 people live in the coastal area and are highly vulnerable to wildfires. The onset of sundowner winds normally occur late afternoon and early evening and gusty winds can last during the night until early morning. The synoptic patterns leading these events are typically associated with increase in in sea-level pressure north of the Santa Ynez Mountains inductive of northerly winds and enhanced stability. The mesoscale circulation around the events is complex and attributable to orographic effects and land-sea contrast, which modify pressure gradients through heating differences and dynamical influences over the course of the day. This study uses the Weather Research and Forecast (WRF) model at 2km resolution to investigate dynamical and physical characteristics of the sundowner winds focusing on case studies. The performance of WRF in simulating these events is evaluated using local station data and a wind profiler. We examine in details the evolution of the marine boundary layer during days when these events evolved and characterize the importance of the development of the mesoscale vortex over outer waters west of Pt. Conception in the cycle of the phenomenon. We show that the diurnal cycle of pressure is largest in inland mountain regions, where low sea-level pressure is observed in the afternoon on account of strong diabatic heating, and transitions quickly to high pressure after sunset with increasing local stability driven by radiative cooling. This study investigates in details the evolution of these events and the importance of local feedbacks in the intensification of the phenomenon. This study provides an important framework for other areas in Southern California with similar geographic characteristics.