Large-to-Mesoscale Mechanisms Associated With Gusty Downslope Winds in Coastal Santa Barbara, California: Observations and Modeling

Among the most significant weather events affecting coastal areas of Santa Barbara (SB) County in California are the late afternoon-to-nighttime episodes of gusty downslope surface winds accompanied by rapid increases in temperature and decreases in relative humidity, locally known as ‘Sundowners'. 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. Gale winds, relative humidity of less than 15% and temperatures above 90oF are not uncommon during Sundowner events even during the winter. Most importantly, Sundowner events have played a significant role in the evolution of all major fires that affected SB. These wildfires caused millions of dollars in property loss and significant impacts to the environment. The towns of Santa Barbara and Goleta, the largest in the County (91,196 and 30,525 inhabitants, respectively) are largely exposed to wildfire hazards with most of the population living in a narrow zone between the mountains and the ocean.

The present study investigates dynamical and physical mechanisms associated with events identified in a database created by the National Weather Service (NWS) as significant downslope windstorms, extending from 1999-2014. Given the major interest in extreme conditions and fire hazards, we investigate dynamical mechanisms of 688 events categorized according to relative humidity (RH). We use the National Center for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR) and show that large-scale mechanisms driving windstorms with extremely dry conditions at the coast (RH < 15%) are characterized by the progressive eastward movement and advancement of an anticyclone toward the continent over Northwest U.S. Isentropic analyses demonstrate that these large-scale conditions increase static stability that is advected to the Santa Ynez Mountains by the northerly winds. These conditions are observed only for the very dry episodes (RH<15%). Mesoscale characteristics of Sundowners, including the onset and temporal evolution of these events are investigated based on 13-days simulations with the Weather Research Forecasting (WRF) Model at 2km resolution. This period was selected because Sundowners were observed in 7 out of 13 days and RH was less than 15% during these events, indicating that this was a period of significant wildfire hazards.

We identify biases in the diurnal cycles of wind, RH and temperature by comparing WRF simulations with hourly data from local stations at different altitudes. We show that the onset of the events depends on variations in the sea-level pressure and also on changes in the height of the vertical wind shear layer with implications for the propagation of gravity waves. These results are of great relevance for weather and forecast of extreme fire hazard condition in the region.