Climatology of Sundowner winds in coastal Santa Barbara, California, USA

Sundowner winds are late afternoon-to-early morning episodes of offshore, northerly gusty downslope surface winds frequently observed in the lee of the Santa Ynez Mountains in southern California, USA. Sundowners typically peak after sunset (reason for the name), are spatially non-uniform and can be accompanied by rapid increases in temperature and decreases in relative humidity with significant impact on fire behavior. Gale winds, single digit relative humidity and temperature exceeding 35^oC are not uncommon during these events, justifying Sundowners' reputation as a critical fire weather regime in the region. Sundowners have been observed under distinct synoptic conditions and year-round, with the highest median frequency of events in spring. The general synoptic setting for these winds is relatively well established and differs from the well documented Santa Ana winds. It is well accepted among local forecasters that Sundowners are primarily driven by a regional-scale north to south pressure gradient across the SYM that control the intensity of the winds. Depending on the magnitude of pressure gradients, these winds can reach gale force. Significant cross mountain flow over the Santa Ynez Mountains is necessary, such that the prevailing flow in the vicinity of Santa Barbara is offshore (northerly), eroding (or displacing) the marine BL that typically exists. Despite a handful of recent studies, knowledge about the spatial variability and diurnal, seasonal and interannual variability of Sundowner winds is limited. This presentation will discuss the climatology of Sundowner winds derived from 30-year downscaling simulations with the Weather Research and Forecasting model.