资源环境科技发展态势分析平台

Sundowner winds are a type of downslope windstorms observed on the southern slopes of the Santa Ynez Mountains (SYM) in Santa Barbara, southern California. The name “Sundowner” is due to the onset of strong winds near sunset. Sundowner winds can reach gale force and are extremely dangerous during wildfires. This study presents a climatology of Sundowners based on a 30 yr high spatiotemporal resolution dataset obtained with the Weather Research and Forecasting model. Combined empirical orthogonal function (EOF) analysis of hourly zonal and meridional winds (10 m) with 1 km grid spacing is performed to characterize the diurnal variability of surface winds. The first two eigenmodes (EOF-1, EOF-2) (58% of total variance) characterize the variability of surface winds in the western and eastern parts of the SYM, respectively. The hourly percentiles of the frequency distribution of the first two time coefficients (PC1, PC2) are used to identify three types of Sundowner regimes. The western (eastern) regime occurs when surface winds project strongly on EOF-1 (EOF-2) but not on EOF-2 (EOF-1). The western regime is characterized by a strong coastal jet around Point Conception with northwesterly winds extending into the Santa Barbara Channel. In contrast, a weak coastal jet not extending into the Santa Barbara Channel is observed in the eastern regime. In the Santa Barbara (SBA) regime, hourly surface winds project strongly on both EOF-1 and EOF-2. The western regime occurs more frequently than the eastern and SBA regimes and peaks during March-May. The eastern regime has maximum in January-February, whereas the SBA regime has a seasonal cycle similar to the western regime. Horizontal and vertical structures of winds and potential temperature show that the lee slope jet and mountain wave activity on the SYM exhibit strong spatial variations in the three regimes.