Abstract
Abstract. Downslope Sundowner winds in southern California's Santa Ynez Mountains favor wildfire growth. To explore differences between Sundowners and Santa Ana winds (SAWs), we use surface observations from 1979 to 2014 to develop a climatology of extreme Sundowner days. The climatology was compared to an existing SAW index from 1979 to 2012. Sundowner (SAW) occurrence peaks in late spring (winter). SAWs demonstrate amplified 500 hPa geopotential heights over western North America and anomalous positive inland mean sea-level pressures. Sundowner-only conditions display zonal 500 hPa flow and negative inland sea-level pressure anomalies. A low-level northerly coastal jet is present during Sundowners but not SAWs.
Highlights
The combination of episodic low relative humidity and strong winds, complex terrain, and fuel conditions coupled with extensive wildland– urban interfaces (WUI) in southern California produces significant wildfire hazards with frequent large, severe, and costly fires (Westerling et al, 2004)
Sundowner + Santa Ana winds (SAWs) events primarily occur during the cool season (October–February) with a secondary peak in April (Fig. 1e)
During the most extreme temperature ramps, reduced relative humidity and increased winds were observed in the foothills and at the coastal plain, supporting the validity of this assumption
Summary
The combination of episodic low relative humidity and strong winds, complex terrain, and fuel conditions (e.g., load, moisture, and continuity) coupled with extensive wildland– urban interfaces (WUI) in southern California produces significant wildfire hazards with frequent large, severe, and costly fires (Westerling et al, 2004). Despite the high impact of fires in the Santa Ynez Mountains on urban communities (i.e., WUI; Martinuzzi et al, 2015) and agricultural operations, little research has focused on the smallerscale Sundowner winds and is limited to case studies (Blier, 1998; Cannon et al, 2017). These studies indicate that different atmospheric processes are involved in Sundowner events compared to classic SAW events at the synoptic scale (Blier, 1998; Cannon et al, 2017). Identifying the nuances that differentiate Sundowners from SAWs may provide additional insight into fire weather forecasts and an understanding of weather–fire–climate interactions (Mensing et al, 1999; Moritz et al, 2010; Williams and Abatzoglou, 2016) in California’s Transverse Ranges
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