Brief Communication: Synoptic-scale differences between Sundowner and Santa Ana wind regimes in the Santa Ynez Mountains, California

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.

Characteristics of Sundowner Winds Near Santa Barbara, CA, From a Dynamically Downscaled Climatology: Environment and Effects Aloft and Offshore

“…The wind speed profile is characterized by a jet between 600 and 1,200 m above sea level (asl) whose magnitude is roughly 9–12 m/s depending on event strength. This is consistent with the northerly low‐level jet evident in composites presented in Hatchett et al (). The modeled jet is also present near the coast at Vandenberg, at the mouth of the Santa Ynez Valley, and at the offshore buoy stations (see Figure and Table for station locations).…”

“…These winds can occur in the lee of fairly modest terrain such as northern New Jersey (Decker & Robinson, ) or the Falkland Islands (Mobbs et al, ). They also occur during Santa Ana conditions in and around coastal southern California (e.g., Cao & Fovell, ; Fovell and Cao, ); however, those are distinct from the phenomena we treat here (Hatchett et al, ).…”

“…The coupling the alongshore coastal jet and the sea breeze also forces air parcels up and over the Santa Ynez range, leading to downslope windstorms on the southern slopes of the Santa Ynez range known as Sundowners. Sundowners exhibit the following notable characteristics (Smith et al, , hereafter referred to as SM18): Propensity to exhibit extreme temperature ramps (Figure ) outside of the typical diurnal cycle at downstream weather stations (i.e., Santa Barbara airport; Hatchett et al, ), while other regional stations do not experience a similar ramp. Association with large human caused conflagrations, including the Painted Cave, Montecito Tea, Jesusita, Sherpa, and Whittier fires (see fire perimeters on Figure , bottom). Initiation of the lee slope jet along the western side of the Santa Ynez range (near Gaviota) and progression of the lee slope high winds from west to east throughout the course of a day with a peak occurrence time in Santa Barbara near sunset (hence the name Sundowner). Seasonal modulation that matches the coastal alongshore north‐northwesterly jet and occurs contemporaneously with either transcritical or supercritical flow around Point Concepcion and the Southern California Bight (Dorman & Koraćin, ; Dorman & Winant, ; Parish et al, ; Parish et al, ; Rahn et al, ; Skyllingstad et al, ). …”

The 2017 North Bay and Southern California Fires: A Case Study

“…A positively tilted mid-tropospheric ridge slowly moved onshore, centered over the US Pacific coast, with this general pattern persisting until 22 December, keeping offshore flow across southern California (e.g., Figure 4a). Both Santa Ana and Sundowner winds can develop under this synoptic configuration [6,45,46]. Widespread strong east-northeast surface winds and RH below 15% developed by 00:00 UTC 5 December and expanded in coverage and intensity by 21:00 UTC 5 December (Figure 4b).…”

Spatial Distribution of Wildfires Ignited under Katabatic versus Non-Katabatic Winds in Mediterranean Southern California USA

“…Owing to the close proximity of densely populated Santa Barbara and surrounding communities, Sundowner-associated fires have been some of the most destructive wildfires regionally, despite relatively smaller sizes (e.g., the 2009 Jesusita Fire consumed 80 homes despite burning only 3534 ha). These conflagrations have often occurred in spring months, suggesting a third key period of fire danger for regional fire management outside of the summer and autumn peaks [17,18]. However, the proportion of the southwestern California fire history attributable to Sundowner events has not been quantified.…”