APRIL 2020 C A R V A L H O E T A L . 1519 The Sundowner Winds Experiment (SWEX) Pilot Study: Understanding Downslope Windstorms in the Santa Ynez Mountains, Santa Barbara, California a,b b a,b a c LEILA CARVALHO, GERT-JAN DUINE, CHARLES JONES, KATELYN ZIGNER, CRAIG CLEMENTS, c c a a d HEATHER KANE, CHLOE GORE, GARRET BELL, BRANDI GAMELIN, DAVE GOMBERG, d d d d e e TODD HALL, MARK JACKSON, JOHN DUMAS, ERIC BOLDT, ROB HAZARD, AND WOODY ENOS a Department of Geography, University of California, Santa Barbara, Santa Barbara, California b Earth Research Institute, University of California, Santa Barbara, Santa Barbara, California c Department of Atmospheric Sciences, San José State University, San Jose, California d National Weather Service, Los Angeles and Oxnard, California e Santa Barbara County Fire Department, Santa Barbara, California (Manuscript received 19 June 2019, in final form 15 January 2020) ABSTRACT Sundowner winds are downslope gusty winds often observed on the southern slopes of the Santa Ynez Mountains (SYM) in coastal Santa Barbara (SB), California. They typically peak near sunset and exhibit characteristics of downslope windstorms through the evening. They are SB’s most critical fire weather in all seasons and represent a major hazard for aviation. The Sundowner Winds Experiment Pilot Study was de- signed to evaluate vertical profiles of winds, temperature, humidity, and stability leeward of the SYM during a Sundowner event. This was accomplished by launching 3-hourly radiosondes during a significant Sundowner event on 28–29 April 2018. This study showed that winds in the lee of the SYM exhibit complex spatial and temporal patterns. Vertical profiles showed a transition from humid onshore winds from morning to mid- afternoon to very pronounced offshore winds during the evening after sunset. These winds accompanied mountain waves and a northerly nocturnal lee jet with variable temporal behavior. Around sunset, the jet was characterized by strong wind speeds enhanced by mountain-wave breaking. Winds weakened considerably at 2300 PDT 29 April but enhanced dramatically at 0200 PDT 29 April at much lower elevations. These tran- sitions were accompanied by changes in stability profiles and in the Richardson number. A simulation with the Weather Research and Forecasting (WRF) Model at 1-km grid spacing was examined to evaluate the skill of the model in capturing the observed winds and stability profiles and to assess mesoscale processes associated with this event. These results advanced understanding on Sundowner’s spatiotemporal characteristics and driving mechanisms. 1. Introduction the southern-facing slopes of the SYM. These winds typically intensify from early evening to morning hours Coastal Santa Barbara County (SB), with a pop- and are known as ‘‘Sundowner winds’’ or ‘‘Sundowners’’ ulation exceeding 130 000 inhabitants, is among the most (Ryan 1996; Blier 1998). exposed communities to wildfire hazards in southern All major wildfires affecting SB coast have exhibited California (Kolden and Abatzoglou 2018). The Santa significant fire spread rates toward the SB wildland- Ynez Mountains (SYM) rise abruptly from coastal SB urban interface due to Sundowners. Some dramatic separating the Pacific Ocean on its south face from the examples of these rapidly spreading wildfires fueled by Santa Ynez Valley (SYV) on its north face. Downslope, strong Sundowners are: Eagle Canyon (September dry and gusty windstorms are frequently observed on 1979), Painted Cave (June 1990), Gap (July 2008), Tea- House (November 2008), Jesusita (May 2009), Sherpa Supplemental information related to this paper is available (June 2016), Whittier (July 2017), and Thomas Fire at the Journals Online website: https://doi.org/10.1175/MWR- (December 2017); the latter is considered the largest D-19-0207.s1. wildfire in Southern California’s history to date. Although ignited in Santa Paula (Ventura County) during persis- 2 Corresponding author: Leila M. V. Carvalho, [email protected] tent Santa Ana winds with gusts as high as 35 m s 1 DOI: 10.1175/MWR-D-19-0207.1 Ó 2020 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses). Unauthenticated | Downloaded 09/27/21 07:32 AM UTC 1520 MONTHLY WEATHER REVIEW VOLUME 148 (Fovell and Gallagher 2018), the Thomas Fire further strong gusts, spatiotemporal variability of winds leeward of spread toward Montecito, SB, on the evening of the SYM, and mountain waves that can enhance wind- 16 December due to the strong Sundowner winds storms posing significant hazards to aviation. Moreover, (Kolden and Abatzoglou 2018). According to the the existing network of surface stations and the wind California Department of Forestry and Fire Protection, profiler installed at KSBA, though useful in evaluating these tragic events have caused injuries and fatalities, local conditions, are insufficient to examine mechanisms destroyed hundreds of properties and cost millions of explaining the observed wind variability downwind of dollars in suppression efforts. Additionally, they led the SYM and in coastal SB. KSBA is located near to massive evacuations and disruption of schools and the coast and is often influenced by a stable and cool businesses, created unbearable air pollution and environ- marine boundary layer (MBL) that significantly influ- mental problems, among many other issues. Therefore, ences winds near ground level. Consequently, significant improved weather warnings of these events, including gradients in winds exist between the coast and foothills greater lead time and spatiotemporal specificity of where of the SYM during Sundowners (Duine et al. 2019). For winds will be strongest is critical in supporting a rapid re- example, Hatchett et al. (2018) evaluated a climatology sponse in case of a wildfire. of Sundowner events (1979–2014) using KSBA and Despite the importance of Sundowners, little is known could not properly identify the remarkable Sundowner about the spatiotemporal variability and mechanisms event that affected eastern SB and Montecito during the driving these events. Contrary to Santa Ana winds that Jesusita Fire (May 2009). That event was clearly de- prevail during fall and winter seasons (Hughes and Hall tectable in other stations on the SYM slopes (Cannon 2010; Jones et al. 2010), Sundowners occur year round et al. 2017) and widely recognized by the NWS LOX and though appear more frequent during spring (Hatchett local meteorologists. These results demonstrate the et al. 2018). Blier (1998) noticed that significant cross- large degree of spatial variability of Sundowners and mountain flow over the SYM ridgeline is necessary for suggest the strong control of the MBL on temperature strong offshore (northerly) winds to erode the marine and winds near ground level (Duine et al. 2019). boundary layer that typically exists. That study also Numerous observational and modeling studies have showed that multiple synoptic conditions can lead to been proposed to explain the dynamics of downslope strong cross-mountain winds, and suggested that these winds in Alpine-type of mountains (Klemp and Lilly forcings maybe seasonally dependent. The multiplicity 1975; Smith 1979, 1985; Durran and Klemp 1987; Durran of synoptic settings conducive to Sundowners was also 1990; Gallus and Klemp 2000; Durran 2003; Vosper identified in Cannon et al. (2017) based on case studies, 2004; Grubisic´ and Billings 2007, 2008; Jiang and Doyle some of them related to major wildfires (such as the 2008; Lawson and Horel 2015). However, few investi- Painted Cave and Jesusita fires). Ryan (1996) found gations have been conducted to understand the inter- that strong mean sea level pressure (MSLP) differences action between downslope winds and a stable and cool between Santa Maria (KSMX) and Santa Barbara MBL in coastal ranges with moderate elevations, which (KSBA) airports are significantly associated with strong is typically observed in SB and other coastal areas in Sundowners (typically with a N-NW direction) in the lee California. Additionally, upstream conditions such as of the western section of the SYM. Sukup (2013) noticed the v-shaped SYV and the presence of the San Rafael that MSLP differences between KSBA and Bakersfield Mountains with elevations exceeding the highest peaks airports (KBFL) could be a better predictor for the in the SYM are important features to understand N-NE Sundowner winds that typically affect the eastern Sundowner winds. parts of the SYM. Cannon et al. (2017) investigated dynamical mecha- The National Weather Service Office in Oxnard (here- nisms associated with significant episodes of Sundowner after, NWS LOX) considers a significant Sundowner event winds based on simulations with the Weather Research when atmospheric conditions favor cross-mountain and Forecasting (WRF) Model at 2-km resolution. They (offshore) winds with sustained wind speeds $30 mph hypothesized that the cross-mountain winds generated 2 2 ($13.4 m s 1) and/or gusts $35 mph (15.6 m s 1)in gravity waves that further accelerated downwind in the stations leeward of the SYM. The NWS LOX has lee of the SYM due to the presence of critical layers utilizedthepressuredifferencecriteria(Ryan 1996; (Durran and Klemp 1987; Durran 1990, 2003). They Sukup 2013) as a rule of thumb to forecast Sundowners. showed that a mean-state critical layer, attributable to While MSLP differences have been successfully used in wind backing and reversal with height in the lower tro- forecasting the potential for Sundowners on synoptic posphere above mountain top, could be one possible time scales, these measures alone are not sufficient to mechanism related to the acceleration of winds. However, predict mesoscale features such as the onset and end of this mechanism could not explain all case-studies Unauthenticated | Downloaded 09/27/21 07:32 AM UTC APRIL 2020 C A R V A L H O E T A L . 1521 investigated in that study. In some situations, self- results of the experiment, focusing on observational an- induced critical layers (identified with Richardson alyses (section 5a) and simulations (section 5b).