We use an 11‐year numerically downscaled climatology to diagnose various characteristics of downslope windstorms known as Sundowners that occur along the Central California coast. At the surface, Sundowners are manifested as strong northerly winds along the southern slopes of the east‐west trending Santa Ynez Mountains that are part of a lee slope jet forced by internal gravity wave breaking aloft. Our analysis shows that barotropic shallow water interfacial waves along an elevated inversion do not play any significant part in Sundowner dynamics. The mountain wave is forced on a diurnal basis by the synoptically driven strong jet of north‐northwesterly winds located just offshore, which propagates into and through the Santa Ynez Valley. The occurrence of Sundowners is associated with a transcritical transition of the barotropic shallow water mode of the marine boundary layer around the Southern California Bight. The strength and presence of the alongshore jet are of primary importance in determining upstream profiles of wind speed and static stability and thus the magnitude and location of most Sundowner events. This is especially true for the relatively common and mild Gaviota‐type events that frequently occur during spring in the western part of the range. We show that in a general sense, there is no distinct eastern or Montecito type of Sundowner event but rather a continuum of Sundowners based on wind direction upstream near ridgetop height. Montecito‐type events tend to occur in conjunction with internal gravity wave breaking over the upstream San Rafael range that enhances mountain wave activity near Montecito.
- Award ID(s):
- 1664173
- NSF-PAR ID:
- 10180056
- Date Published:
- Journal Name:
- Atmosphere
- Volume:
- 10
- Issue:
- 3
- ISSN:
- 2073-4433
- Page Range / eLocation ID:
- 155
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract The impact of upstream terrain on the diurnal variability of downslope windstorms on the south‐facing slopes of the Santa Ynez Mountains (SYM) is investigated using numerical simulations. These windstorms, called Sundowners due to their typical onset around sunset, have intensified all major wildfires in the area. This study investigates the role of the orography upstream of the SYM in the diurnal behavior of Sundowners. Two types of Sundowners are examined: western sundowners (winds with dominant northwesterly direction) and eastern Sundowners (winds with dominant northeasterly direction). By using semi‐idealized simulations, in which we progressively reduce the upstream terrain, we show that the onset of the lee slope jet occurs in the late afternoon only when the flow approaches the SYM from the northeast, after interacting with a considerably higher mountain barrier. We demonstrate that during the eastern regime, the progressive reduction of the upstream terrain results in strong lee slope winds throughout the day. Conversely, the diurnal cycle of downslope winds during the western regime is less sensitive to the reduction of the upstream terrain. The Sundowner diurnal cycle during the eastern regime can be explained by boundary‐layer processes in the valley and the blocking effect of high mountains upstream of the SYM. These results contribute to a better understanding of the influence of upstream orography in the cycle and intensity of downslope windstorms in coastal mountains.
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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 designed 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 midafternoon 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 transitions 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.
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3390/atmos10030155
