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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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Multiscale WRF Modeling of Meso‐ to Micro‐Scale Flows During Sundowner Events
Abstract Two Sundowner events observed during the Sundowner Wind Experiment (SWEX) project are analyzed using a realistically forced large eddy simulation (LES) employing a multiscale Weather and Research Forecasting (WRF) model configuration with domain grid spacings ranging from 11,250 to 30 m centered over the Santa Barbara, CA region to examine their meso‐ to micro‐scale drivers. The main drivers of both events are increasing mountaintop stability and the mountain wave activity exhibiting a hydraulic jump and near‐surface critical layer. Another important finding is ascent of the downslope flows over the turbulent adiabatic layers at the coastal regions. In both events, the strong downslope flow warms and dries the air descending the southern slopes of the SYM adiabatically generating a deepening adiabatic layer that is 0.4 to as much as 1 km deep during peak Sundowner intensity over the coastal regions. This layer, exhibiting turbulence within and atop, is characterized with the strong downslope flow atop with much weaker, and at times, reversed flow beneath over the coastal regions. This flow structure, along with regions of turbulence within and atop the adiabatic layer, is indicative of a mountain lee‐wave rotor. Coastal locations in both events remain relatively unaffected. Further investigations are needed to determine whether or not this is consistent across all Sundowner events observed during the SWEX project and whether turbulence helps diffuse or accelerate the flows.
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- Award ID(s):
- 2236504
- PAR ID:
- 10571418
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Atmospheres
- Volume:
- 130
- Issue:
- 3
- ISSN:
- 2169-897X
- 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.more » « less
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Abstract In wildfire-prone coastal Santa Barbara, California, downslope winds observed on the southern slopes of the east-west oriented Santa Ynez Mountains are known as Sundowner winds (or Sundowners). One important feature of Sundowners is the remarkable spatial and temporal variability in lee slope jet characteristics. Besides the intensity of the flow approaching the mountain range, the acceleration of the lee slope jet can be influenced by reflected gravity waves associated with one or more of the following mechanisms: a self-induced critical level, an inversion close to mountaintop, and the presence of a mean-state critical level (MSCL). The relative contribution of these mechanisms to the enhancement of Sundowners is yet unknown. This study uses 32-yr simulations (hourly, 1-km grid spacing) complemented with observations collected during the Sundowner Winds Experiment (SWEX) to better quantify the relative contribution of these mechanisms and to quantify the importance of MSCLs. We show that when an MSCL is present below 5 km, less atmospheric forcing is necessary to attain similar lee-slope jet strengths compared to when MSCLs are absent or above 5 km. This was evidenced from simulations and verified with observations. Although MSCLs during Sundowners occur year-round, their relative frequency increases in summer, when temperatures are high and fuels are dry, enhancing wildfire risk. Properly identifying these processes contributes to improved understanding and predictability of Sundowners and many other hazardous downslope windstorms in coastal environments.more » « less
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