AOSC Seminar by Dr. Gert-Jan Duine, 09/25/2025
AOSC Seminar
Gert-Jan Duine
UCSB, Earth Research Institute
Title
Advancing Understanding of Coastal Downslope Windstorms: Results from the Sundowner Winds EXperiment (SWEX)
Abstract
Downslope windstorms are among the most hazardous meteorological events contributing to the rapid growth of wildfires. These events remain difficult to forecast due to their high spatiotemporal variability. Observations of downslope windstorms, mountain waves, lee slope jets, and hydraulic jumps are particularly scarce in wildfire-prone environments. These challenges all come together in the region of coastal Santa Barbara, California, where the relatively shallow, but prominent Santa Ynez Mountains facilitate Sundowner winds, which is the local name for downslope windstorms in the area. Sundowner winds, or Sundowners, thank their name to their typical onset ranging from late afternoon to early evening. As many downslope windstorms, Sundowners are influenced by the upstream environment, mountain wave dynamics and boundary layer characteristics. Additional complications to Sundowners are a narrow valley and higher mountain range upstream of the Santa Ynez Mountains, and the adjacent cool Pacific Ocean downstream of the Santa Ynez Mountains. This geographic complexity introduces intriguing interactions between a deep, warm and dry continental boundary layer, and a shallow, cool, and stably stratified marine boundary layer. Sundowner winds have been associated with all major wildfires along the SYM that affected populated coastal areas. Improving understanding of their spatiotemporal evolution is therefore critical for both wildfire spread prediction and hazardous weather forecasting. The Sundowner Winds EXperiment (SWEX) was carried out in the spring of 2022 to enhance understanding of Sundowner wind mechanisms. SWEX collected a unique set of high-resolution observations as it added 18 flux towers, 5 lidars, 3 wind profilers, a microwave radiometer and 4 ceilometers in the foothills, slopes and mountains. A total of 10 intensive observational periods (IOPs) and three enhanced observational periods (EOPs) were carried out during Sundowner wind conditions. These missions included the simultaneous release of radiosoundings at 4 locations, mobile lidar measurements, and airborne observations of temperature, humidity, and winds (in-situ and remote sensing). The extensive and novel use of instrumentation during SWEX gave an unprecedented view on downslope windstorms, mountain waves, and boundary layers in coastal mountainous terrain. In this talk, I will highlight results obtained from the SWEX field experiment and subsequent numerical modeling efforts (kilometer and subkilometer scales). SWEX is funded by the US National Science Foundation, and was a multi-institutional effort including nine US universities, NCAR, the National Weather Service and local fire agencies.
Bio
Gert-Jan Duine is an Assistant Project Scientist at the Earth Research Institute of UC Santa Barbara in California, where he spends most of his time on unravelling the main mechanisms behind Sundowners winds using the SWEX field experimental data and numerical simulations (mesoscale and LES). He obtained his Bachelor and Master degrees in boundary layer meteorology at the Wageningen University in the Netherlands, and his PhD degree at the University of Toulouse, France, in 2015, where he investigated the characteristics of valley flows during periods of stable stratification. From 2016 and 2018 he was a postdoc at the University of Virginia in Charlottesville Virginia (Department of Environmental Sciences) and since 2018 he is at UC Santa Barbara. Besides Sundowner winds, Gert-Jan currently works on other projects related to coastal fog, wildfires, and the collection of eddy covariance measurements to support these projects, as he remains interested in many topics related to meteorology.
Contact
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AOSC Seminar
Pre-seminar refreshment: N/A
Seminar: 3:30-4:30pm, Room: ATL 2400(only when in-person)
Meet-the-Speaker: 4:30-5:00pm, Room: ATL 3400(only when in-person) [For AOSC Students only]
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