The Sundowner Experiment

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The Sundowner Experiment
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All of the major wildfires in Santa Barbara have been dramatically influenced by Sundowners, including the recent Thomas Fire pictured here. (Photo: Mike Eliason)

By Julie Cohen, UC Santa Barbara

The Santa Ynez Mountains rise abruptly from the coast above Santa Barbara and have an east-west orientation. This unusual positioning sometimes results in strong, gusty offshore downslope winds known as Sundowners. 

Although conditions for these winds can be predicted in some cases, mechanisms driving their spatial and temporal variability remain largely unknown. Existing scientific understanding of Sundowners largely relies on a few case studies using regional models. 

Now, for the first time, a team of scientists from UC Santa Barbara and San Jose State University has evaluated these winds in real time and studied how they vary with elevation. Understanding these aspects can help improve future Sundowner forecasts on the ground — an extremely relevant and valuable tool during wildfires. 

The experiment determined that Sundowners are associated with a nocturnal downslope low-level jet with a strong and narrow riverlike flow, a north-south orientation and speeds of about 30 mph at elevations between 328 and 490 feet above sea level.  

“This was the first time that we effectively observed the evolution of a nocturnal jet over land in coastal Santa Barbara,” said Leila Carvalho, a professor in UCSB’s Department of Geography and the principal investigator on a National Science Foundation project called “Understanding Extreme Fire Weather Hazards and Improving Resilience in Coastal Santa Barbara, California.” This latest Sundowner experiment was funded by a UCSB Faculty Research Grant. “We have seen the jet in high-resolution model simulations before, but we had no way of confirming these features. Meteorological observations in Santa Barbara are limited to surface stations and do not provide weather information above the ground,” Carvalho added.

Sundowners happen year-round but are more common from March to May or early June, peaking from early evening to early morning, when onshore winds weaken. They are often associated with warm temperatures in the evenings accompanied by low relative humidity. The effect of the winds and temperature vary dramatically with the distance from the mountains. However, all of the major wildfires in Santa Barbara have been dramatically influenced by Sundowners, including the recent Thomas Fire. 

During the last weekend in April, when winds gusted to between 40 and 50 mph in the mountains, the researchers launched 13 radiosondes — weather balloons that can reach 75,000 feet into the atmosphere — at three-hour intervals. They were able to capture atmospheric profiles of winds, temperature, pressure and relative humidity.

“Observing the formation and evolution of the low-level jet was extremely important because ultimately these jets can cause mountain waves, which can be strong enough to cause ‘rotors’ or eddies that are dangerous for aviation,” Carvalho explained.  “These waves can intensify the gustiness of the winds as well. Therefore, being able to identify and forecast the spatial and temporal evolution of these jets is essential to improve the forecast of Sundowner winds, particularly during wildfires.”

The team’s future research will focus on evaluating these features in models. By simulating these events using a regional model, for the first time, they should be able to compare simulation profiles with actual observations made in the area.

“In the near future, our major goal is to have a comprehensive field campaign in the region,” Carvalho said. “Santa Barbara has faced many disasters lately, and our research aims at improving understanding and advancing our modeling and forecasting capabilities of these events.”

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Luvaduck May 09, 2018 08:10 AM
The Sundowner Experiment

Interesting to find out why, but does it help anyone come up with a way to keep them from happening when there's a forest fire? That would be the real break-through.

oceandrew May 09, 2018 09:00 AM
The Sundowner Experiment

I think you have it backwards. It's not how to prevent sundowners when a fire exists but how to prevent fires when sundowners exist... or at least prepare for them and get to them before they are out of control. Don't you think that makes more sense?

DH May 09, 2018 08:45 AM
The Sundowner Experiment

Ummm, how on earth would one even begin to prevent sundowner winds?

CoastWatch May 08, 2018 11:16 PM
The Sundowner Experiment

If you travel the Gaviota section of 101 anytime after sundown until midnight, you will experience this affect... It can occur anytime, but has been really strong these last 6 weeks.

richyrich May 08, 2018 11:28 AM
The Sundowner Experiment

This is valuable research, the more prepared we are for extreme weather events the better we can be prepared. And we need to stop blaming other human beings for the cause of fires. They start for thousands of different reasons, but only get out of control when weather conditions support it.

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