Study of sundowner winds offers wildfire forecasting insights

sundowner winds
Sundowner winds have worsened all major wildfires in Santa Barbara County. Image: courtesy Leila Carvalho

By Kathleen Wong, UC Natural Reserve System

Nestled between the blue Pacific and the steep rise of the Santa Ynez Mountains, the beach town of Santa Barbara is extraordinarily picturesque. But the surroundings that make for such handsome photos also have a sinister side. They’re responsible for one of the area’s greatest dangers—the sundowner winds closely linked to catastrophic wildfires.

Named for their propensity to intensify as the sun sets, sundowner winds are a product of Santa Barbara’s unique geography. When strong winds blow from north to south and crash into the Santa Ynez Mountains, they act like a fast-flowing river encountering a boulder: the air splashes over the ridgeline and tumbles downhill, gaining momentum and losing moisture as it travels. On the coastal side of the mountains, this flowing air breaks up like a big wave, generating turbulence. By the time they reach foothill neighborhoods, these dry winds can reach speeds of 50 miles an hour or more—perfect for fanning a stray spark into a massive conflagration.

“All the fast spreading and destructive fires that have scared the Santa Barbara community over the years have been driven by sundowners,” says Leila Carvalho, a UC Santa Barbara professor of meteorology and climatology.

“The wind is very, very intense and turbulent. It can change very rapidly in intensity. That makes fire behavior extreme: it’s difficult for firefighters to predict how fast it is going to go,” says UCSB geography professor Charles Jones.

sundowner winds
Sundowner winds pouring down the Santa Ynez Mountains are illuminated by smoke from the 2017 Whittier Fire. Photo Credit: Image: Mark von Tillow

Understanding sundowner conditions

This spring, Carvalho and Jones led researchers from ten different institutions in a six-week effort to measure sundowner conditions across the Santa Barbara region. Funded by the National Science Foundation, the Sundowner Wind Experiment (SWEX) will paint the most detailed portrait yet of an atmospheric phenomenon first described in the 1990s. The result should be more accurate red flag warnings of increased fire danger, and more strategic deployments of firefighters.

“Our goal was to understand more about the conditions that favor sundowners. This experiment will allow us to understand more about processes and mechanisms,” says Jones, who plans to use the findings to refine his model of atmospheric conditions in the Santa Barbara region.

“The problem is that the sundowner winds are not strong everywhere at the same time. The experiment generated data to evaluate how this variability occurs, when, and what kinds of conditions drive it,” Carvalho says.

SWEX has provided the hard data needed to answer many outstanding questions about sundowner behavior. Until now, scientists have had to rely on information from the relatively few research-grade weather stations scattered around the Santa Barbara region.

What scientists do know is that sundowners are not the same as their famous neighbor, the Santa Ana winds. Besides their potential to drive relative humidity into the single-digits while pouring down California mountains, the two have little in common. Sundowners are most common in spring, while Santa Anas tend to blow during late fall and early winter. Santa Anas usually affect a broad swath of southern California, while sundowners are typically confined to the Santa Barbara coast. And Santa Anas tend to peak in the wee hours of the morning, as opposed to the after-sunset schedule common with sundowners.

sundowner winds
Researchers installed meteorological towers in the backcountry of the Santa Ynez Mountains to characterize typical as well as sundowner weather conditions. Image: courtesy Leila Carvalho

A meteorological battle plan

The logistics for SWEX resembled those of a military campaign. Researchers and students from seven universities around the country, the National Center for Atmospheric Research (NCAR) in Colorado, and the Naval Postgraduate School (NPS) in Monterey planned the campaign and the deployment and installation of sophisticated instruments on land and in the air. They did not work alone. Local assistance came from the Santa Barbara County Fire Department, Montecito Fire Department, and U.S Forest Service. In addition, daily meetings with the Los Angeles/Oxnard office of the National Weather Service informed the timing and location of measurement efforts.

The campaign itself ran from April 1 through May 15, the peak of sundowner season. During this period, SWEX ran 10 successful missions observing sundowner events, plus three more missions observing more typical conditions—the foggy mornings, light afternoon sea breezes, and light evening winds familiar to most Santa Barbarans.

Each of those 13 occasions involved choreographing meteorological measurements across the regions. One major tool used to measure wind conditions was LiDAR. LiDAR shoots laser beams into the sky and measures the time required for light reflected by airborne particles to return. The technique yields a precise measurement of wind profiles. The researchers set up six LiDAR instruments on towers in the rugged backcountry of the Santa Ynez and San Rafael Mountains, as well as across the Santa Barbara coast and the Santa Ynez Valley. Other ground instruments included flux towers measuring the magnitude, humidity, and flow of air; microwave radiometers observing humidity and temperature profiles; and infrasound instruments measuring air turbulence.

sundowner winds
As part of the SWEX effort, a small plane equipped with LiDAR flew around the Santa Barbara region while dropping wind-measuring instruments. Image: courtesy Leila Carvalho

More instruments were driven in circles through the valley and along the coast: the car-mounted Jackson State University Meteorological Unit collected temperature and humidity near ground level, while the University of Virginia’s Wind LiDAR on Wheels obtained wind profiles from ground level to about 5,000 feet.

To expand their understanding of air column behavior, the researchers also released weather balloons from four sites across the region. Every three hours around the clock, students from participating institutions released helium-filled balloons the size of beanbag chairs. These hauled compact instrument packages called radiosonds aloft to measure both winds and weather conditions.

Observations from the air

Overhead, the Naval Postgraduate School’s Twin-Otter plane navigated choppy conditions to fly three other LiDARs in figure-eight formations over the valley and the Santa Barbara Channel. The plane also deployed dropsondes: devices equipped with parachutes that gather profiles of wind speed, temperature, and humidity as they fall.

All of the instruments on board the Twin-Otter were used to characterize an atmospheric phenomenon called mountain waves. Carvalho likens the behavior of air cresting a mountain ridgeline to water in a rushing river that has encountered a big obstacle such as a boulder. “When that flow encounters the rock, it flies up a bit and then it gains potential energy, gravity energy. And on the other side it accelerates down very strongly, and splashes up and down as it breaks on the other side” in a wavelike pattern. Plane passengers often experience such waves as turbulence when flying over mountains. Understanding the behavior of mountain waves associated with sundowners is particularly important because the turbulence can transport embers ahead of fire lines during a wildfire, and pose hazards to aviation and firefighters.

Together, all of these instruments revealed remarkable contrasts in winds, temperature, and humidity in the region, illustrating why forecasting fire-weather conditions is extra challenging in Santa Barbara.

sundowner winds
Mobile LiDAR and other instruments for SWEX were carried on a small plane that traced figure-eight formations (straight purple and red lines) plus a vehicle that drove in circles along the coast and into the Santa Ynez Valley (wavy blue and purple lines). Image: courtesy Leila Carvalho

A base in the valley

The NRS’s Sedgwick Reserve served as SWEX headquarters in the Santa Ynez Valley. Located in the midst of a critical area for the study, the reserve provided space for a wind profiler, a microwave radiometer, a flux tower, and weather balloon releases.

“Sedgwick was the perfect location for us to do research. They have a landscape we are interested in. We could leave instruments that cannot be abandoned in a secure, protected area with power. And they had locations where our students could stay overnight. Altogether, Sedgwick was scientifically and logistically very desirable,” Carvalho says.

Patience rewarded

SWEX was delayed two years by the pandemic. Conditions during the 2022 campaign, however, proved well worth the wait. Jones and Carvalho originally budgeted to measure seven sundowner days plus four non-sundowner days for comparison. Mother Nature had other ideas.

“It turned out the season was so active, so off the charts, that we ended up running 12 and a half missions. We used all the resources we had and more, because we did not want to miss a thing. But we got amazing, amazing data,” Carvalho says.

Matching data to mechanisms

The researchers are now sifting through the reams of data they’ve gathered. The information will help ground-truth a computer model Jones runs simulating atmospheric conditions in the Santa Barbara region. The model already provides area forecasts up to several days in advance. SWEX will help the researchers identify what aspects of the model work well, which areas need adjustments, and determine whether improvements such as a higher resolution model can increase the model’s accuracy.

sundowner winds
Students released balloons carrying air wind and weather-measuring equipment from Sedgwick Reserve (shown here) and other sites around the county. Image: courtesy Leila Carvalho

Among the mysteries they hope to settle: why sundowners tend to arise after dark. During the day, winds from the Santa Ynez valley tend to blow one to two kilometers high, well above the mountaintop. But the rapid cooling of the valley after sunset, models suggest, pulls those winds to lower elevations. If they fall far enough to encounter the mountains as an obstacle, sundowners occur. Data from SWEX should help verify all of these mechanisms.

Fuel for sunset winds

Sundowners can gain a double dose of mountain wave energy thanks to the San Rafael Mountains. Even taller than the Santa Ynez range, these form the northeastern edge of the Santa Ynez Valley. At night, they accelerate down the San Rafael Mountains and hurtle across the valley before encountering the wall of the Santa Ynez Mountains.

“This explains why even before sunset winds start picking up in the Gaviota region,” past the western edge of the Santa Ynez Mountains, “but in Montecito or Santa Barbara they tend to pick up after sunset: the San Rafael Mountains are playing a role,” Carvalho says.

While the science is fascinating in and of itself, those who stand to benefit most from SWEX are emergency wildfire responders and local residents. Being better able to predict the behavior of these capricious winds through certain canyons and at certain times will help firefighters determine where to marshal their forces and when to evacuate neighborhoods at risk. At a time when a drier, warmer climate has made wildfires both more frequent and fierce, these improvements in forecasting cannot come fast enough.

Related links

The Sundowner Winds Experiment, Climate Variations and Change research group, UC Santa Barbara

The Sundowner Winds Experiment (SWEX) update, video talk by Leila Carvalho

Earth, wind, and fire, UC Santa Barbara

SWEX, National Center for Atmospheric Research

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