A study found that monsoon storms around the state are occurring less often but with more intensity. Tucson is just outside the affected area, likely due to the mountain ranges that surround the city.

Mike Christy / Arizona Daily Star

Monsoon storms are rumbling through central and southwestern Arizona less often but with more intensity than they were 60 years ago, according to a new study by researchers at the University of Arizona and Universidad Nacional Autónoma de México.

This includes areas south and west of the Mogollon Rim, such as the Phoenix metro area, Casa Grande, Ajo, Yuma, Bullhead City, Kingman, Gila Bend, Lukeville, as well as Luke Air Force Base, the Barry Goldwater Air Force Range and the Tohono O’odham Reservation.

Tucson is just outside the affected area and that’s likely due to the mountains that surround the city. Storms come off the mountain ranges and gather strength as they move from this area, researchers said.

“The monsoon is the main severe weather threat in Arizona. Dust storms, wind, flash flooding, microbursts — those are the things that are immediate dangers to life and property,” said Chris Castro, associate professor of hydrology and atmospheric science at the UA and co-author of the research, in a statement.

The team counted the number of extreme weather events between 1950 and 1970 and compared them with events between 1991 and 2011. They compared the observations with the Weather Research and Forecasting Model and found that they agreed.

Researchers chose to compare periods before and after significant anthropogenic climate change had occurred (those studying climate cite the 1980s as the pivot point) to see what effect it might have on local weather patterns over time, said Hsin-I Chang, UA research assistant professor in hydrology and atmospheric sciences and co-author.

“Having less frequent but more intense storms is consistent with what is expected throughout the world due to climate change,” Castro said.

“Scientists need to be honest,” he said. “How we’re realizing (the consequences of climate change) in Arizona is in the form of its largest, most intense events,” the monsoon.

This is the first time scientists have looked into how climate change might be directly affecting the monsoon in the region.

Currently, scientists use global-scale models to understand climate patterns. But when trying to understand changes in geographic areas with complex terrain, such as monsoon season in Arizona, the global models just don’t cut it.

The resolution of those models is too low, or coarse, Chang said. This includes models with a resolution of 10 miles or more.

So the team used a more detailed model, with a higher resolution of 1½ miles, and found that it was much more accurate at matching observed weather from the past.

“You just can’t trust coarser simulations to represent changes in severe weather. You have to use the high-resolution model,” Castro said in a statement.

Now that they know their model is reliable, they can say with more certainty that when a storm hits, we can expect it to be more intense, Castro said.

This research was funded by the U.S. Department of Defense Strategic Environmental Research and Development Program and the Universidad Nacional Autónoma de México PAPIIT.

“How can we sustain this kind of intensity?” Chang said. “This will affect safety.”

These extreme storms have the potential to damage city infrastructure.

“The Department of Defense is very interested,” Castro said. “There are fixed assets important for national security.”

The Department of Defense owns about 3 million acres of Arizona land, concentrated in the southwestern part of the state.

“Air Force bases have protocol to shut down and lock up in the event of extreme weather,” Chang said. “They can’t do it in an hour.”

And these changes in weather patterns won’t affect everyone in the same way.

About 27 percent of Arizona is tribal land, and a large swath exists within the area monsoon storms have intensified. This is especially true for the Tohono O’odham tribe.

“We (in cities) have a range of infrastructure and socio-economic challenges, but it’s even worse on the (Tohono O’odham) Nation,” Castro said. “Flooding, winds, homes not in good enough condition to withstand extreme weather. They’re highly vulnerable.”

Next, the research team will investigate if the monsoons are changing in Mexico.

Those involved in the research include: Thang M. Luong, lead author and doctoral student during the time of the research; Castro, Chang and Timothy Lahmers of the UA Department of Hydrology and Atmospheric Sciences; and David K. Adams and Carlos A. Ochoa-Moya of the Universidad Nacional Autónoma de México.

Contact Mikayla Mace at mmace@tucson.com or (520) 573-4158. Or follow @mikaylagram on Twitter.