Study shows more intense but less frequent storms
An international research team says monsoon storms in the Southwest have become less frequent but more intense, bringing more extreme wind and rain to central and southwestern Arizona than just a few decades ago.
The study, led by scientists with the University of Arizona, compared precipitation records from 1950 to 1970 with data from the 1991-2010 period to verify their climate model, scaled down to capture changes at a resolution of 1.5 square miles. At that level of detail the changes over time became apparent, while models using a 10 square mile grid aren’t able to accurately recreate the precipitation trends.
That shows how important finely scaled models will be for capturing the local impacts of climate change. In a press release, the University of Arizona scientists outline their findings:
“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 co-author Christopher Castro, a UA associate professor of hydrology and atmospheric sciences.
“This is one of the first studies to look at long-term changes in monsoon precipitation,” Castro said. “We documented that the increases in extreme precipitation are geographically focused south and west of the Mogollon Rim–and that includes Phoenix.”
Having less frequent but more intense storms is consistent with what is expected throughout the world due to climate change, Castro said.
“Our work shows that it certainly holds true for the monsoon in Arizona,” he said.
First author Thang M. Luong conducted the research as part of his doctoral work at the UA. He is now a postdoctoral researcher at King Abdullah University of Science and Technology, Thuwal, Saudi Arabia.
The paper, “The More Extreme Nature of North American Monsoon Precipitation in the Southwestern U.S. as Revealed by a Historical Climatology of Simulated Severe Weather Events,” by Luong, Castro, Hsin-I 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, México D.F. was published July 3 in the early online edition of the Journal of Applied Meteorology and Climatology.
The U.S. Department of Defense Strategic Environmental Research and Development Program and the Universidad Nacional Autónoma de México PAPIIT funded the research.
The researchers wanted to identify risks from warm-season extreme weather, especially those to Department of Defense installations in the American Southwest.
Existing global and regional climate change models don’t represent the North American monsoon well in either seasonal forecasts or climate projections, the research team wrote.
Looking at the average precipitation over the entire monsoon season doesn’t show whether monsoon storms are becoming more severe now compared with 60 years ago, Castro said.
Therefore Luong, Castro and their colleagues looked for extreme rainfall events during 1950-1970 as compared with 1991-2010. Average precipitation was about the same, but 1991-2011 had more storms with very heavy rain.
“What’s going on in the changes to the extremes is very different from what goes on in the changes to the mean,” Castro said. “Big storms, heavy flooding — we found out those types of extreme precipitation events are becoming more intense and are becoming more intense downwind of the mountain ranges.”
The team tested a common computer model of the atmosphere to try to replicate the historical changes in monsoon storm intensity. The model, similar to one used by the National Weather Service for forecasts, produces results similar to what would be observed on radar or satellite imagery by realistically simulating the physical structure of monsoon thunderstorms.
A key innovation of the UA research was the level of detail–the team tested several different levels of resolution. Only by using the high resolution of 1.5 miles (2.5 km) could the model replicate the actual rainfall recorded for the two 20-year periods being compared.
The recorded data showed only rainfall. The high-resolution models indicated rainier monsoon storms were accompanied by higher winds and more downbursts.
“Because the models get the precipitation right, it gives us confidence that the models get the winds right too,” Castro said.
He said that in Phoenix, monsoon storms used to be late in the evening but are now happening earlier.
The time shift makes the storms more dangerous, he said. “It’s when people are more likely to be out on the roads.”
The team’s next step, Castro said, is investigating whether the North American Monsoon is changing in Mexico.