UTSA researchers examine urban effects on thunderstorms in Southeastern cities

JUNE 7, 2022 — To better understand the impact that urban environments have on storms and rainfall, two researchers from The University of Texas at San Antonio (UTSA) collaborated with colleagues at Louisiana State University (LSU) to observe how metro areas throughout the Southeastern United States enhance thunderstorm frequency.

Neil Debbage, an assistant professor of geography and environmental sustainability in the UTSA College of Liberal and Fine Arts, and graduate geography student Jewel Uzquiano worked with doctoral student Robert Forney and geography professor Paul Miller of LSU. The collaboration resulted in a NASA-supported research paper that was published in the May 2022 issue of Urban Climate.

Debbage has studied the effects of large cities on climatology for some time. Urban cores have long been constructed with heat-absorbing materials like concrete, steel and asphalt. Cities also have a higher concentration of machines that pump heat into the atmosphere and generally have a lower concentration of cooling vegetation. This increased temperature in large metro areas provides a source of unstable air, which has the capability of producing clouds leading to more frequent rainfall and thunderstorms. This is commonly known as the urban rainfall effect.

Although the understanding of the urban rainfall effect has grown more robust over the decades, studies have generally focused on a limited number of large cities. Aiming for more accurate comparisons, the UTSA-LSU collaborative examined weakly-forced thunderstorms from 2001 to 2015 in 32 urban environments across the Southeastern U.S. The team’s focus included large metro areas such as Atlanta, Miami and New Orleans as well as smaller ones such as Hickory, N.C., Jackson, Miss., and Huntsville, Ala.

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“This suggests that the urban rainfall effect is potentially widespread throughout the Southeast.”

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In this photo, air flowing over the downtown area of Chicago is forced upward forming clouds. This is one factor that contributes to the phenomenon more commonly known as the urban rainfall effect. Photo courtesy of Kyle S. Mattingly

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To accomplish this feat, the team developed a methodology to create an Urban Convection Ratio (UCR) for each city. The UCR compared the frequency of weakly-forced thunderstorms—which are storms driven largely by instability and moisture rather than the synoptic-scale lifting of air and wind shear—to the frequency of storms anticipated as a result of geophysical factors alone, allowing the researchers to isolate urban influences on thunderstorm activity.

Of the 32 cities in the study, these 10 had the largest UCRs:

1. New Orleans, La.
2. Memphis, Tenn.
3. Tampa, Fla.
4. Raleigh, N.C.
5. Knoxville, Tenn.
6. Atlanta, Ga.
7. Sarasota, Fla.
8. Miami, Fla.
9. Norfolk, Va.
10. Birmingham, Ala.

Although coastal metros such as New Orleans have historically made it more challenging to precisely identify urban effects on rainfall—due to their increased interaction with sea breezes and tropical disturbances—Debbage said the cities at the top of the list absolutely warrant additional investigation.

“I think this study sets the stage for future modeling work to more fully understand the specific physical mechanisms that explain the large UCRs in New Orleans, Memphis and Tampa,” he explained.

While New Orleans, Memphis and Tampa could prove to be informative study sites for the urban rainfall effect, two other top 10 entrants offered even more striking insights.

“One of the more notable findings was that several relatively smaller cities like Knoxville and Norfolk exhibited significant thunderstorm enhancements,” Debbage said. “This suggests that the urban rainfall effect is potentially widespread throughout the Southeast.”

The researchers were also surprised to find that two urban areas they examined actually suppressed thunderstorm activity: Winston-Salem, N.C., and Hickory, N.C., which ranked third-to-last and last, respectively, in UCR size among the cities studied. In Hickory’s case, the group surmises that the city’s interactions with the mountainous topography of the Appalachians to the west could reduce thunderstorms. In addition to cities with large UCRs, these two areas certainly merit further analysis.

Moving forward, Debbage hopes to apply the UCR approach to additional urban environments beyond the Southeast. One of the driving forces behind his research is urban flood resiliency. A better understanding of the urban rainfall effect will inform city leaders and first responders on how to prepare for potentially hazardous weather events.

“To fully prepare for flooding, urban planners and emergency managers likely need to account for urban amplification of precipitation,” Debbage explained. “It is, in many respects, a double whammy, where urban environments can increase the initial rainfall and increase runoff once the precipitation reaches the ground.”

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In addition to research on urban climatology, natural hazards, weather modeling and sustainability, Debbage regularly teaches courses in weather, climate, physical geography and GIS mapping.

In fact, his graduate student, Uzquiano, was responsible for the GIS visualizations and cartography for this research project.

UTSA is a Tier One research university that is committed to tackling society’s grand challenges through world-class education and research programs. With an emphasis on transdisciplinary collaboration, innovation and entrepreneurship, it is leveraging its research and development capabilities to benefit the San Antonio community and the world beyond.

— Shea Conner

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