Hazardous weatherUrban Land, Aerosols Amplify Hazardous Weather, Steer Storms toward Cities

Urban landscapes and human-made aerosols—particles suspended in the atmosphere—have the potential to not only make gusts stronger and hail larger; they can also start storms sooner and even pull them toward cities, according to new research exploring the impact of urban development on hazardous weather, led by scientists at the U.S. Department of Energy’s Pacific Northwest National Laboratory. 

By modeling two thunderstorms—one near Houston, Texas, and another in Kansas City, Mo.—atmospheric scientist Jiwen Fan teased out the separate and synergistic effects that urban landscapes and human-caused aerosols can have on storms, rain and hail. 

In the case of the Kansas City storm, urban land and aerosols worked together to amplify the frequency of large hail by roughly 20 percent. In Houston, an otherwise gentler thunderstorm saw amplified, longer-lasting rainfall that developed sooner, among other changes.

Fan shared her findings at the American Geophysical Union’s 2020 fall meeting, on Tuesday, Dec. 1, and answered questions virtually on Tuesday, Dec. 15.  

“The novelty of our study is that we consider both urban land and aerosols together,” saidFan, “instead of their separate impacts.”

In previous work, researchers have shown that urban land shapes weather, both through its topographical nature and the heat it produces. Cities are often warmer than their surroundings, because buildings not only absorb and retain the sun’s heat differently than trees and agricultural land, but also block wind flow. 

Yet many studies focus primarily on how cities and aerosols change precipitation and temperature, or only examine the influence of those factors separately, rather than their joint effect. 

Simulated Storms Reveal Changes in Hazardous Weather
Fan modeled two very different types of storms: Kansas City’s violent, rotating, hail-filled thunderstorm, and Houston’s gentler, sea breeze-induced thunderstorm. She simulated multiple versions of the same storms, with and without cities and aerosols present, to isolate the effects of these two distinct factors. 

In Houston, afternoon showers swelled as urban land and aerosols worked synergistically to amplify rainfall. Compared to simulations without cities, rain drenched Houston roughly a half-hour sooner, increasing its total by an extra 1.5 millimeters. Sea breeze winds blew stronger as well, whipped up by the influence of urban land. 

When cooler, denser air from the sea breeze flowed toward Houston, it brought moisture with it and clashed with warmer, lighter city air. The two mixed upon meeting, creating stronger convection compared to simulations without urban land.