Urban Land, Aerosols Amplify Hazardous Weather, Steer Storms toward Cities
Houston’s thunderstorm clouds began as warm clouds with only liquid drops, but the strengthened sea breeze caused a quickened transition to mixed-phase clouds, named for their simultaneous mixture of water vapor, ice particles and supercooled water droplets. Even after the sea breeze trickled off, said Fan, residual heat from the city continued feeding storm convection well through the night, causing longer-lasting rain. Contrast that with Fan’s simulation where the city was removed, showing a weaker sea breeze and a storm that dissipated sooner.
Aerosols played a larger role in enhancing precipitation than urban land in Houston. As mixed-phase clouds formed and convection grew stronger, numerous ultrafine particles were transformed into cloud droplets. This transformation enhanced the conversion of water vapor into cloud condensates, thereby increasing latent heating and further strengthening the storm.
In the case of the Kansas City storm, heat from the city was carried downwind, where it met the already formed storm at the northern urban-rural boundary. When warmer, drier air met with cooler, moister rural air, it intensified convergence, creating turbulent mixing and a more violent storm that moved toward urban land.
In contrast with Houston’s thunderstorm, Kansas City’s aerosols did not influence storm initiation or propagation, nor did they, on their own, greatly influence hail. But, when simulated alongside urban land, the two amplified hail, synergistically producing a more hazardous hailstorm. Because of this relationship, said Fan, it’s important to consider both urban land and aerosols when exploring the impact cities have on weather and associated hazards.
Hail alone inflicts billions of dollars of damage in the U.S. and, according to the National Oceanic and Atmospheric Administration, it is possible for especially large hailstones to fall at over 100 miles per hour.
Accounting for Aerosols
Urban land and aerosols shape weather differently, according to Fan, depending on other environmental conditions, like whether air is already polluted.
“The aerosol effect really depends on the background concentration,” said Fan. “If the environment is already polluted, adding more aerosols doesn’t seem to affect much. But if you’re already in clean a condition and you add aerosols, it may produce a large impact.”
Houston’s busy shipping channel and nearby oil refineries, three of which are in its metro area, regularly discharge aerosols into the atmosphere, said Fan. Humidity, too, she added, can amplify the aerosol effect.
Fan hopes her work may lead to more accurate predictions of hazardous weather, mitigating the death and damage dealt by storms. She plans to more deeply explore how sprawling urbanization will shape severe storms in future climate scenarios.
This work was supported by the U.S. Department of Energy’s Office of Science and the National Science Foundation, and was supported by the National Energy Research Scientific Computing Center, a DOE Office of Science user facility.
