The Shape and Layout of Cities Affect the Weather

May 18, 2016 | Joanne Kennell

A stormy sky above a city
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And the quality of air you breathe.

Thanks to the phenomenon known as the urban heat island effect, cities are often quite balmy compared to their surroundings. In fact, cities can get so hot that they can begin to influence weather and affect air quality. But it comes down to more than just the heat that cities release — it’s also a result of their shape and layout.

When winds blow over a city, buildings interact with the air and generate turbulence. This turbulence spreads up into the atmosphere and down into the streets, which transports more heat, humidity, and pollutants from the ground.

"Most city representations used in weather models are based on data obtained from tower measurements made at a particular location within the city, which current models approximate as a rough patch of land," said Marco Giometto, the first author of the study, in a press release. "The transport of heat, humidity, or pollutants is then computed using mathematical relationships."

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However, the Ecole Polytechnique Fédérale de Lausanne (EPFL)-led study published in the Journal of Boundary Layer Meteorology, shows that the way cities are represented in current weather and air quality models doesn’t capture the important process of heat and energy transfer in the lower portion of the atmosphere.

Giometto and his colleagues performed a series of simulations of the wind flow over and within a neighborhood in the city of Basel, Switzerland, and compared the results against wind tower measurements collected within the same region. By accounting for the different streets and buildings in the Basel neighborhood, the researchers were able to show that approximating the city simply as a uniform patch of land can lead to large errors in the dispersal of smoke, smog, and other pollutants.

"Weather models obviously can't include detailed representations of all large cities," said Giometto. High-resolution simulations require time and resources that are not available to weather forecasters.

Rather, Giometto suggests that scientists should research and develop new, more accurate ways to describe urban settings that can then be integrated into a computer model. Particularly, focusing on the dispersive terms. In the mathematical equations that govern the movement of air, dispersive terms distribute pollutants, heat, humidity, and energy.

"You can picture them as mini air-circulations, locked in between buildings that transport warm and polluted air up from ground-level on one side and draw down cleaner and cooler air on the other," said Andreas Christen, a co-author of the study, in the release.

Unfortunately, a single weather station is unable to measure these dispersive terms directly, which is why computer simulations are necessary. "We need accurate computer simulations of the wind over cities to estimate dispersive terms," explained Giometto.

The hope is that this information will result in the development of more accurate weather and air quality forecasts for urban residents. Not only that, they could also help make cities more energy efficient, and maybe even a little less hot.

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