Structure of cities shapes their weather: study

The features that make cities unique can help in understanding how they affect weather and disperse air pollutants, a new study has found.

Compared to their surroundings, cities can be hot enough to influence the weather, researchers said. Industrial, domestic and transportation-related activities constantly release heat and after a warm day, concrete surfaces radiate stored heat long into the night. These phenomena can be strong enough to drive thunderstorms, they said.

According to scientists, it is not only about the heat cities release; it us also about their spatial layout. By channelling winds and generating turbulence hundreds of metres into the atmosphere, the presence and organisation of buildings also affect weather and air quality.

Researchers led by Ecole Polytechnique Federale de Lausanne (EPFL) in Switzerland have shown that the way cities are represented in today's weather and air quality models fails to capture the true magnitude of some important features such as the transfer of energy and heat in the lower atmosphere.

They also found that processes that atmospheric sensors are unable to sense are essential to more accurately represent cities in weather models. When wind blows over a city, buildings interact with the moving air mass, generating turbulence, much like sticking your fingertips into a stream causes visible vortices to form on the water surface, researchers said.

This turbulence spreads up into the atmosphere and down into the streets. As a result, more heat, humidity and pollutants are transported upwards from the ground. At the same time, more of the wind's turbulent energy dissipates between streets, in gardens, or in other open spaces.

"What we showed in our work is the importance of taking into account the spatial variability of cities - the unique features that make Paris Paris and London London," said Marco Giometto from EPFL.

"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 Giometto.

"The transport of heat, humidity, or pollutants is then computed using mathematical relationships. These relationships implicitly assume that the city is geometrically regular, which is a stringent assumption," he added.

Researchers performed a series of detailed simulations of the wind flow over and within a neighbourhood of the city of Basel and compared results against wind tower measurements collected within the same area.

By accounting for the spatial variability of streets and buildings in the same Basel neighbourhood they were able to show that, for certain parametres that play a role in the local weather and the dispersal of smoke, smog, or other pollutants, approximating the city as a uniform patch of land can up errors, researchers said. The findings were published in the journal of Boundary Layer Meteorology.