In heatwaves in July 2016, temperatures in New York City sometimes soared 10°C higher than the local countryside, according to the study by Prathap Ramamurthy and others at the City College of New York. Their research also revealed that New York’s heat island has internal boundary layers, which current weather models are unable to predict, according to the authors.

The study "challenges our current models that are overwhelmingly used to predict environmental flows in complex coastal urban areas," said Ramamurthy. "Secondly, it shows that megacities like New York are highly vulnerable to extreme heat."

More than half the world’s population lives in urban areas, and cities account for some three-quarters of the world’s greenhouse gas emissions. Urban climates are known to be different to rural climates because of the urban heat island effect: elevated temperatures that result from the use of materials such as concrete, which are poor at retaining moisture but have a high thermal storage capacity.

The heat island effect is thought to be a reason why heatwaves – along with their consequences, such as higher mortality, and stress on infrastructure – are felt more strongly in urban areas. Even so, the dynamics of the heat island effect, particularly in coastal areas, are poorly understood. "Most climate models and even regional weather forecasts do not have representation for urban areas," said Ramamurthy.

To address the problem, Ramamurthy and colleagues continuously monitored the urban boundary layer – a layer of the atmosphere created by a city, and encompassing its heat island – of New York City during July 2016. The team used sky-scanning radiometers and ground-based weather stations, among other instruments. By coincidence, this month saw three heatwaves in the city, and according to NASA was the hottest month globally since records began.

The temperature difference between New York’s heat island and surrounding rural areas during the month was nearly twice the decadal average, the results showed, while the individual heatwaves occasionally saw the difference rise to 10°C. There were elevated temperatures not just at ground level, but up to 2500 metres above the city.

In addition, the researchers found rare evidence for an internal boundary layer in those neighbourhoods populated by tall buildings. Here, the moist sea breeze appeared to interact with the hot urban surface, creating a bottom layer some 100 metres deep that was cut-off from the atmosphere above it.

"The thermal internal boundary layer (TIBL) was a bit surprising as our weather prediction models were unable to predict them," said Ramamurthy. "The TIBL will affect various processes like cloud formation, thunderstorms, transport of pollutants, etc."

Ramamurthy says the take-home message for physical scientists would be to advance our current understanding of coastal urban climates. "More environmental [and] meteorological observations are necessary to better represent the physics," he added.

Now the researchers are continuing to observe the urban boundary layer, with existing and new instruments. "Our primary goal is to understand and model the physics and derive its implication to critical processes that impact human health and energy use," Ramamurthy said.

The study is published in Environmental Research Letters (ERL) .

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