Skyscrapers make hurricanes much worse, study finds

In August 2017, Hurricane Harvey – a category 4 tropical cyclone – ripped into the US city of Houston causing catastrophic flooding, killing 68 people and causing $125bn in damage, tying with 2005’s Hurricane Katrina as the costliest cyclone on record.

In the four days the hurricane battered southern Texas, many areas received between 40 – 60 inches (100 – 150cm) of rain, which inundated thousands of homes in and around Houston and displaced over 300,000 people.

While recent studies are showing how a warming climate has made tropical cyclones significantly more intense, both in terms of rainfall and wind speed (see below), new complimentary research indicates Houston’s urban landscape directly contributed to the huge impacts of the storm.

A research team from Princeton University found the Houston area’s risk of extreme flooding during the hurricane was 21 times greater due to urbanisation.

Writing this week in the journal Nature, they say computer models indicate the same hurricane would have resulted in “significantly less rainfall”, in the same geographic area, “but with open fields and crops in place of streets and skyscrapers”.

The authors say “surface roughness” of the environment due to the presence of manmade structures creates a “drag effect” on hurricanes.

They write: “This effect occurs when high winds strike tall buildings, creating friction. In the case of Hurricane Harvey, winds shifted warm surface air farther up into the atmosphere, thereby creating an optimal situation for cloud formation and precipitation.”

“When Hurricane Harvey blew into Houston, it literally got snagged on the city’s tall skyscrapers and towers,” said co-athour Gabriele Villarini, associate professor of civil and environmental engineering at the University of Iowa.

“The friction caused by high winds buffeting tall buildings created a drag effect that influenced air and heat movement and resulted in optimal conditions for precipitation.”

As well as using computer models, the team looked at year-on-year changes in flood peaks and compared them to rainfall levels and population growth – a proxy for urbanisation.

They compared these results to flood peaks from the 1950s, a period predating major urban development in Houston.

They found as Houston’s urban centre grew up and out, so too did the risk of severe flooding.

They recorded a greater risk of higher flood peaks and more serious and dangerous flooding overall. As the city’s miles of impervious pavements and streets increased, heavy rainfall could not be absorbed into the ground.

“Urbanisation is generally associated with a significant reduction in storm-water infiltration,” Professor Villarini said.

“Houston has experienced one of the most impressive urban-development booms in US history, and with growth comes an increase in impervious surfaces. This increase in urbanisation, combined with the region’s flat clay terrain, represents a very problematic mix, even with flood-mitigation measures in place.”

While the authors say the findings cannot be directly extrapolated to every coastal urban area in the US, they say the study does provide insight into the “unexpected costs of development”.

“Hurricane Havey’s impacts on Houston highlight hazards to coastal cities along the Gulf Coast and Eastern Seaboard of the United States,” said co-author James Smith, a Princeton professor of civil and environmental engineering.

“An unfortunate repeat performance from Hurricane Florence this year underscores the problems of extreme tropical cyclone rainfall in urban settings.”

Impacts of climate change

The study comes as new supercomputer simulations by climate scientists at the US Department of Energy’s Lawrence Berkeley National Laboratory have shown climate change intensified the amount of rainfall in recent hurricanes such as Katrina, Irma, and Maria by between 5 to 10 per cent.

The research, also published this week in Nature, says if those hurricanes were to occur in a future world warmer than present, there would be even more rainfall and stronger winds.

Using three future climate scenarios based on increasing levels of greenhouse gases, the Berkley team’s simulations indicated rainfall could increase by 15 to 35 per cent. Wind speeds increased by as much as 25 knots, although most hurricanes saw increases of 10 to 15 knots.

“We’re already starting to see anthropogenic factors influencing tropical cyclone rainfall,” said lead author Christina Patricola, a scientist in Berkeley Lab’s earth and environmental sciences department.

“And our simulations strongly indicate that as time goes on we can expect to see even greater increases in rainfall.”