New Climate Model Better Predicts Future Rainfall in Urban Environments

Researchers have designed a new climate model incorporating the intermingling effects of greenhouse gases and urban design to predict the impact of new buildings and municipal works projects.

Construction taking place in Denver, Colorado. (Chris Marshall/CNS)

(CN) — A combination of urban expansion and greenhouse gases may be behind some of the extreme weather events that hammer a new metro area every few years, according to a new climate model.

Previous studies have looked at the impact each of these factors exhibit individually, but this is the first time anyone investigated their combined effects. By modeling the interactions between these two forcing agents, researchers believe city planners can become better informed about the impact of new building proposals and municipal works projects.

A team from Arizona State University released their findings Monday in a new study published in the journal Environmental Research Letters.

“The major significance of the study is the projected broad enhancement of extreme precipitation across future U.S. metropolitan regions as a result of urban development and climate change,” explained Matei Georgescu, associate professor in ASU’s School of Geographical Sciences and Urban Planning and lead author of the study, in an email. “The results highlight an opportunity for cities to immediately consider planning strategies that improve the uptake of precipitation that is expected across U.S. cities in future decades.”

According to the authors’ model, growing cities all over the U.S. like Denver, Phoenix and Houston are increasingly vulnerable to the extreme whether events that lead to flash floods in unprepared urban centers. In one September 2013 flood, places along Colorado’s Front Range received as much rainfall as they normally receive in an entire year, causing over $1 billion in property damage.

Growing metros are vulnerable to what’s known as the urban heat island effect — as cities grow, they become warmer than the surrounding area. This effect has several causes, such as dark asphalt soaking up solar radiation and energy used by the city’s inhabitants being expended as waste heat. That expended heat forces air to rise and condense faster, bringing with it increasing levels of rain. The result is flooding for unprepared municipalities.

“Destabilization of the urban boundary layer resulting from development of the urban heat island (UHI) circulation, modification of convective structures due to the rough urban form, and the presence of cloud condensation nuclei in more polluted urban environments are broadly considered responsible for urban-induced precipitation modification,” note the authors in their study.

The urban heat island effect can be partially mitigated by creating more green spaces inside cities such as rooftop gardens and parks, and the researchers’ new model could help direct those types of projects where they’re needed most.

“If we trust the models’ capability to simulate average and extreme precipitation so well, and our results demonstrate such simulation skill, then we can conduct simulations that include future urbanization, future greenhouse gasses, separately and then together, and trust what the model will tell us,” Georgescu said in a related statement.

Previous studies on future precipitation over major cities looked at changes in weather patterns caused by greenhouse gasses and urban development in isolation, but this is the first time anyone has modeled their combined effects.

“Our results were based on a technique called dynamical downscaling, whereby we provided information to our Regional Climate Model from two different Global Climate Models (GCMs) to account for projected increases in greenhouse gases,” Georgescu said in an email. “The consistency in our results, despite the differences in the two GCMs that we used to provide information to our Regional Climate Model shows that our results are robust.”

He explained that it’s not just about reducing greenhouse gas emissions, of fundamental importance is how cities are designed and built. The area they cover, how tall the buildings are, how densely those buildings are packed, how much electricity the inhabitants use — all these factors have an important impact on future precipitation in urban environments.

The authors concluded that mean and extreme precipitation days are likely to increase relative to the first decade of the century across all regions studied, and thus city planners need to be ready. They believe further research is needed to facilitate building more resilient cities capable of handling these increasingly extreme weather events.

“The results provided highlight the important effects of these dual drivers of precipitation change (urban development and climate change), especially extreme precipitation change,” Georgescu explained in an email. “It underscores the critical importance of modifying existing cities to undertake stormwater management adaptation decisions that ensure future flood risk and impacts are minimized.  These results also raise concerns about potential intra-city impacts, highlighting the need for similar studies as we have conducted, but at much higher resolution.”

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