The Urban Cool Island Effect stumps deniers at WUWT

There is a new paper with the title: "The urban heat island effect and city contiguity". The authors, Neil Debbage and J. Marshall Shepherd from the University of Georgia analysed the difference between urban areas in the USA and their rural surrounds. I don't have time to go into much detail except to say that, as with other studies, they found that some places have an Urban Cool Island Effect. Not all cities are hotter than their surrounds.

Here's the map from Figure 4 in the paper:

Fig. 4. Map of the annual average UHI intensity (°C) in 2010 for the 50 most populous MSAs in the United States.

Urban heating and what drives it

I was surprised that Anthony Watts hasn't copied and pasted a press release about a new paper on urban heat island effect. It's one of his very favourite subjects. I guess he missed it.

The paper has been published in Nature and I spotted the press release about it a couple of days ago when scanning ScienceDaily.com. The article describes research from scientists Lei Zhao, Xuhui Lee, Ronald B. Smith and Keith Oleson. It's an international collaboration. The scientists hail from Nanjing University of Information Science and Technology, Yale University and the National Center for Atmospheric Research in Boulder, Colorado.

What they did was look in some detail at the various contributions to warming associated with the growth of towns and cities. In particular, they analysed data for 65 cities across North America.

Urban heating has implications not just from a comfort perspective but from a health perspective. Research on the subject can help cities mitigate against this human-caused warming.

After dark, the main contributor to urban heating is the buildings and roads etc releasing heat stored during the day. That confirmed what was already thought.

During the daytime, however, it's related to the efficiency of convection and humidity. As described in the abstract:  "If urban areas are aerodynamically smoother than surrounding rural areas, urban heat dissipation is relatively less efficient and urban warming occurs (and vice versa)."

There's more. Here is an extract from ScienceDaily.com

Their results reaffirmed the consensus view that, regardless of the local climate, the release of heat stored in human-built structures is the dominant contributor to UHI during the nighttime.

But during the daytime, researchers found, convection is the dominant factor -- particularly in "wetter" cities of the southeastern United States. In those places, the smooth surfaces of buildings and other human-made features are far less conducive to heat diffusion than the densely vegetated areas that surround them. Overall, in wetter climates urbanization reduces convection efficiency by 58 percent.

"The 'rougher' surfaces of the vegetation triggers turbulence, and turbulence removes heat from the surface to the atmosphere," said Lei Zhao, a doctoral student at F&ES and lead author of the study. "But where there is a smoother surface, there is less convection and the heat will be trapped in the surface."

Convection plays a key role in drier cities, too -- albeit with far different consequences. In those settings -- including in urban areas of the U.S. Southwest where surrounding vegetation is typically shorter and scrubbier -- the rural areas are less effective at dissipating heat. As a result, the urban landscapes are actually 20 percent more efficient in removing heat than their rural surroundings, triggering a 1.5-degree C cooling within the cities.

That's interesting. I wonder if Anthony Watts has compared temperature trends in drier cities and surrounding rural areas with those in wetter cities and surrounding rural areas?

---

Lei Zhao, Xuhui Lee, Ronald B. Smith, Keith Oleson. "Strong contributions of local background climate to urban heat islands". Nature, 2014; 511 (7508): 216 DOI: 10.1038/nature13462