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.

Here is some of what the authors wrote:

In contrast to the intense UHIs of Salt Lake City,Miami and Louisville, the negative values in Fig. 4 indicate that a city was actually cooler than its natural surroundings. Riverside and Las Vegas exhibited the strongest urban cool islands (UCIs) of −1.37 and −0.76 °C, respectively. This “oasis effect” is largely due to the increased presence of moisture and heightened potential for evaporative cooling within the cities relative to the surrounding desert landscapes (Brazel, Selover, Vose, & Heisler, 2000). The Riverside UCI was a particularly extreme case that was partially influenced by the seasonality of aridity (Fig. 5), as the summertime UCI peak occurred during a phase of extremely arid conditions according to the De Martonne index. 

This finding has surprised and confused deniers at WUWT, where Anthony copied and pasted the press release (archived here). Anthony didn't comment on the cooling effect. He might not have noticed.

This is from a paper I wrote about last year. In contrast to the reason proposed by Debbage and Shepherd (evaporative cooling), Lei Zhou and colleagues argue that in some dry regions, cities are cooler because of convection:

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.

From the WUWT comments

The readers at WUWT have come up with various notions to explain the cooling effect. As far as I can tell, none of them have managed to locate the paper but that doesn't stop them from speculating.

lsvalgaard asks:

September 18, 2015 at 11:28 am
How can the UHI effect be negative?

Kev-in-Uk weirdly speculates the towns act as a heat sink. He's got that back to front. The towns that soak up heat are hotter, not colder.

September 18, 2015 at 11:33 am
I wondered the same thing – I can only assume (without time to read the paper) that it is suggesting that certain urban environments act as some form of heat soak (or sink) for the atmospheric temperature thereby lowering it? Weird! Will have to read it later!

Data Soong wonders if Las Vegas is getting a sea breeze because of its elevation :)

September 18, 2015 at 12:37 pm
That’s the first thing I thought. It appears as though they are calculating the difference between the urban area, and the surrounding area, which is partially from UHI effects, but can also be from other effects such as land/sea breezes, different elevations, etc.

Stephen Rasey doesn't "believe" it:

September 18, 2015 at 12:48 pm
A very heavily irrigated Urban Area with a high density of trees relative to its surroundings would be plausible.
But it strains credibility that even the thickest of urban forests can counteract the high density of asphalt and concrete roads, parking lots, roof areas, glass boxed high-rises, and stupendous use of electrical and fossil fueled energy.
I live in West University, Texas, and tree rich enclave of Houston (Inner loop, SW of downtown). When you fly into Hobby on a clear night, right side window seat, West U is a black lake within a sea of city lights. The comparison is striking.
If cities are bright at night, then it is hard to believe they are anything but strong positive Heat Islands. 

Gunga Din says it's all down to siting:

September 18, 2015 at 2:33 pm
Some of the surface stations were in air-conditioned rooms rather than near the AC’s exhaust vents?

KTWO reckons its the climate conspiracy:

September 18, 2015 at 3:52 pm
I’m guessing they wanted negatives to roughly balance the positives. That would allow net UH to be discounteded as a cause of any rising world temperature. Every rise MUST be attributed to a GHG. And mostly to CO2.
The entire study probably hinges on how “we………..quantified each city’s urban morphology with spatial metrics.”

co2islife thinks science says surface temperature is solely determined by atmospheric CO2 and that the temperature on every spot on earth should be identical all year around, in all seasons, day and night.

September 18, 2015 at 11:30 am
CO2 is 400 ppm everywhere you go. CO2 can’t cause temperature differentials. Constants can’t cause change. These climate “scientists” should be forced to explain how 400 ppm in the city can cause a temperature differential with the 400 ppm rural areas. How can a constant 400 ppm cause such temperature differentials. Also, those temperature records are made during the daytime. How does CO2 cause daytime temperature records when CO2 is transparent visible light, the light that warms the day. 

TonyL is wrong. Other people have found some cities are cooler than their surrounds:

September 18, 2015 at 3:51 pm (excerpt)
They have invented or discovered something that nobody has ever suspected. A negative UHI effect. I wonder if they know how badly their new discovery conflicts with all known physics.

References and further reading

Neil Debbage, J. Marshall Shepherd. "The urban heat island effect and city contiguity." Computers, Environment and Urban Systems, 2015; 54: 181 DOI: 10.1016/j.compenvurbsys.2015.08.002 (pdf here)

• Study ranks U.S. cities based on the urban heat island effect on temperatures - press release at ScienceDaily.com

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 (pdf here)

From the HotWhopper archives

• Urban heating and what drives it - July 2014
• Urban heat confuses deniers at WUWT - August 2015
• Anthony Watts is Extremely Hot on China - April 2013
• Anthony Watts visits Greenland and finds Airport UHI disease! - August 2013
• Sequel to: Anthony Watts visits Greenland and finds Airport UHI disease! - September 2013
• Denier weirdness: Russian steam pipes are causing global warming, sez Anthony Watts @wattsupwiththat - December 2013