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Study finds convection an important part of urban heat effect.

With more than half the world's population living in cities, it's important to understand how climate in urban areas can be different from that in surrounding regions. NCAR is utilizing software that can simulate atmospheric conditions in both densely packed downtown areas and residential neighborhoods, which will allow researchers to explore how human health is affected during heat waves.

Part of NCAR's Community Earth System Model, the software was recently used in tandem with satellite observations to quantify the causes of the urban heat island effect--in which temperatures are warmer in cities than in less urbanized surrounding areas. The research, which studied 65 North American cities, confirmed what most scientists had believed about the heat island effect at night: the evaporation of moisture from trees and crops cools the air, so a lack of vegetation in cities prevents this cooling effect.

In the daytime, however, the study revealed the importance of convection, which created widely varying temperatures depending on the humidity of the region. In humid areas, the software showed that a lack of atmospheric turbulence was a significant factor in keeping cities warmer. Specifically, aerodynamically smooth building surfaces and other urban architecture resulted in less vertical air movement than the uneven surfaces of vegetation in less urban areas. Consequently, less heat is removed from the air in cities, increasing daytime temperatures by about 3[degrees]C.

"The 'rougher' surfaces of the vegetation trigger turbulence, and turbulence removes heat from the surface to the atmosphere," explains the study's lead author, Lei Zhao of Yale University's School of Forestry & Environmental Studies (F&ES). "But where there is a smoother surface, there is less convection and the heat will be trapped in the surface."

This "convection efficiency" was also found to be influential in drier areas, but in a much different way. In these regions, vegetation outside of cities is shorter and sparser, and less heat is lost from rural areas. This creates what study coauthor Xuhui Lee of F&ES calls a "paradoxical phenomenon," where in urban areas "we sometimes see urban heat sinks instead of the typical heat island." The study showed that urban temperatures in these drier regions actually cooled by about 1.5[degrees]C.

The study, which was published last year in Nature, suggested that the heat island effect could be minimized if urban structures such as parking lots, roads, and buildings were lighter in color in order to amplify surface albedo. [Source: Yale School of Forestry and Environmental Studies]

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Publication:Bulletin of the American Meteorological Society
Date:Apr 1, 2015
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