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Pavement Affects Weather, Aids Build Up of Urban Pollutants

Highways (Jeramey Jannene/Flickr)A team from the National Center for Atmospheric Research in Boulder, Colorado found the intense commercial development in Houston, Texas can change weather patterns in that city, which makes it easier for pollutants to accumulate during warm summer weather instead of being blown out to sea. Their findings will appear in an upcoming issue of the Journal of Geophysical Research–Atmospheres.

The researchers — from the U.S., China, and Japan — combined atmospheric measurements with computer simulations to examine the impact of pavement on breezes in Houston. They found that pavement soaks up heat and keeps land areas relatively warm overnight, which reduces the contrast between land and sea temperatures during the summer.

This reduced temperature contrast causes a reduction in nighttime winds. The built-up structures also interfere with local winds and contribute to relatively stagnant afternoon weather conditions.

In Houston study, NCAR’s Fei Chen and colleagues focused on the onset of a nine-day period of unusually hot weather, stagnant winds, and high pollution in the Houston-Galveston area that began on 30 August 2000. Extensive atmospheric measurements were taken during that summer by researchers participating in a field project known as the Texas Air Quality Study 2000. The study team also created a series of computer simulations with NCAR’s Advanced Weather Research and Forecasting model.

Fei and his colleagues focused on wind patterns, which are driven by temperature contrasts between land and sea. Chen and colleagues found the paved surfaces of Houston absorb more heat during the day and are warmer overnight. At night, they found the city’s temperatures are similar to those offshore. The lack of a sharp temperature gradient has the effect of reducing winds.

During the day, the researchers noted, the hot paved urban areas tend to draw in air from offshore. However, this air is offset by prevailing wind patterns that blow toward the water, resulting in relatively little net movement in the atmosphere over the city.

In addition to the pavement, buildings and other structures block local winds far more than does the relatively smooth surface of croplands or a natural surface like grasslands, which tends to further reduce breezes.

One of the teams computer simulations suggested that if Houston were covered with cropland instead of pavement, inland air would heat up more than marine air during summer days and cause a sea breeze to blow onshore in the afternoon. Conversely, as the inland air became cooler than marine air overnight, a land breeze would blow offshore, potentially blowing away pollution.

The study found as well that drought conditions can worsen air pollution, since the dry soil acts like pavement and tends to heat up more quickly than wet soil during the day. Like pavement, dry soil releases more of that heat overnight, reducing the temperature contrast between land and water and thereby reducing nighttime breezes.

Photo: Jeramey Jannene/Flickr

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