Quality Worsened by Paved Surfaces
7, 2011 -- New research focusing on the Houston area suggests that
widespread urban development alters weather patterns in a way that
can make it easier for pollutants to accumulate during warm summer
weather instead of being blown out to sea.
study, led by the National Center for Atmospheric Research (NCAR),
could have implications for the air quality of fast-growing coastal
cities in the United States and other mid-latitude regions overseas.
the proliferation of strip malls, subdivisions and other paved areas
may interfere with breezes needed to clear away smog and other pollution.
combined extensive atmospheric measurements with computer simulations
to examine the impact of pavement on breezes in Houston.
found that, because pavement soaks up heat and keeps land areas relatively
warm overnight, the contrast between land and sea temperatures is
reduced during the summer.
in turn causes a reduction in nighttime winds that would otherwise
blow pollutants out to sea.
built structures interfere with local winds and contribute to relatively
stagnant afternoon conditions.
developed area of Houston has a major impact on local air pollution,"
says NCAR scientist Fei Chen, lead author of the new study. "If
the city continues to expand, it's going to make the winds even
weaker in the summertime, and that will make air pollution much
cautioning that more work is needed to better understand the impact
of urban development on wind patterns, Chen says the research can
eventually help forecasters improve projections of major pollution
might also consider new approaches to development as cities work to
clean up unhealthy air.
will be published this month in the Journal of Geophysical Research,
published by the American Geophyiscal Union.
was funded by the U.S. Air Force Weather Agency, the U.S. Defense
Threat Reduction Agency, and the National Science Foundation (NSF),
urbanization and coastal zone populations in Houston and other port
cities around the globe make our ability to understand and predict
complex interactions between the urban canopy and local sea-breeze
circulation ever more critical," says Brad Smull of NSF's Division
of Atmospheric and Geospace Sciences." This study represents
a significant step toward that objective."
to NCAR, the authors are affiliated with the China Meteorological
Administration, the U.S. National Oceanic and Atmospheric Administration
and the University of Tsukuba in Japan. The research is built on a
number of previous studies on the influence of urban areas on air
known for its mix of petrochemical facilities, sprawling suburbs and
traffic jams that stretch for miles, has some of the highest levels
of ground-level ozone and other air pollutants in the United States.
and federal officials have long worked to regulate emissions from
factories and motor vehicles in efforts to improve air quality.
study suggests that focusing on the city's development patterns and
adding to its already extensive park system could provide air quality
benefits as well.
you made the city greener and created lakes and ponds, then you
probably would have less air pollution even if emissions stayed
the same," Chen explains. "The night-time temperature
over the city would be lower and winds would become stronger, blowing
the pollution out to the Gulf."
adds that more research is needed to determine whether paved areas
are having a similar effect in other cities in the midlatitudes where
sea breezes are strongest.
cities from Los Angeles to Shanghai are striving to reduce air pollution
levels. However, because each city's topography and climatology is
different, it remains uncertain whether expanses of pavement are significantly
affecting their wind patterns.
Houston study, 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 Aug. 30, 2000.
chose that date partly because they could draw on extensive atmospheric
measurements taken during the summer of 2000 by researchers participating
in a field project known as the Texas Air Quality Study 2000.
campaign was conducted by the National Oceanic and Atmospheric Administration,
the U.S. Department of Energy, universities and the Texas Natural
Resource Conservation Commission.
to the real-world measurements, the study team created a series of
computer simulations with a cutting-edge software tool, NCAR's Advanced
Weather Research and Forecasting model.
his colleagues focused on wind patterns, which are driven by temperature
contrasts between land and sea.
was covered with cropland instead of pavement, as in one of the computer
simulations, inland air would heat up more than marine air during
summer days and cause a sea breeze to blow onshore in the afternoon.
the computer simulations showed that as the inland air became cooler
than marine air overnight, a land breeze would blow offshore, potentially
blowing away pollution.
the actual paved surfaces of Houston absorb more heat during the day
and are warmer overnight.
results in stagnation for three reasons:
- At night, the city's temperatures are similar to those offshore.
The lack of a sharp temperature gradient has the effect of reducing
- During the day, 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.
- Buildings and other structures break up local winds far more than
does the relatively smooth surface of croplands or a natural surface
like grasslands. This tends to further reduce breezes.
very existence of the Houston area favors stagnation," the article
also found that drought conditions can worsen air pollution.
is because dry soil tends to heat up more quickly than wet soil during
the day. It releases more of that heat overnight, reducing water-land
temperature contrast and therefore reducing nighttime breezes.
observations taken in 2000 with computer simulations of Houston-area
winds and temperatures, the researchers were able to confirm that
the Advanced Weather Research and Forecasting model was accurately
capturing local meteorological conditions.
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