New research focusing on
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.
The international 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 reason: the
proliferation of strip malls, subdivisions and other paved areas may interfere
with breezes needed to clear away smog and other pollution.
The researchers combined
extensive atmospheric measurements with computer simulations to examine the
impact of pavement on breezes in Houston.
They 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.
This in turn causes a
reduction in nighttime winds that would otherwise blow pollutants out to sea.
In addition, built
structures interfere with local winds and contribute to relatively stagnant
“The 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
While 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 events.
Policy-makers might also
consider new approaches to development as cities work to clean up unhealthy
The article will be
published in the Journal of Geophysical
The research was funded by
the U.S. Air Force Weather Agency, the U.S. Defense Threat Reduction Agency,
and the National Science Foundation (NSF), NCAR’s sponsor.
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.”
In addition 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 pollution.
Houston, 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
State and federal
officials have long worked to regulate emissions from factories and motor
vehicles in efforts to improve air quality.
The new 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.
“If 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.”
Chen 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.
Coastal 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.
For the 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.
They 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.
That campaign was
conducted by the National Oceanic and Atmospheric Administration, the U.S.
Department of Energy, universities and the Texas Natural Resource Conservation
In addition 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.
Fei and his colleagues
focused on wind patterns, which are driven by temperature contrasts between
land and sea.
If Houston 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
Conversely, the computer
simulations showed that as the inland air became cooler than marine air
overnight, a land breeze would blow offshore, potentially blowing away
In contrast, the actual
paved surfaces of Houston
absorb more heat during the day and are warmer overnight.
This results in stagnation
for three reasons:
night, the city’s temperatures are similar to those offshore. The lack of a
sharp temperature gradient has the effect of reducing winds.
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.
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.
“The very existence
of the Houston
area favors stagnation,” the article states.
The study also found that
drought conditions can worsen air pollution.
This 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.
By comparing 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.