U.S. Water News Online
WASHINGTON -- Houston, Texas, makes some of its own rain,
and it is probably not alone among large coastal cities, according to
a new study. Researchers at NASA and the University of Arkansas
anticipate that the impact of "urban heat islands" on weather
patterns will become greater in the 21st century.
Urban heat islands are a known phenomenon, resulting from
pavements and buildings in big cities, which cause warm air to rise.
In coastal cities, like Houston, this warmer air interacts with sea
breezes, which creates heavier and more frequent rain in and downwind
of the city, according to the study.
The Houston-area study used data from the world's only space-based
rain radar, on NASA's Tropical Rainfall Measuring Mission (TRMM)
satellite, and dense clusters of rain gauges on the ground. An
analysis of the rain gauge data, both prior to and since
urbanization, also suggests there have been observed increases in
rainfall since Houston became urbanized.
The report appears in the journal, Earth Interactions, published
jointly by the American Geophysical Union, the American
Meteorological Society, and the Association of American Geographers.
Authors J. Marshall Shepherd of NASA's Goddard Space Flight Center in
Greenbelt, Maryland, and Steve Burian of the University of Arkansas
in Fayetteville, believe that the impact of large coastal cities on
weather and possibly climate will become increasingly important as
more people move into urban areas, with even greater concentrations
in coastal zones. A recent United Nations report estimates that 60
percent of the worldâ¤s population will live in
cities by 2025.
While previous studies have shown that urban heat islands do
create more heavy rain in, and downwind of, cities like Atlanta, St.
Louis, and Chicago, this is the one of the first to provide evidence
of such an effect around a U.S. coastal city. It is also likely the
first to use satellite-derived rainfall data for a coastal urban
High concentrations of buildings, roads, and other artificial
surfaces retain heat, leading to warmer surrounding temperatures, and
create urban heat-islands. Rising warm air may help produce clouds
that result in more rainfall around cities. In addition, buildings of
different heights cause winds to converge, driving them upward and
helping to form clouds. The study finds that this urban heat
island/rain effect may be even more pronounced near coasts. In
coastal cities like Houston, sea breezes also create rising air and
clouds. The combination of urban converging winds and sea breezes may
further enhance the development of thunderstorms.
"Recent publications have shown evidence of increased lightning
activity over and downwind of Houston," Shepherd said. "Since
lightning and rainfall are so closely related, we decided to use
TRMM's Precipitation Radar and a network of rain gauges to see if
urban-induced abnormal rainfall existed."
Using data from 1998-2002, the researchers found that the mean
rainfall rates during the warm season were 44 percent greater
downwind of Houston than upwind, even though both regions share the
same climate. They also found that rainfall rates were 29 percent
greater over the city than upwind of it. Rainfall rates can be
indicator of enhanced thunderstorm activity.
To rule out any effect on thunderstorm development from the
coastline's curvature near Houston, the researchers divided the
entire Texas coast into seven zones that extended 100 kilometers [60
miles] inland and included four or five major inlets or bays. Their
analysis of rainfall data in these zones from 1998 to 2002 showed
that abnormal rainfall only occurred over and downwind of Houston,
which suggested that the effect of the urban landscape was
significant. At the coastlines, TRMM satellite data were important,
because they allowed the researchers to access rainfall data in areas
where there were no gauges and records, including over the ocean.
The study was funded by NASA.
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