May 27, 2003
Coastal Cities Turn Up the Heat on Rainfall
The old song, asking rain to "go away" and "come again another day," may
get even older for people who live in large coastal cities, according
to new NASA-funded research.
According to the study, urban heat islands, created from pavement and
buildings in big coastal cities like Houston, cause warm air to rise
and interact with sea breezes to create heavier and more frequent rainfall
in and downwind of the cities. Analysis of Houston-area rain-gauge data,
both prior to and since urbanization, also suggests there have been observed
increases in rainfall as more heat islands were created.
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.
Authors, J. Marshall Shepherd of NASA's Goddard Space Flight Center,
Greenbelt, Md., and Steve Burian, a University of Arkansas, Fayetteville,
Ark. researcher, believe the impact large coastal cities have on weather,
and possibly climate, will become increasingly important as more people
move into urban areas, with even greater concentrations in coastal zones.
The paper is in the current American Meteorological Society and American
Geophysical Union's journal, Earth Interactions.
A recent United Nations report estimates 60 percent of Earth's population
will live in cities by 2025. Previous related studies have shown urban
heat islands create heavier rainfall in and downwind of cities like Atlanta,
St. Louis and Chicago. However, this is one of the first studies to provide
evidence of such an effect around a U.S. coastal city. It is also the
first to incorporate specific satellite-derived rainfall data for a coastal
urban area.
Urban areas with high concentrations of buildings, roads and other artificial
surfaces retain heat, which leads to warmer surrounding temperatures
and creates heat islands. Rising warm air, promoted by the increased
heat, may help produce clouds that result in more rainfall around cities.
Buildings of different heights cause winds to converge, driving them
upward, helping form clouds. The study shows the 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 coastal sea breezes may enhance thunderstorm
development.
"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," he said.
Using data from 1998 to 2002, the researchers found mean rainfall rates,
during the warm season, were 44 percent greater downwind of Houston than
upwind, even though the regions share the same climate. They also found
rainfall rates were 29 percent greater over the city than upwind. Rainfall
rates indicate how hard it rains and can be an indicator of enhanced
thunderstorm activity.
To rule out any effects from the coastline curvature near Houston on
thunderstorm development, the researchers divided the entire Texas coast
into seven zones extending 100 kilometers (62 miles) inland and including
four or five major inlets or bays. Analysis of rainfall data in these
zones showed abnormal rainfall only occurred over and downwind of Houston,
which suggested effects from the urban landscape were significant. At
the coastlines, TRMM satellite data were important, because they allowed
researchers to assess rainfall data in areas where there were no gauges
and records, like over the ocean.
A companion paper by the researchers, presented in March at a Geological
Society of America meeting in Kansas City, Mo., stated urban areas also
affect the timing of rainfall. Compared to upwind areas, there were nearly
two times as many occurrences of rainfall from noon to midnight in the
urban area. This finding has significant implications for flood control
in Houston, Burian said.
NASA's Earth Science Enterprise, which supported this study, is dedicated
to understanding the Earth as an integrated system and applying Earth
System Science to improve prediction of climate, weather and natural
hazards using the unique vantage point of space.
###
Contacts:
David E. Steitz
Headquarters, Washington
(Phone: 202/358-1730)
Krishna Ramanujan
Goddard Space Flight Center, Greenbelt, Md.
(Phone: 301/286-3026) |
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![Satellite Images of Houston Metro Area](https://webarchive.library.unt.edu/eot2008/20081012235430im_/http://earthobservatory.nasa.gov/Newsroom/NasaNews/ReleaseImages/20030527/houston-smaller-area_m_tn.jpg)
![Satellite Images of Houston Metro Area](https://webarchive.library.unt.edu/eot2008/20081012235430im_/http://earthobservatory.nasa.gov/Newsroom/NasaNews/ReleaseImages/20030527/houston-large-area_m_tn.jpg)
Satellite Images of Houston Metro Area
These images show the Houston metropolitan area, where buildings, roads
and other built surfaces create urban heat islands that can affect local
rain patterns. The images were taken by ASTER (Advanced Spaceborne Thermal
Emission and Reflection Radiometer), an imaging instrument that is flying
on Terra, a satellite launched in December 1999 as part of NASA's Earth
Observing System (EOS). Credit: NASA/JPL
![U.S. City Lights](https://webarchive.library.unt.edu/eot2008/20081012235430im_/http://earthobservatory.nasa.gov/Newsroom/NasaNews/ReleaseImages/20030527/image-3_tn.jpg)
U.S. City Lights
As urban areas continue to grow, as indicated by this view of U.S. city
lights from space, it is important to understand their affects and feedbacks
on weather and climate. In this image human-made lights highlight particularly
developed or populated areas of the Earth's surface, including the Houston
metropolitan area. Credit: NASA Goddard Space Flight, Center Scientific
Visualization Studio
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