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David E. Steitz
Headquarters, Washington             May 27, 2003
(Phone: 202/358-1730)

Krishna Ramanujan
Goddard Space Flight Center, Greenbelt, Md.
(Phone: 301/286-3026)

RELEASE: 03-172

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.

For more information and images on the Internet, visit: 

http://www.gsfc.nasa.gov/topstory/2003/0523urbanrainfall.html

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