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