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.