TRMM Sees Rain From Hurricanes Fall Around The World

Researchers at the University of Miami's Rosenstiel School of Marine and Atmospheric Science, Miami, and the National Oceanic and Atmospheric Administration Atlantic Oceanographic and Meteorological Laboratory's Hurricane Research Division, Miami, used data from NASA's Tropical Rainfall Measuring Mission (TRMM) satellite to show how rain falls at different rates in different areas of a storm. The results were published in the July issue of the journal Monthly Weather Review.

The results are already being used in a model developed at the Hurricane Research Division to estimate rainfall accumulation related to tropical cyclones. The findings are important because they may help in the development of better forecasts.

The TRMM satellite offers the best measurements of how and where rain falls around tropical cyclones. This is because its orbit is low to the Earth, allowing more detailed information on storms, and it was designed to cover the tropics.

Tropical cyclones consist of winds rotating around low-pressure centers in the tropics that can develop into everything from tropical storms to Category 5 hurricanes.

From 1998 through 2000, TRMM observed 260 tropical cyclones in six major ocean basins. Researchers found that the rainfall intensity and where the heaviest rains fell varied depending on a storm's wind speeds, its location and the environment of each basin.

Scientists looked at three types of tropical cyclones, based on a standard system for classifying these storms. Tropical storms have wind speeds of less than 73 miles per hour (mph). Category 1 and 2 hurricanes blow with winds of 74 to 110 mph, and Category 3 to 5 hurricanes' winds range above 110 mph.

"This study is important because we know very little about the rainfall distribution in tropical cyclones," said lead author of the study, Manuel Lonfat, a University of Miami researcher. "It revolutionizes our understanding of the distribution of rain in tropical cyclones," he added. Lonfat is a NASA Earth System Science Fellowship recipient.

"More than 50 percent of deaths in the U.S. from tropical cyclones over the last 30 years are related to freshwater flooding. So this is currently a very large problem for the forecasting community," Lonfat said.

When all storms were averaged together the most intense rainfall occurred within 50 kilometers (about 31 miles) of a storm's center, with evidence of very large rain rates as far as 300 to 400 kilometers (about 186 to 250 miles) from the center.

When all storms were averaged and analyzed basin by basin, storms in the North Indian basin were the wettest, and East-central Pacific storms were the driest. The Atlantic and West Pacific storms showed similar rain rates: this at first surprised the researchers since Western Pacific storms tend to be bigger and were presumed to be wetter.

Researchers also found that the storms were not symmetric, meaning that rain fell at different rates in different areas of a storm. If a round storm were divided into four equal parts through the center, called quadrants, in general it was found that the heaviest rainfall occurred in one of the front quadrants. However, the heaviest rainfall shifted from the front-left to the front-right quadrant as a tropical cyclone's intensity increased.

Tropical storms were less symmetric, while stronger hurricanes had a more symmetric inner core. In the Southern Hemisphere, the heaviest rain occurred to the front-left of the storm's path, while in the Northern Hemisphere the heaviest rainfall peaked in the front-right quadrant.

Normally, the only way to accurately measure rain falling from a hurricane is when it gets close enough to the coast to be picked up by National Weather Service radars, or by rain gages. Since TRMM is space-based, researchers can assess the rainfall over vast tracts of ocean, where these storms spend most of their lives.