By listening to raindrops splash on the ocean surface with underwater microphones, scientists at the University of Washington and NASA Goddard Space Flight Center, Greenbelt, Md., are exploring how the ocean and the atmosphere interact.

"Rainfall is a fundamental component of climate and affects the heat, momentum, water and gas exchange between the atmosphere and oceans," said Dr. Jeffrey Nystuen, University of Washington oceanographer and research team member for NASA's Tropical Rainfall Measuring Mission (TRMM), who leads the acoustic experiment.

Until now however, there have been no good ways to measure ocean rainfall. "The acoustical method may work and provide this crucial information," he said.

By placing microphones that Nystuen calls Acoustic Rain Gauges attached to buoys 65 feet (20 meters) deep in the Pacific Ocean, the team can tell the size of raindrops by the way they sound when they hit the sea surface. Knowing the size of raindrops is important in determining the type of clouds that form during a storm and how the rain will fall.

Raindrops produce two distinct sounds that are picked up by Nystuen's instrument. The Acoustic Rain Gauge hears a slapping sound as the drop strikes the water surface, and then picks up a ringing sound as air bubbles are trapped beneath the surface.

The bubble makes the most important sounds that Nystuen uses to determine raindrop size. The slapping sounds are more unique, relying on the size, shape and speed of the raindrops, and are most relevant for larger drops, he said.

By placing hydrophones on deep ocean moorings, the team can listen to the ocean and determine when it is raining, how hard it is raining and even what type of rain is falling. This is because different size raindrops produce different sounds underwater.

Raindrops produce two types of sound underwater. One is the slapping sound as the drop strikes the water surface, and the second is a distinct ring from air bubbles trapped underwater by the splash. For most raindrops, the sound of the bubble is, by far, the louder sound. Small raindrops are especially good at trapping air bubbles underwater and are surprisingly loud.

Nystuen has used the Acoustic Rain Gauges in conjunction with other scientific experiments, which have taken place in the North Atlantic Ocean, the South China Sea and the central tropical Pacific Ocean.

Currently acoustic data are being collected in the central and eastern tropical Pacific Ocean in collaboration with NOAA's Pacific Marine Environmental Laboratory (PMEL). The results of the collaborative studies are promising. "An ongoing goal is to determine how well the acoustic technique works," Nystuen added.

"Knowledge of rain distribution and intensity is important not only to farmers and flood control planners, but also to meteorologists, oceanographers and climatologists," Nystuen said.

"This is because the formation of a raindrop releases energy into the air. This energy or heat release is one of the primary sources of energy driving atmospheric circulation. Understanding the global patterns of distribution and intensity of rainfall is needed to improve weather and climate forecasting."

Unfortunately, rainfall is also very difficult to measure, especially over the ocean where few people live and where rain gauges commonly used on land don't work. It is well known that rain falling onto a tin roof makes a lot of noise, but so does rain falling onto water. In fact, rain falling onto water is one of the loudest sources of underwater sound.

Although there are man-made or biological noises which sometimes are loud and could potentially interfere with the acoustical measurement of rain, these noises are generally intermittent or geographically localized.

hen rain is present, the sound from rain dominates the underwater sound field. Perhaps someday scientists will be able to measure oceanic rain by listening to it from below the ocean surface.

By studying rainfall regionally and globally, and the difference in ocean and land-based storms, NASA's TRMM Mission is providing scientists the most detailed information to date on the processes of these powerful storms, leading to new insights on how they affect global climate patterns.

TRMM's complement of state-of-the- art instruments will provide extremely accurate measurements of the distribution and variability of tropical rain and lightning, and the balance of solar radiation absorbed and reflected by Earth's atmosphere.

The acoustic tests are part of NASA and NASDA's (the Japanese Space Agency) Tropical Rainfall Measuring Mission (TRMM). Launched over two years ago, the TRMM satellite orbits Earth to measure tropical rainfall and perfecting other climate models.

Tropical rainfall, which falls between 35 degrees north latitude and 35 degrees south latitude, comprises more than two-thirds of the rainfall on Earth.

Each of the microphones were placed by NASA in association with NOAA and its Office of Global Programs, the National Science Foundation and the Office of Naval Research -- Ocean Acoustics Division.

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