Rhythm of rain not soothing to fish
Scientists use underwater racket as weather gauge

Although a human swimming underwater might not realize it, rain falling onto water is one of the loudest sources of underwater sound in the world’s oceans.

Jeff Nystuen prepares to submerse his acoustic rain gauge.

Oceanographer Jeffrey Nystuen and engineers at the UW’s Applied Physics Laboratory are trying to take advantage of the cacophony. Underwater microphones listening for raindrops hitting the ocean surface could be a new kind of rain “gauge,” one that might allow scientists to determine the intensity of rainfall and the size of drops.

These kinds of measurements are important to scientists involved in weather and climate forecasts because rainfall is such a fundamental component of climate.

“When raindrops form they release energy in the form of heat,” Nystuen says. “That heat is one of the primary sources of energy driving atmospheric circulation. Understanding the patterns of distribution and intensity of rainfall helps us figure out the Earth’s complicated climate system.”

Rain falling over the oceans is, however, very difficult to measure because gauges used on land obviously won’t work and there are such vast areas to be monitored.

NASA’s Tropical Rainfall Measuring Mission is, for example, using a satellite and a variety of other instruments to study rain, lightning and storms over the tropical oceans. Nystuen’s acoustic rain gauges are part of the field programs being conducted in conjunction with the project.

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 that are trapped underwater by the splash.

Different sizes of raindrops generate distinctive sound signals. Small drops, the size typical in what people call “drizzle,” generate small, uniform-sized bubbles that resonate at a higher frequency than other drops and are surprisingly loud underwater. Medium-sized drops, on the other hand, do not generate bubbles and are actually quieter underwater than the small drops. The largest raindrops, which often occur at the beginning of a heavy downpour, create bubbles of many sizes. The resulting sound includes frequencies not found when small drops alone are falling. In addition, the largest drops create loud sounds when they strike the surface.

Nystuen’s acoustic rain gauges were designed, built and programmed at the UW’s Applied Physics Laboratory by support engineers Bob Drever, Fred Karig and Tim Wen. The 4-foot long plastic cylindrical devices come with a tiny computer, batteries and a hydrophone (an underwater microphone) at one end.

Meant to be left at sea for a year, the devices are programmed to “listen” every 10 minutes for rainfall. If it’s raining, the devices record acoustic samples.

Nystuen is working with Mike McPhaden’s climate group at NOAA’s Pacific Marine Environmental Lab at Sand Point and presently has five acoustic rain gauges, mounted 65 feet below the surface, on several buoys that carry instruments monitoring ocean changes that signal El Niño and La Niña events.

Although there are sometimes man-made or biological noises that could potentially interfere with the acoustical measurement of rain - for example, the noise from harbors or snapping shrimp - they are generally intermittent or geographically localized, Nystuen said. Even the sounds of breaking waves can be sorted from the sounds of falling rain.

Information from the instruments now deployed in the tropical Pacific should be available later this year after scientists visit the moorings and retrieve the data. Ideally - if the technique proves useful - future instruments will be designed to transmit the data so that flood managers, farmers, meteorologists, oceanographers and others interested in the distribution and intensity of rainfall can make timely use of the information. ¶

Sandra Hines, News & Information