Research Where Every Drop Counts

Posted: November 16, 2005 at 1:00 am, Last Updated: November 30, -0001 at 12:00 am

By Lynn Burke

Over the past few months, the importance of weather satellites really hit home as TV news reports showed tropical weather systems explode into hurricanes one after another. These satellites not only relayed pictures showing the hurricanes’ size, one, the Tropical Rainfall Measurement Mission (TRMM), also measured precipitation.

TRMM, a joint U.S.–Japanese mission, is the first satellite equipped with space-borne radar for this task. Because this measuring method is relatively new, scientists are looking for ways to validate the satellite data, which in the long run will improve weather forecasting and determine changes in global rainfall patterns possibly brought on by climate change.

Associate Research Professor Eyal Amitai of Mason’s Center for Earth Observing and Space Research is a member of one collaborative team contributing to the validation effort. Amitai, along with Jeffrey Nystuen of the University of Washington and Emmanouil Anagnostou of the University of Connecticut, is measuring precipitation by listening to the underwater sound of raindrops with the use of acoustic rain gauges (ARGs). (Learn more about how a raindrop’s size is related to the sound it makes by visiting NASA’s Earth Observatory web site.)

Eyal Amitai
Eyal Amitai
Creative Services photo

According to the research team, studies of the distribution and intensity of rain will almost certainly depend on satellite rainfall measurement techniques, but these techniques are not without their problems.

“Validation of the satellite measurements requires reliable surface measurements,” they say. “Over land, rain gauges and weather radars are deployed, but these instruments are generally not available in oceanic regions.”

With more than $500,000 in funding from the National Science Foundation, the researchers have deployed ARGs in the Mediterranean off Greece’s coastline and a dense rain gauge network within the range of a high-resolution coastal radar to examine the method’s validation potential. One of the things they are looking at is whether the depth at which the ARGs are located affects results.

In early 2004, four ARGs were deployed at 60, 200, 1,000 and 2,000 meters underwater on a single mooring. Preliminary results, presented by Amitai at the American Meteorological Society 32nd Conference on Radar Meteorology in Albuquerque, N.M., in late October, show that acoustic detection of rain events and storm structure as recorded by all ARGs is in agreement with the radar observations.

“For several years we have known that a high correlation exists between radar reflectivity and the underwater sound of rainfall based on observation in shallow water, but now, for the first time, such correlation is found in the deep sea. The new results demonstrate the potential for evaluating ground and space-based radar rainfall observations using underwater sound because the listening area is proportional to the ARG depth, and for deep ARGs, it is in the order of the radar footprint,” says Amitai.

Mason is also part of a consortium of researchers from six countries in the European Union’s VOLTAIRE (Validation of Multisensors Precipitation Fields and numerical modeling in Mediterranean Test Sites) Project, which is working to develop methods for validating precipitation fields measured by ground and space-based radars.

“VOLTAIRE’s results will help prepare us and European participants for the Global Precipitation Measurement Mission, the next generation of such missions,” says Amitai, who represents Mason, the only U.S. partner in this project.

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