ESRL Quarterly Newsletter - Fall 2009

Chasing Shadows

ESRL researchers, colleagues develop a new technique for detecting tsunamis from space

Long, shallow tsunami waves racing through open ocean change the roughness of the sea surface ahead of them, and ESRL scientists demonstrated this summer that such “shadows” can be measured by satellite-borne radars. The finding could one day help save lives through improved detection and forecasting of tsunami intensity and direction at the ocean surface.

“We’ve found that roughness of the surface water provides a good measure of the true strength of the tsunami along its entire leading edge,” said Oleg Godin of ESRL’s Physical Sciences Division. “This is the first time that we can see tsunami propagation in this way across the open ocean.”

Large tsunamis crossing the open ocean stir up and darken the surface waters along the leading edge of the wave, according to the study. The rougher water forms a long, shadow-like strip parallel to the wave and proportional to the strength of the tsunami. That shadow can be measured by orbiting radars and could one day help scientists improve early warning systems. The research was published in the journal, Natural Hazards and Earth System Sciences, and Godin’s co-authors are from the Physical Sciences Division, ZelTechnology, LLC, and the University of Colorado.

The new study challenges the traditional belief that tsunamis are too subtle in the open ocean to be seen at the surface, and it confirms a theory that Godin published five years ago. It also helps explain anecdotes that have long puzzled tsunami scientists, and inspired Godin. In 1994, a tsunami shadow was captured on video from shore, moments before the wave struck. Written reports from 1946 similarly document a shadow that accompanied a deadly tsunami on April 1.

Godin tested his theory during the deadly 2004 Indian Ocean tsunami, the result of the massive Sumatra-Andaman earthquake. Godin and colleagues analyzed altimeter measurements of the 2004 tsunami from NASA’s Jason-1 satellite. The data revealed clear evidence of increased surface roughness along the leading edge of the tsunami as it passed across the Indian Ocean between two and six degrees south latitude.

The new study identifies a third way of detecting tsunamis. Today, tsunamis that threaten coastal U.S. communities are picked up by an extensive buoy network, NOAA’s Deep-ocean Assessment and Reporting of Tsunamis (DART) system. DART uses ocean-floor sensors to measure the pressure changes that accompany passing tsunami waves. A second method uses space-borne altimeters to detect tsunamis by measuring small changes in sea surface height—but only a handful of these instruments are in orbit and observations are limited to points along a line.

Godin’s team found that relatively common scientific instruments, microwave radars and radiometers, can pick up the contrast between a rough, dark tsunami “shadow” and the smooth, bright water on either side. From orbit, microwave radars and radiometers can observe a band of ocean hundreds of kilometers wide and thousands of kilometers long. If programmed correctly, they could potentially map an entire tsunami, Godin said.

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