NEAL CONAN, host:
Scientists at the National Oceanic and Atmospheric Administration announced that their satellite captured pictures of the devastating Indian Ocean tsunami, that those pictures now allow them to measure the height of the wave. At this point, as scientists did not receive the data in real time and the satellites are not trained on specific areas, these images can't be used to forecast the hazardous effects of tsunamis; however, they may lead to improvements in forecasting. NOAA geophysicist Walter Smith joins us now from his offices in Silver Spring, Maryland, to tell us what he and the satellite saw and what we can learn from these images.
Thanks very much for being with us.
Mr. WALTER SMITH (NOAA Geophysicist): Good afternoon, Neal. It's a pleasure to be with you.
CONAN: And how do you measure a wave?
Mr. SMITH: Well, these satellites carry a radar device, and as they go around the Earth, the radar looks down at the ocean's surface, measures the distance from the satellite to the surface. At the same time, we figure out exactly where the satellite is above the center of the Earth, and then the difference of those two measurements gives us the height of the sea level with respect to the solid Earth. It's kind of an amazing technology. The satellites are 500 or more miles up in space, and we can measure sea level to about an inch.
CONAN: And it's also extraordinary; this technology was also used to actually make maps of the ocean floor using those measurements.
Mr. SMITH: That's correct. That's work that I've done with colleague David Sandwell at the Scripps Institution of Oceanography. The sea-level height varies because of mountains and valleys on the ocean floor, as you just said, and because of blobs of warm and cold water that can show us where hurricanes might intensify or an El Nino might be coming on. And in the case of data from December 26th, it also showed us the passing of the tsunami.
CONAN: And from those satellite images, what did this wave look like?
Mr. SMITH: Well, we get a slice across the wave along the satellite's path of flight, and there were actually four different satellites that saw this, operated by NASA, the French National Space Agency, the US Navy and the European Space Agency. And in our work in NOAA, we put all these data together with a model run by the NOAA Tsunami modeling group. My colleagues, Vasily Titov in the Tsunami group and Remko Scharro in our Laboratory for Satellite Altimetry, have worked to put this together. We get slices--as the satellite passes over the ocean, we get sort of a brief glimpse along the path of the satellite of what the advancing tsunami looked like.
And the first one we saw--these satellites were just there by chance, I should say. The first one we saw was two hours after the earthquake, when we see something about two feet high, and it's just reaching Sri Lanka on the east coast of India by that time. When I say two feet, I don't mean on the beaches where those people were affected, but out in the deep open water. Later we see 16 inches or so, about three and a quarter hours after the earthquake, and even as much as eight hours or so after the earthquake, when the tsunami has gone all the way across the Indian Ocean, we still see things that are large enough for the satellite to see, several inches, still in the Bay of Bengal.
This is important because we don't have any deep open ocean measuring devices in the Indian Ocean, and so this satellite measurement is all we can use to calibrate the models and understand how the tsunami moves across the basin.
CONAN: By the way, if you'd like to take a look at those satellite images gathered by NOAA, you can go to our Web site at npr.org and see them there.
Again, something two feet high in deep water in the ocean can translate into this terribly destructive wave when it hits the beach. How is that possible?
Mr. SMITH: Well, that's--I'm not the hydrodynamicist who's the expert on that. My understanding is, in very deep water--and when I say 'very deep,' we're talking about three miles deep in this part of the Indian Ocean--these things are moving at something like 500 miles an hour. The wave length is so very long from peak to trough of these two-feet-high things that if you were out there on a ship you would never know the wave had passed you. You wouldn't feel it. But all that water moving so very rapidly, when it tries to fit into a small space in shallow water as it approaches the coast, that's when things pile up very high and suddenly get very destructive.
CONAN: Mm. Now as you said, this can't be used as a forecasting device. For one thing, as I understand it, these measurements are far more valuable in deep water than they are in shallow water?
Mr. SMITH: I'm not sure that that's correct. We may be mixing apples and oranges here. The ability to map ocean-floor depths by this technique works better in deep water than...
Mr. SMITH: ...in shallow water, but as far as the sea-level measurements, they do work pretty much all the way up to the coast. The reason this is not a good hazard monitoring and forecasting technique is--well, there are several. First, these satellites were there just by chance. We don't have continuous coverage of the whole globe by this technique. Secondly, we don't get the data down from space and into our computers in real enough time to make a useful warning. And even if we could do those things with some future system, the--an important part of a warning system is that you don't issue false alarms. Your listeners who remember Aesop's fable about the boy who cried wolf will be reminded of that.
What we do now at NOAA is to have, in the Pacific warning system, detailed gauges on the ocean bottom that can find a tsunami that's very small. So after a large earthquake occurs, we can see a tsunami, even if it's a small one, and then we can be confident that we don't need to issue an alarm. When those tsunameters, as we call them, were put in place in the Pacific, our false-alarm rates were cut by 75 percent. So I think something that can see small tsunamis as well as big ones is probably what we need for a global monitoring system.
CONAN: Walter Smith, thanks very much. It's fascinating.
Mr. SMITH: Thank you, Neal.
CONAN: Walter Smith, geophysicist from the NOAA Laboratory for Satellite Altimetry. He joined us from his office in Silver Spring, Maryland. Again, if you'd like to see the satellite images gathered by NOAA, just go to npr.org.
In Washington, I'm Neal Conan, NPR News.
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