Radar satellites capture tsunami wave height
From: George (george_at_wtfiswrongwithyou.com)
Date: 01/08/05
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Date: Sat, 08 Jan 2005 02:15:50 GMT
http://www.newscientist.com/article.ns?id=dn6854
A pair of satellites that happened to be over the Indian Ocean just after the
massive Sumatran earthquake on 26 December have captured the only measurements
of the tsunamis' height in deep water. The images, which will be released within
days, will help improve future forecasts of the deadly waves.
The US-French satellites, called TOPEX/Poseidon and Jason-1, passed over the Bay
of Bengal two hours after the massive earthquake struck just off the coast of
Indonesia.
That is "just about the time the leading edge of the tsunami was hitting Sri
Lanka and India", says Lee-Lueng Fu, project scientist for the satellites at
NASA's Jet Propulsion Laboratory in Pasadena, California, US.
The satellites did not observe coastal areas. But for eight minutes, they used
radar to measure the sea level along a 3000-kilometre-long track of ocean. This
image of the bay as the tsunami rolled through it provides a unique and valuable
continuous measure of the waves. In contrast, the floating sensors suggested as
part of any future early-warning system can only provide measurements at single
points, says Fu.
"There are no other observations of a similar nature anywhere in the world," he
told New Scientist.
Fast as jets
The satellites saw the first two wavefronts produced by the main quake, spaced
500 to 800 kilometres apart. These waves reached a maximum height of 50
centimetres in the open ocean, only reaching their full devastating height when
entering the shallow waters of the coast.
By comparison, wind-driven waves typically reach 10 metres. However, these waves
only involve water close to the ocean surface and are separated by at most
several hundred metres.
Tsunami waves have a very deep reach indeed - 4000 metres or more. "These waves
involve the entire depth of the ocean. Their energy is thousands of times more
than a monster storm," Fu says. And even though tsunamis barely ripple the
surface, they speed through the water as fast as jet airliners.
Fu hopes the observations will improve tsunami warning systems by providing
vital new data to calibrate tsunami models. Just after an earthquake,
seismometers around the world help pinpoint the source. That data is then sent
to tsunami modellers who use the information to compute the probable height and
arrival of any resulting waves.
The key number in the modelling is wave height in deep water, as this indicates
energy, and that is exactly what the radar recorded. Fu adds that, because the
waves take time to travel to land, "when you see waves in the open ocean this
large, you have time to warn people on the coast".
The observations also showed smaller features. Piggybacking on top of the main
waves were smaller ripples about 100 or 200 kilometres apart. "This is the first
observation of details of tsunami waves," says Fu.
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