After reviewing data from four Earth-orbiting radar satellites, NOAA scientists
today announced they were able to measure the height of the devastating tsunami
that erupted in the Indian Ocean. The ability to make depth surveys from space
may lead to improvements in the models that forecast the hazardous effects of
tsunamis. NOAA is an agency of the U.S. Department of Commerce.

The height goes down over time as the wave spreads over the ocean and the energy
is expended on shore. At 2 hours after the quake, it was 60 cm (about 2 feet)
high. By 3 hours 15 minutes after the quake, that dropped to around 40 cm (about
16 inches) high. By 8 hours 50 minutes after the quake, the wave spread over
most of the Indian Ocean and was quite small in most areas — 5 to 10 cm (2 to 4
inches) — about the limit of the satellite resolution. However, the wave was
still large enough after all that elapsed time that it was still bouncing around
in the Bay of Bengal still appears about 25 cm high (10 inches) as measured by
the satellites.

Several days after the Indian Ocean tsunami, scientists from NOAA’s Laboratory
for Satellite Altimetry in Silver Spring, Md., examined data from the four
spacecraft — the TOPEX/Poseidon and Jason, operated jointly by NASA and the
French space agency, CNES; the European Space Agency’s Envisat and the U.S.
Navy’s Geosat Follow-On.

“By chance, these satellites were in the right place at the right time,” said
Walter H.F. Smith, a geophysicist at the NOAA LSA, which routinely monitors
sea-level variations from space using radar measurements taken by several
satellites. The measurements are used to make simple charts of unexplored ocean
basins, and to forecast the intensification of hurricanes, the onset of an El
Niño, and other weather and climate events.

Upon further investigation of satellite data, NOAA researchers were able to
detect the Indian Ocean tsunami in profiles of sea level along the satellites’
flight paths by comparing the sea level seen on Dec. 26, 2004, with the sea
level measurements the satellites picked up a few days, or weeks, earlier.

Also, U.S. and French teams, working in parallel with data from the joint
NASA/French Space Agency’s Jason and TOPEX/Poseidon, independently confirmed the
satellites’ measurements of the height of the tsunami waves. “These observations
are unique and of tremendous value for testing and improving tsunami computer
models and developing future tsunami early warning systems,” said Lee-Lueng Fu,
of NASA’s Jet Propulsion Laboratory in Pasadena, Calif.

Smith emphasized that LSA does not get the satellite data until several hours
after a tsunami has developed — too late to be used as a real-time forecast.

“Right now, this technique is not a first line of defense in tsunami hazard
monitoring and warnings, but it gives scientists a window to tsunami activity in
the deep and in remote parts of an ocean basin, too far away from coastal tide
gauges and other instruments that could detect it,” Smith added.

Smith said NOAA researchers proposed that the best application of satellite data
to improve tsunami hazard forecasts would be a reconnaissance mapping of the
ocean floor from space. “The detailed shape of the sea bed, all across the ocean
basin, determines the focusing — or diffusion — of tsunami energy barreling
toward the coast.”

At NOAA’s Pacific Marine Environmental Laboratory in Seattle, scientists have
developed a computer model, showing how the tsunami moved across the Indian
Ocean. Combined with the data from LSA, the PMEL model showed the changing sea
levels along the flight paths of the four satellites, which crossed the ocean
basin after the earthquake.

PMEL scientists apply the tsunami computer model to provide hazard assessments
for communities at risk, and develop a real-time forecast system for NOAA’s
Tsunami Warning Centers.

Vasily Titov, an oceanographer at PMEL, said the satellite observations would be
used in ongoing research to improve the understanding of how tsunamis move
across the ocean.

The NOAA Satellite and Information Services is America’s primary source of
space-based oceanographic, meteorological and climate data. It operates the
nation’s environmental satellites, which are used for ocean and weather
observation and forecasting, climate monitoring, and other environmental
applications. Some of the oceanographic applications include sea-surface
temperature for weather forecasting and sea-surface heights for prediction of El
Niño and hurricane intensification.

NOAA is dedicated to enhancing economic security and national safety through the
prediction and research of weather and climate-related events and providing
environmental stewardship of the nation’s coastal and marine resources.

Relevant Web Sites


[ (164KB)]
Tsunami wave height as measured by satellites two hours after the event. Please
credit “NOAA”.