Ocean Surface A Boon
For Extreme Event Forecasts, Warnings
(16 July 2008) For humans in the path
of destructive hurricanes and tsunamis, an accurate warning of the pending
event is critical for damage control and survival.
Such
warnings, however, require a solid base of scientific observations, and a new
satellite is ready for the job.
The Ocean Surface Topography Mission
(OSTM)/Jason 2 adds to the number of eyes in the sky measuring sea surface and
wave heights across Earth's oceans. The increased coverage will help
researchers improve current models for practical use in predicting hurricane
intensity, while providing valuable data that can be used to improve
tsunami-warning models.
"When it comes to predicting hurricane
intensity, the curve in the last 40 years has been somewhat flat, with little
advance in how to reduce error in predicted intensity," said Gustavo Goni, of
the National Oceanic and Atmospheric Administration (NOAA) in Miami. Maps of
sea surface height created from satellites, however, could help change the
curve.
Satellites that measure sea surface height have been running
operationally nonstop since November 1992. But more than one is needed to fly
at the same time in order to identify all the features that could be
responsible for intensification of tropical cyclones all over Earth. The
OSTM/Jason 2 mission will help make the additional coverage
possible.
NASA, university and NOAA investigators, including Goni, work
to transform sea surface height information obtained from satellites, such as
OSTM/Jason 2, into maps of ocean heat content. Forecasters can use the maps to
develop models to predict how hurricanes will strengthen.
Determining
heat content from sea surface height is possible because warm water is less
dense and hence sits higher than cooler water. In some regions, such as inside
and outside the Gulf Stream current, the temperature differences result in more
than a one-meter (three-foot) difference in sea surface height. Goni and
colleagues use this established concept to estimate from sea level variations
how much heat is stored in the upper ocean in areas where hurricanes typically
develop and intensify.
While sea surface height may not necessarily be
the most significant parameter for hurricane intensity forecasts, researchers
now know that if sea surface height is accounted for in current forecast
models, errors in forecasts for the most intense storms are reduced. For weak
storms, the reduction in error is not very significant. However, for storms in
the strongest category 5 range, the heat content in the upper ocean derived
from sea surface height becomes increasingly important. "This is a good thing,
because these are the storms that produce the most damage," Goni
said.
"OSTM/Jason 2 will help us to keep the necessary coverage that we
need to identify ocean features that can be linked to tropical cyclone
intensification, because with only one satellite we may miss some of them,"
Goni said.
Upper ocean heat content derived from sea surface height is
now used in operational and experimental forecast models in all seven ocean
basins where tropical cyclones exist.
In December 2004, two satellites
happened to be in the right place at the right time, capturing the first
space-based look at a major tsunami in the open ocean. Within two hours of a
magnitude 9 earthquake in the Indian Ocean southwest of Sumatra, the Jason 1
and Topex/Poseidon satellites fortuitously passed over the path of the
resulting tsunami as it travelled across the ocean. It measured the leading
wave, travelling hundreds of miles per hour in the open ocean, at about 0.5
meters (1.6 feet) tall.
Wave height measurements like those of the
Indian Ocean tsunami do not provide an early warning because the information is
not relayed to ground stations in real time. That's the job of early warning
systems operated by NOAA and other global organisations that currently employ a
network of open-ocean buoys and coastal tide gauges. Sea surface height
measurements of tsunamis can, however, help scientists test and improve
ground-based models used for early warning. One such system developed at NASA's
Jet Propulsion Laboratory (JPL), Pasadena, Calif., and undergoing tests at
NOAA's Pacific Tsunami Warning Center, Ewa Beach, Hawaii, could become
operational within about three years.
Most tsunamis are caused by
undersea earthquakes. Using the JPL-developed system, when seismometers first
identify and locate a large earthquake, scientists can use GPS measurements to
search around the earthquake's source to see if land has shifted, potentially
spurring a tsunami. Scientists can then immediately compile the earthquake's
size, location, and land movement into a computer program that generates a
model tsunami to determine the risk of a dangerous wave. After the wave passes,
scientists can search through wave height data from satellites and verify what
the model predicted.
"Satellite data play the crucial role of verifying
tsunami models by testing real tsunami events," said JPL research scientist
Tony Song. "If an earthquake generates a tsunami, does the satellite data match
observations on the ground and model predictions?"
"One of the unique
pieces of satellite observations is the large-scale perspective," said JPL
research scientist Philip Callahan. Tsunamis can have waves more than 161
kilometres (100 miles) long. Such a wave would likely go unnoticed by an
observer in a boat on the ocean's surface. But satellite altimeters like
OSTM/Jason 2 can see this very long wave and measure its height to an accuracy
of about 2.5 centimetres (one inch).
Scientists' ability to test tsunami
warning models will be aided by OSTM/Jason 2. With the Topex/Poseidon mission
now ended, the currently orbiting Jason 1 has now been joined by and will
eventually be replaced by OSTM/Jason 2. This will help ensure that future
tsunamis will also be observed by satellites as well as by buoys and tide
gauges.
"The biggest value in satellite measurements of sea surface
height is not in direct warning capability, but in improving models so when an
earthquake is detected, you can make reliable predictions and reduce damage to
property and people," Callahan said.
(source: NASA Jet Propulsion
Laboratory)
