Hurricane Dean tracked from space (Forwarded)

ESA News
http://www.esa.int

21 August 2007

Hurricane Dean tracked from space

ESA satellites are tracking the path of Hurricane Dean as it rips across the
Caribbean Sea carrying winds as high as 260 km/h. The hurricane, which has
already claimed eight lives, is forecast to slam into Mexico's Yucatan
Peninsula on Tuesday morning.

Dean was upgraded early Tuesday to a Category 5 -- the highest on the
Saffir-Simpson scale -- before pummelling the peninsula. Knowing the
strength and path of hurricanes is critical for issuing timely warnings;
satellites are the best means of providing data on the forces that power the
storm, such as cloud structure, wind and wave fields, sea surface
temperature and sea surface height.

Instruments aboard ESA's Envisat and ERS-2 satellites allow them to peer
through hurricanes. Envisat carries both optical and radar instruments,
enabling researchers to observe high-atmosphere cloud structure and pressure
in the visible and infrared spectrum.

The Medium Resolution Imaging Spectrometer (MERIS) optical instrument shows
the swirling cloud-tops of a hurricane, while radar instruments such as the
Advanced Synthetic Aperture Radar (ASAR) pierce through the clouds to show
how the wind fields shape the sea surface and estimate their likely
destructive extent.

ERS-2 uses its radar scatterometer to observe the hurricane's underlying
wind fields. The scatterometer instrument works by firing a trio of
high-frequency radar beams down to the ocean, then analysing the pattern of
backscatter reflected up again. Wind-driven ripples on the ocean surface
modify the radar backscatter, and as the energy in these ripples increases
with wind velocity, backscatter increases as well. Scatterometer results
enable measurements not only of wind speed but also of direction across the
water surface.

What makes ERS-2's scatterometer especially valuable is that its C-band
radar frequency is almost unaffected by heavy rain, so it can return useful
wind data even from the heart of the fiercest storms.

Dr Ad Stoffelen of the Royal Netherlands Meteorological Institute (KNMI),
which processes ESA's scatterometer images, said: "Observed winds from
hurricane Dean by ESA's ERS-2 scatterometer are provided to meteorologists
within the hour. This C-band radar wavelength scatterometer peeks right into
the 'eye' of a hurricane like Dean, providing timely and precise information
on its position and force.

"The wind field derived from the ESA ERS-2 scatterometer measurements are
distributed via a EUMETSAT (European Organisation for the Exploitation of
Meteorological Satellites) project to a registered database of a few hundred
users, originating from all over the world, includng the Americas,
Australia, Asia and Europe. Scatterometer winds are used directly by shift
meteorologists in forecast rooms and to initialise Numerical Weather
Prediction models aiding the forecasting of hurricanes 5 days ahead."

Another Envisat instrument called the Radar Altimeter-2 (RA-2) uses radar
pulses to measure sea surface height (SSH) down to an accuracy of a few
centimetres. Near-real time radar altimetry is a powerful tool for
monitoring a hurricane's progress and predicting its potential impact
because anomalies in SSH can be used to identify warmer ocean features such
as warm core rings, eddies and currents.

Water temperatures are the main underlying energy reservoir that power
hurricanes; together with the correct atmospheric conditions, temperatures
need to exceed 26 C in order to form and maintain a tropical cyclone.
Because warm water expands, scientists can locate warm underwater ocean
features by detecting bulges in the ocean surface height, as detected by
RA-2.

The thermal energy of warm water, which partly powers a hurricane, is known
as tropical cyclone heat potential (TCHP). Warm waters may extend to at
least 100 metres beneath the surface in many of these oceanic features,
representing waters of very high heat content. Several hurricanes have
intensified when their tracks pass over eddies or other masses of warm water
with high TCHP values.

The US National Oceanic and Atmospheric Administration (NOAA) is using
Envisat RA-2 results along with those from other space-borne altimeters to
chart TCHP and improve the accuracy of hurricane forecasting.

Envisat's Advanced Along Track Scanning Radiometer (AATSR) works like a
space-based thermometer, acquiring the temperature of the sea surface down
to a fraction of a degree. It also returns useful atmospheric data,
measuring the temperature of the top of hurricane clouds -- the higher into
the atmosphere they extend, the colder they are.

AATSR information can be correlated with MERIS data cloud height and
development to gain a good estimate of the hurricane's precipitation
potential, and improve understanding of how this relates to its overall
intensity. Condensation of water vapour releases latent heat, which warms
the vicinity of the hurricane eye. This in turn evaporates more surface
water and feeds the heat engine powering the hurricane.

The International Charter 'Space and Major Disasters' has been activated to
provide Earth Observation satellite data for assessing the damage of
Hurricane Dean in Belize.

[NOTE: Images supporting this release are available at
http://www.esa.int/esaCP/SEM5LLWZK5F_index_1.html ]