As Hurricane Isabel converges on the US East Coast, a veteran ESA spacecraft has
provided meteorologists with crucial insights into the underlying pressure
system powering the storm.

An entire flotilla of satellites is being kept busy tracking Hurricane Isabel in
visible and infrared light, as well as gathering additional measurements of
local sea surface temperature, wind and rainfall levels. ESA spacecraft ERS-2
has made the picture more detailed still by discerning the wind speed and
direction around the hurricane’s cloud and rain-wracked heart.

ERS-2 instruments include a C-band scatterometer, which works by sending a
high-frequency radar pulse down to the ocean, then analysing the pattern of
backscatter reflected back again. Scatterometers are particularly useful in
measuring wind speed and direction at the sea surface, by detecting signature
scatter from ripples on the water caused by wind.

ERS-2’s scatterometer is less sensitive than comparable space-based instruments
to rain or bad weather, and can gather data both day and night. This makes it
invaluable as an early detector of Atlantic storms — especially in the current
hurricane season.

The Isabel data was obtained mid-afternoon Wednesday at one of ESA’s ground
stations in Gatineau Canada, then rapidly delivered to meteorology offices
worldwide. At the Reading-based European Centre for Medium-Range Weather
Forecasts (ECMWF), it was analysed against the surface wind pattern predicted by
their existing software simulation of Isabel, run on powerful supercomputers.

"The ERS wind data is very valuable to us," said Hans Hersbach of ECMWF. "It
shows differences with our analysis, for instance a lack of inward wind flow
into the centre. By assimilating the data into our analysis we improve our
forecasting skills.

"The ESA scatterometer data was routinely assimilated into our analysis after
1997, until it become no longer available early this century. Now the service
has been resumed we are making use of it once more."

ESA’s ERS-2 has been in orbit since 1995, but the service from the scatterometer
was interrupted in 2001. A degradation in attitude control prevented access to
the data. Meteorologists lost a valuable window on the weather — until this
summer, when after two-and-a-half-years of effort, new processing software
developed by the Belgian Royal Military Academy (RMA) compensated for the
degradation and regained access to scatterometer measurements.

The software algorithm was installed in ground stations at Kiruna in Sweden,
Maspalomas in the Canary Islands, Gatineau in Canada as well as Frascati in
Italy, with an additional installation planned for West Freugh in Scotland. The
new service began at the end of August, just in time for Hurricane Isabel’s
dramatic arrival.

To maintain future continuity of scatterometer coverage, a new more advanced
scatterometer instrument called ASCAT is part of the payload for ESA’s MetOp
mission, currently due to launch in 2005.

Inside a hurricane

Hurricanes are large powerful storms that rotate around a central area of
extreme low pressure. They arise in warm tropical waters that transfer their
heat to the air. The warmed air rises rapidly, in the process creating low
pressure at the water surface. Winds begin rushing inwards and upwards around
this low-pressure zone.

Currently classed at Category Two on the five-point Saffir-Simpson Hurricane
scale, Isabel originated in the eastern Atlantic last week. It is currently
moving northwest at only about 24 kilometres an hour but winds within it are
rotating at about 160 km per hour. Meteorologists forecast the hurricane will
make landfall in North Carolina on Thursday.

Related links

* ERS-1 and 2

* MetOp



[Image 1:]
This reduced resolution (1200 metre) image shows Hurricane Isabel off the US
East Coast, acquired by MERIS on board ESA’s Envisat spacecraft at 14.58 UTC on
17 September 2003. Envisat is ESA’s latest Earth Observation satellite, launched
February 2002. Its orbit is a half hour ahead but otherwise identical to that of
ERS-2, whose scatterometer also acquired useful meteorological measurements of
the hurricane’s heart.

[Image 2:]
A map of the wind field at the heart of Hurricane Isabel, using data acquired at
15.23 UTC on 17 September 2003 by the C-band scatterometer on board ESA’s ERS-2
spacecraft. The map indicates wind direction and also wind velocity — the more
lines on each bar, the highest the velocity. The data provides insight into the
pressure system powering the hurricane.

Credits: ESA

[Image 3:]
This reduced resolution (1200 metre) image of Hurricane Isabel was acquired by
MERIS on board ESA’s Envisat spacecraft on 16 September, and shows the edge of
the hurricane approaching the US East Coast.