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Wednesday, September 2, 1998 Published at 23:00 GMT 00:00 UK


Hurricane Bonnie was taller than Mt Everest

Bonnie's stormclouds - vertical scale is exaggerated

Bonnie was the most studied hurricane ever and observations of it from space show that it had a central cloud tower twice the height of Mt Everest. Our Science editor Dr David Whitehouse reports:

Hurricane Bonnie which just caused so much havoc to the United State's eastern seaboard has become the most observed hurricane in history.

It cut a towering figure twice as tall as Mt Everest a week ago when it was scanned by the first satellite rain radar as part of a co-ordinated observing campaign.

The radar on the Tropical Rainfall Measuring Mission (TRMM) satellite observed a chimney of clouds as Bonnie slowed and gathered strength north of Hispaniola and east of the Bahamas.

A DC-8 aircraft also carried a series of instruments into the storm. It made four sets of flights in front of, through, and over Bonnie - several in co-ordination with TRMM overflights.

[ image: Heading towards America]
Heading towards America
Bonnie held some surprises for scientists as they review the data collected from the large array of instruments used to studying the storm.

The GOES-8 weather satellite was also watching Bonnie. It was looking almost straight down on the storm and could not see its interior structure. Only with TRMM's radar could the towering grandeur of the central cloud chimney be appreciated.

Towering Clouds

Bonnie's cumulonimbus storm cloud, towered 18km (59,000ft) into the sky from the eye wall of the hurricane. By comparison, the highest mountain in the world, Mt Everest, is 9km (29,000ft) and the commercial jets fly at barely one-half the height of the Bonnie's cloud tops.

Scientists believe that towering cloud structures like this are probably precursors to hurricane intensification. This was the situation with Bonnie whose central pressure dropped from 977 to 957 millibars in the subsequent 24 hours.

"It looks like a skyscraper in the clouds," said Dr. Christian Kummerow, TRMM Project Scientist at NASA's Goddard Space Flight Center.

[ image: Measurements made from aircraft]
Measurements made from aircraft
"This is the first time that TRMM's precipitation radar has seen structure of this type in a hurricane approaching the US East coast."

"Clouds this tall are rarely observed in the core of Atlantic hurricanes," said Dr. Bob Simpson, former director of the National Hurricane Center in Miami and the National Hurricane Research Project.

This huge cloud probably happened because, at the time the data was collected, Bonnie was moving very slowly. The lack of movement kept funnelling warm moist air into the upper atmosphere, thus raising the entire height of the tropopause, which is normally at around 45-52,000ft (13.7-15.9km).

[ image: The TRMM spacecraft]
The TRMM spacecraft
The tropopause marks the upper limits of Earth's densest layer of atmosphere, the troposphere, the layer starting at the surface where we live.

"TRMM has flown over 100 tropical cyclones since its launch in November of 1997," said Kummerow.

"This enormously enhances our database of cloud structures within tropical storms during their growth and decay phases. It also greatly improves the more restricted observations we have obtained from aircraft radar and allows for the systematic study of this hurricane behaviour which appears to precede their intensification."

As the height of the hurricane season approaches, TRMM scientists are looking forward to the continuing analysis of Atlantic hurricanes.

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