Tornado puzzle: what actually may trigger the hook signature on Dopplers

Are heavy rains the key for triggering tornadoes? Some scientists may have some
interesting correlations.

*Heavy Rain Triggers Destructive
Tornadoes<http://dsc.discovery.com/news/2008/08/22/tornado-rain.html>
*Michael Reilly, Discovery News

*Aug. 22, 2008* -- When the spring skies blacken over the Midwestern United
States and the clouds begin to rotate ominously, residents know a
tornado<http://science.howstuffworks.com/tornado.htm>could be on its
way.

But just how the deadly twisters form is a mystery that has puzzled
scientists for decades. They know a range of atmospheric conditions must be
in place, including strong, closely paired up- and downdrafts, windshear at
high altitudes, and usually a foreboding supercell thunderstorm, spinning
slowly in the sky.

Amid this stormy cocktail, new research suggests that the heavy rains that
often accompany supercells may be key in triggering
tornadoes<http://blogs.discovery.com/news_earth/2008/01/tornadoes-globa.html>to
form. According to Robert Davies-Jones of the National
Severe Storms Laboratory <http://www.nssl.noaa.gov/> in Oklahoma, rain
falling in heavy sheets from supercell storm cloud provides a strong push to
updrafting air, causing it to spin up into a fully fledged twister.

"You can have a rotating storm but until you have rain you don't get a
tornado," Robert Davies-Jones of the National Severe Storm Laboratory said.

Scientists have known this simple fact since 1953, when radar first allowed
meteorologists to peer into supercells. In tornadic storms, they noticed the
rain swirled into a hook shape. The feature, called a 'hook echo' quickly
became known as a tell-tale radar marker for a tornado.

But the swirling rain was regarded as a by-product of a tornado, merely the
effect of its powerful corkscrewing winds. Davies-Jones believes the
opposite may be true.

"The hook echo is usually thought to be a passive feature of tornadoes," he
said. "I'm saying it's not passive, it's an active mechanism for tornado
formation."

His work is published in the August issue of *Journal of the Atmospheric
Sciences*.

Davies-Jones ran computer simulations of supercell storms to see if falling
rain could provide the needed kick that turned diffuse updrafts rising off
the warm plains into tight-spinning, lethal tornadoes. As the rain
falls<http://science.howstuffworks.com/question479.htm>out of a
rotating supercell cloud it is also twisting, and as it falls he
found it transfers the rotational energy into the updrafting air adjacent to
it.

The rain also acts as a sort of wall, confining the swirling, rising air. As
it continues to head skyward the air inside the rain curtain stretches out
like a figure skater raising her arms. The spinning speeds up, and a tornado
is born.

"The mechanism is a good one," David Lewellen of the University of West
Virginia said. "But until these things are seen more conclusively out in the
field, it's not at all clear whether rain is involved in the formation of
most tornadoes, a few, or none at all."

Lewellen points out that rain is only one of myriad weather conditions that
have to be just right for a tornado to form. A massive field campaign of
experiments is scheduled for the spring season in 2009 and 2010 that he
hopes will determine if the implications in the models are correct.