Blog Archive

Friday, August 22, 2008

Heavy Rain Triggers Destructive Tornadoes

by Michael Reilly, Discovery News, August 22, 2008

When the spring skies blacken over the Midwestern United States and the clouds begin to rotate ominously, residents know a tornado 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 to form. According to Robert Davies-Jones of the National Severe Storms Laboratory in Oklahoma, rain falling in heavy sheets from a 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 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.

Link to article: http://dsc.discovery.com/news/2008/08/22/tornado-rain.html


NOAA 2008 Monthly Tornado Statistics

Number of Tornadoes Number of Tornado Deaths Killer Tornadoes

20082007200620053 Year Avg.20082007200620053 Year Avg.20082007200620053 Year Avg.

PrelimActualPrelimActualFinalFinal
PrelimFinalFinalFinal
PrelimFinalFinalFinal
JAN 136 84 29 21 47 33 34 7 2 1 4 2 4 1 1 2 1
FEB 230 148 87 52 12 10 25 59 22 0 0 7 12 3 0 0 1
MAR 150 126 214 171 150 62 128 4 27 11 1 13 3 10 7 1 6
APR 189 189 187 165 245 132 181 0 9 38 0 16 0 3 9 0 4
MAY 597 460 282 250 139 123 171 43 14 3 0 6 10 4 1 0 2
JUN 394 ? 152 128 120 316 188 7 0 0 0 0 4 0 0 0 0
JUL 120 ? 55 69 71 138 93 1 0 0 0 0 1 0 0 0 0
AUG 59 ? 87 73 80 123 92 ? 1 1 4 2 ? 1 1 3 2
SEP ? ? 63 51 84 133 89 ? 0 1 1 1 ? 0 1 1 1
OCT ? ? 115 87 76 18 60 ? 5 0 0 2 ? 3 0 1 1
NOV ? ? 7 7 42 150 66 ? 0 10 28 13 ? 0 3 5 3
DEC ? ? 22 19 40 26 33 ? 1 2 0 1 ? 1 2 0 1
Total 1875 1007 1300 1093 1106 1264 1159 121 81 67 38 62 34 26 25 13 21

Note:? means final number not yet available.
Important! Prelim. numbers represent tornado reports. Columns marked Final represent total tornadoes.
2008 numbers updated through May.

Click to see the larger Daily Tornado Trend Image

Link: http://www.spc.noaa.gov/climo/torn/monthlytornstats.html

No comments: