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
	2008		2007		2006	2005	3 Year Avg.	2008	2007	2006	2005	3 Year Avg.	2008	2007	2006	2005	3 Year Avg.
	Prelim	Actual	Prelim	Actual	Final	Final		Prelim	Final	Final	Final		Prelim	Final	Final	Final
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.

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