April is the Cruelest Month   
       
   April 13, 2012: In the opening lines to The Waste Land, T.S. Eliot wrote   
   "April is the Cruelest Month."   
       
   You might agree if you live in the southeastern United States. Last April, a   
   historic outburst of 202 tornadoes turned broad swaths of that part of the   
   country into a disaster zone.   
       
   "The event of April 27th and 28th 2011 was the costliest convective storm in   
   U.S. history," said Kevin Knupp, professor of atmospheric sciences at the   
   University of Alabama-Huntsville. And he doesn't just mean costly in terms of   
   property damage -- 316 people lost their lives.   
       
   Of the 202 twisters that day, 62 tore through Alabama, where Knupp works. Ten   
   of them were ranked EF 4 and 5 on the Enhanced Fujita Scale.  Three tornadoes   
   churned paths more than 120 miles long, and a large number of the twisters cut   
   swaths more than a half mile wide.   
       
   Knupp saw the results first-hand, and he's been studying them ever since.   
   Aided by a team of graduate students and colleagues, he's sifted through   
   gigabytes of data1 collected by NASA and NOAA satellites and local ground   
   sensors.   A year later, they have drawn some interesting conclusions.   
       
   A new ScienceCast video recaps the events of April 27-28, 2011, and explores   
   what researchers have learned from the outbreak.   
       
   One discovery was how rapidly an EF-5 spun up near the small town of   
   Hackleburg, Alabama.   
       
   "The Hackleburg storm got its act together really quickly," says Knupp. This   
   particular twister formed only 50 minutes after the underlying thunderstorm   
   appeared. For comparison, the average time for tornado formation is 2 hours.   
   The twister blasted through north Alabama with winds over 200 mph, killing 72   
   people.   
       
   Knupp's team believes that something called a "thermal boundary" set the stage   
   for the birth of the killer. Cool, moist air on one side of the boundary   
   formed a low cloud base -"kind of like a wall," he explains.  Warm air from   
   the storm ran into the wall and swept upward. Updrafts are a key ingredient of   
   tornadoes.  In this case, updrafts as swift as 75 feet per second were   
   recorded.   
       
   They also noticed that many of the tornadoes seemed to cluster in space and   
   time.  Knupp offers the example of Arab and Guntersville, neighboring   
   communities not far from NASA's Marshall Space Flight Center: "16 tornadoes   
   touched down in the area -- 13 of them in a half hour period."   
       
   VIDEO: http://www.youtube.com/watch?v=WIKAV7B2oTk   
       
   In advance of a tornado, storm clouds approach Huntsville, AL, in April 2011.   
   More: A Story from the Tornado ZoneCould local topography have attracted the   
   twisters?   
       
   There does appear to be a link between the shape of the landscape and the path   
   of these tornadoes.   
       
   "Arab-Guntersville is in a valley between two mountain ridges, and valleys can   
   channel the flow of air, like in a breezeway," notes UA-Huntsville atmospheric   
   scientist Tim Coleman, a member of the research team.   
       
   Coleman also noticed a correlation between the slope of the terrain and the   
   onset of damage tracks. "Winds intensified on the downward slopes of mountains   
   in the area-and that is sometimes where the damage path starts." He has   
   observed this phenomenon in past tornadoes in east Tennessee and southern   
   Virginia.   
       
   This tornado-topography hypothesis might seem obvious, but researchers have   
   not always had enough data to test it-that is, not until April 2011.   
       
   Prompted in part by the outbreak, Coleman has studied tornado paths all over   
   the southeast  and found some interesting repeats. For example, just north of   
   Birmingham, several violent tornadoes have tracked within a 10-mile wide band   
   since 1977. A similar track appears to the north and west of Huntsville.   
       
   "More research is needed to see if topography really plays a role," cautions   
   Coleman. "The more we can learn, the more accurate we can make tornado   
   warnings."   
       
   They're likely to get more data in the near future.  It is April, after all.   
       
       
   Author: Dauna Coulter| Production Editor: Dr. Tony Phillips | Credit:   
   Science@NASA   
       
   More information   
   Next-Gen Weather Satellites to Improve Tornado Warnings -- NOAA and NASA are   
   working on a series of next-generation weather satellites called "GOES-R" that   
   will improve warnings of deadly tornadoes and other severe weather.   
       
   A Story from the Tornado Zone -- Science@NASA   
       
   1Sources of data and images used in the study   
       
   MIPS (Mobile Integrated Profiling System): gives vertical profiles of the   
   atmosphere and maps the winds   
       
   MAX (Mobile Alabama X-band dual polarimetric Doppler Radar):  a dual   
   polarization radar that can be quickly deployed to observe storms   
       
   ARMOR (Advanced Radar for Meteorological and Operational Research): a   
   dual-polarization radar that can accurately map rain intensity, precipitation,   
   and debris associated with tornadoes   
       
   LMA (lightning mapping array, a NASA development): provides total lightning,   
   which often indicates the relative intensity of thunderstorms   
       
   Doppler radar   
       
   Other sources   
       
   ground surveys: conducted by NWS and UAH personnel on 27 April to determine   
   the damage path characteristics of the tornadoes   
       
   aerial surveys with 1 ft resolution: an outside contractor, Atlantic Group,   
   provided these surveys, which provided highly detailed pictures of the   
   variability in tornado damage characteristics and the relationship to   
   topography   
       
   NOAA aerial surveys: provided highly detailed pictures of the variability in   
   tornado damage characteristics and the relationship to topography   
       
   visual documentation (videos and images): used to relate radar signatures to   
   tornado properties such as intensity, size, and general appearance   
       
   ASTER: NASA's Advanced Spaceborne Thermal Emission and Reflection Radiometer,   
   to image the entire tornado path in a timely manner (especially in remote   
   areas) to facilitate planning for ground surveys   
       
       
   Regards,   
       
   Roger   
      
   --- D'Bridge 3.79   
    * Origin: NCS BBS (1:3828/7)