The Mysterious Rumble of Thundersnow   
   February 24, 2011: NASA atmospheric scientists got an unexpected chance to   
   study a curious phenomenon called "thundersnow" when a recent storm unleashed   
   it right over their heads.   
   [...]   
   The Weather Channel's Jim Cantore is surprised by an episode of Thundersnow in   
   Dec. 2006. Credit: weather.com [YouTube video] Walt Petersen and Kevin Knupp   
   have traveled far and wide to study winter storms. They never dreamed that the   
   most extraordinary one they'd see - featuring freakish thundersnow, a 50-mile   
   long lightning bolt, and almost a dozen gravity waves -- would erupt in their   
   own back yards. The storm hit Huntsville, Alabama, on the evening of January   
   9th.   
       
   "This incredible storm rolled right over the National Space Science and   
   Technology Center where we work," says Knupp. "What luck!"   
       
   Snowstorms usually slip in silently, with soft snowflakes drifting noiselessly   
   to Earth. Yet this Alabama snowstorm swept in with the fanfare of lightning   
   and the growl of thunder.   
       
   Eyewitness Steve Coulter described the night's events: "It was as if a wizard   
   was hurling lightning behind a huge white curtain. The flashes, muted inside   
   thick, low hanging clouds, glowed purplish blue, like light through a prism.   
   And then the thunder rumbled deep and low. This was one of the most beautiful   
   things I've ever experienced.'"   
       
   It was a once-in-a-lifetime scene for anyone lucky enough to see it, but   
   especially enthralling to scientists seeking the keys to nature's unique   
   displays of power. Petersen and Knupp, with the help of graduate students from   
   the University of Alabama-Huntsville, had their research equipment primed and   
   ready.   
   [...]   
   The "instrument pen" at the National Space Science and Technology Center where   
   researchers gathered data on thundersnow sizes, shapes and fall rates. The   
   facility is operated for the Global Precipitation Measurement Mission. Photo   
   Credits: Patrick Gatlin and Matt Wingo of UAH. Larger images: #1, #2   
   From his at-home workstation, Petersen can monitor lightning detector networks   
   and control radars, which he used to measure and record the storm. But when   
   the storm first hit his response was a little less scientific: "I was so   
   excited that I ran outside in my house slippers to take pictures," he recalls.   
   At around 10:30 p.m., he heard the first rumble of thundersnow. "My first   
   thought was, 'excellent, a bonus!'"   
       
   What made this snowstorm act like a thunderstorm? Petersen explains:   
       
   "You rarely have lightning in a snowstorm. But in this case, some unique   
   conditions set the stage for it. Moist air at the bottom of the storm was   
   lifted up, rapidly forming snow and ice. Some of the snow even grew in pellet   
   forms called 'graupel,'" he says.   
       
   Snowflakes and ice pellets of different sizes ascended at different rates--and   
   they began to exchange charges. The process isn't fully understood, but it   
   could be a result of particles rubbing together (like wool socks on carpet).   
   As the cloud charged up, it began to act less like an ordinary winter   
   snowstorm and more like a summer thunderstorm.   
   [...]   
   A negative image of thundersnow flakes. "Taking pictures of the snow flakes   
   and inverting the images helps us to better define their shape (or 'habit')   
   and thus to better interpret the way they grew- which tells us about physics   
   of the thundersnow process," says Walt Petersen of NASA MSFC. [larger image]   
   It's no coincidence that the thundersnow was accompanied by massive roller   
   coasters of air known as gravity waves. These waves are similar to waves in   
   the ocean, but roll through the air instead of water.   
       
   "There was a nearly constant, uniform progression of gravity waves, starting   
   at Monte Sano, a small mountain a few miles east of us, and moving westward,   
   right over our building," says Knupp, who spent most of the storm's duration   
   with his eyes riveted on instrument displays inside the team's mobile X-band   
   radar van. "An easterly flow of air on the other side of the mountain ridge   
   bumped into and was pushed over Monte Sano, forming 11 separate waves, about   
   one per hour."   
       
   He believes the clockwork up and down motion of the waves created variations   
   in the updrafts responsible for the heavy snow, leading to the charge   
   separation that generated lightning. Unfortunately, he was knee-deep in   
   computer displays instead of snow when the storm's most impressive lightning   
   bolt set the sky aglow.   
       
   "This bolt reached from the tower on Monte Sano Mountain all the way to   
   Molton, Alabama, about 50 miles away," says Knupp. "And I missed it."   
       
   Was he disappointed?   
       
   "I felt cheated, but it was worth the trade off. I learned some interesting   
   things."   
       
   Spoken like a true scientist.   
       
       
   Author: Dauna Coulter | Editor: Dr. Tony Phillips | Credit: Science@NASA   
       
   More Information   
   Thundersnow Hits the Deep South -- nasa.gov   
       
   Dr. Knupp is a Professor of Atmospheric Science and director of severe weather   
   research at UA-Huntsville.   
       
   Dr. Petersen is an atmospheric scientist at NASA's Marshall Space Flight   
   Center. He leads a NASA-funded group that travels around the world gathering   
   precipitation data that will support a network of Earth-monitoring satellites   
   being developed by NASA called the Global Precipitation Measurement (GPM)   
   mission. The snowstorm provided an excellent opportunity for Petersen's team   
   to take detailed measurements of precipitation and use those observations as a   
   type of database or model to simulate what the constellation of GPM satellites   
   would see from space. By combining the observations at the ground with those   
   of the polarimetric radar, Petersen's team expects to learn a great deal about   
   the processes responsible for creating the snowfall, and more accurately   
   measure the water content of the snow from space and the rate at which that   
   snow-water equivalent accumulates on the ground.   
       
   Both Knupp and Petersen work at a Huntsville, Alabama, research center known   
   as the National Space Science and Technology Center that houses scientists   
   from NASA, UA-Huntsville, and other entities.   
       
   The team used university and NASA instruments including two lightning detector   
   networks, an advanced dual polarization Doppler radar at Huntsville   
   International Airport, and the National Weather Service Doppler radar at Hytop   
   in Jackson County. Knupp also sent the university's mobile dual polarization   
   Doppler radar unit to set up outside of New Market in northeastern Madison   
   County.   
       
       
   Regards,   
       
   Roger   
      
   --- D'Bridge 3.59   
    * Origin: NCS BBS (1:3828/7)