Lack of Twisters Aside, VORTEX2 Gets Useful Data
* Heats Stays Away: Full Forecast Through the Fourth *
Meteorologist Patrick Marsh huddles over a computer screen at NOAA's Storm Prediction Center in Norman, Oklahoma, anxiously tracking the progress of the armada of researchers taking part in the largest tornado research project in history, known as "VORTEX2." It is June 10, only three days before the end of the first field campaign in the $10 million research program to investigate how tornadoes form.
As the leader of the VORTEX2 Operations Center, Marsh works out of a small room stuffed with a half-dozen computers. He and his colleagues provide situational awareness support for the approximately 100 researchers who are hunting powerful thunderstorms known as "supercells" in the Great Plains region. The scientific armada is represented on Marsh's screen by multi-colored dots, each of which is slowly ticking west across Kansas to meet up with incipient storms in far southeastern Colorado. It is early evening, and the scientists are racing against darkness and atmospheric conditions that are marginal at best for tornadoes.
Keep reading for a sampling of the outcomes from VORTEX2...
Marsh and his colleagues doubt that the target storm will produce a tornado, given the lack of necessary atmospheric ingredients on this day - such as abundant moisture, wind shear, and instability. But like many days during the 2009 VORTEX2 field campaign, the Colorado cells are the best the atmosphere has to offer researchers during a season that has produced remarkably few tornadoes.
As the teams reach the storms, they are not impressed. "Yawn," writes one of the mobile Doppler radar teams on a chat program. Another vehicle terms the storm "pathetic." Scientists set up their instruments to collect data anyway, on what turns out to be a rather ordinary severe thunderstorm. They abandon their efforts around sunset after more than an hour of data collection and head for dinner.
Such a scenario was a familiar one for VORTEX2 scientists, who collected data on only one tornado, in Goshen County, Wyoming, during the entire field campaign.
Peak of tornado season was more like a valley
VORTEX2 was designed to take advantage of the climatological peak of tornado season, which is from mid-May to early June, yet tornadoes were hard to come by during that period this year. Harold Brooks, a researcher with NOAA's National Severe Storms Laboratory in Norman and an authority on tornado statistics, said that the traditional peak of tornado season may have had the fewest tornadoes of at least the past 50 years, with closer to 100 tornadoes recorded compared to the normal of about 300.
"We were way below normal," Brooks said. "It's not very often that you see a late May that's this tornado poor."
According to a new study, dry conditions in the fall and winter may have contributed to the lack of tornadoes in southern parts of tornado alley this spring. The study, from researchers at the University of Georgia and Purdue University, found a statistically significant relationship between fall and winter drought conditions and the number of days with tornadoes in the following spring in northern Georgia and other parts of the Southeast.
The study found that fall and winter drought conditions were correlated with below-normal tornado days in the following spring, with non-drought years having nearly twice as many tornado days than drought years. The authors hypothesized that lack of soil moisture could limit the amount of moisture available to the atmosphere and lower the amount of instability.
Lead author Marshall Shepherd told CWG that he sees no reason why the results would not hold for other regions, although he urged caution in concluding that dry conditions in areas such as Texas, Oklahoma and parts of Kansas this past fall and winter were responsible for the paucity of tornadoes this spring.
"We don't believe this is plausibly something that is just linked to the Southeast," Shepherd said. "We really need to replicate this study in highly tornado prone areas."
Brooks said he is intrigued by the study's reported connection to soil moisture. "Essentially, soil moisture is a source of one of the primary ingredients for thunderstorm formation (moisture)," he said in an email conversation. He expressed skepticism about the connection to the lack of tornadoes during VORTEX2, however, since much of the region targeted by VORTEX2 scientists had experienced above average precipitation prior to the spring.
Testing a "Goldilocks" hypothesis
Regardless of why they had such tough luck with tornadoes this year, the scientists taking part in VORTEX2 believe they still gathered potentially indispensable data on what causes tornadoes to form, in part because they collected data on so many storms that looked like they would produce tornadoes, but which failed to do so in the end.
Paul Markowski, a meteorologist at Penn State University who serves on VORTEX2's steering committee and directs a team of mobile observation platforms taking part in the experiment, said the datasets the teams collected on supercells that didn't produce tornadoes were "real gems for us," because they can be compared to data sets on tornado producing storms to identify key differences.
Markowski said scientists are now zeroing in on the subtleties of supercell thunderstorms, after many past hypotheses of tornado formation failed to pan out.
In particular, scientists are investigating a hypothesis that the air temperature and precipitation characteristics in certain parts of the storm, including the temperature of a current of air that wraps around the backside of the storm, known as the rear flank downdraft, help determine whether or not a supercell will produce a tornado.
The downdraft is required in order to instigate rotation at the ground level, Markowski said, but the air in the downdraft has to be just the right temperature in order to produce a tornado.
If the downdraft is too cold compared to the air being drawn into the storm, it can prohibit tornado production by preventing low-level air from being lifted and spun up into a tornado (cold air is more dense and heavier than warm air). The size of water droplets and hail stones falling nearby may determine the temperature of the downdraft air, Markowski and other VORTEX scientists said.
"You need to have downdrafts, but you don't want the downdraft to be too cold," Markowski said, calling it a "Goldilocks problem." Preliminary data from the Goshen County tornado, for example, shows that the temperature of the downdraft was only one to two degrees Celsius cooler than the air temperature ahead of the storm, which supports the hypothesis.
Marsh, who spent the entire field campaign as he did on June 10 - inside the VORTEX2 Operations Center, said that "hardcore physical evidence" is needed to prove the hypothesis.
For Markowski and his probe vehicles, that means charging right into the heart of the storm. Obtaining robust data on small-scale differences in temperature and precipitation requires data platforms to sample the conditions in and around the area of rotation, what radar aficionados know as the "hook echo."
"You're sampling the air that is going right into the vortex," Markowski said, emphasizing that gathering data in that part of the storm will help complement data from the mobile radars and other ground-based systems that were deployed as part of VORTEX2.
Markowski's own probe vehicle looked like it had escaped from a war zone after it limped out of the Goshen County storm, having encountered softball-sized hail that took out its front windshield, side mirrors, and damaged its rooftop instrument array. Luckily, windshield replacements for each probe were budgeted for. "It's pretty much impossible to be collecting data in those regions... without taking on some big hail," Markowski said.
The hunt for more tornadoes will resume next spring, when researchers hope Mother Nature behaves a bit more like normal.
The writer rode along with NOAA for a week of the VORTEX2 field campaign. See his previous VORTEX2 reports.
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