What does that tell us about how thunderstorms make tornadoes?
We’ve known for decades that all supercell thunderstorms have a gust front, which is the boundary between the moist, warm air that is flowing into the storm and the generally cooler air coming down out of the storm. But what we noticed in several cases recently is that thunderstorms that are making, or are about to make, tornadoes, have a secondary front, which is like a second wave of air rushing down from aloft. A strong downdraft has an important function: It brings the rotation to the ground. But for a tornado to form, you still need to tilt the rotation into the vertical, and this requires a nearby updraft. The intensity of the downdrafts and updrafts is vital, because in the end there needs to be a lot of stretching, which is when you take that existing rotation and turn it into something really violent like a tornado. It’s like a figure skater pulling in her arms and spinning faster and faster.
In the Goshen County tornado, we have a strong suspicion that the development of this secondary surge or front sparked the genesis of the tornado. We need to test this. If, after looking at more cases, we can demonstrate a causal link, then perhaps in the future a forecaster observing the development of a secondary surge will have an increased ability to forecast tornadogenesis.
The data analysis emerging from VORTEX2 also identifies another possible trigger, a “descending reflectivity core.” What is that, and how does it work?
Some supercell thunderstorms have a descending core of intense rain and hail wrapping around the west side of the storm. That’s what we call a descending reflectivity core, or DRC. This DRC drags rotating air downward from maybe four or five kilometers up and might cool the air in various places. As you drag the air downward, you create rotation and antirotation in different parts of the storm, and that seems to occur around the time of tornadogenesis. Right now these two features, the DRC and the secondary surge, hold the most hope for explaining why some supercells are able to generate rotation near the ground and why the low-level rotation is turning into a tornado when it does.
Why was 2011 the deadliest tornado season we’ve seen in 75 years? Were the storms stronger than usual last year?
In recent years, we’ve become very used to tornadoes causing a relatively small number of deaths. A few dozen is
typical. Unfortunately, while some of that may be due to better forecasts, some of it is also due to luck. Last year, the tornadoes hit larger places. They hit Tuscaloosa, they hit Joplin. The total number of tornadoes may have reached 1,800, which is exceptional, but the big spike in deaths was really based on a few individual points. Just one tornado in Joplin killed almost three times the yearly average of the last few decades. The Joplin tornado was rated EF5, but there wasn’t some added degree of destruction. The difference between Greensburg, Kansas [where an EF5 tornado killed 11 people in 2007], and Joplin was how many people got hit, not the strength of the tornado.
You were busy sifting through data from VORTEX2 in 2011. Was it frustrating to sit out such a volatile tornado season?
We did go out a couple of times. One day we got some fascinating data in a strong tornado in Oklahoma that had winds of about 200 miles per hour. We observed this tornado as it crossed a lake, and in the radar we saw this very clear central eye and a very strange wind, because it was lifting up a huge amount of water. We saw for the first time ever, I think, a tornado surge, like a hurricane surge. Then, as the tornado made landfall—and that’s a term we usually use with hurricanes—it just started shredding the forest. The eye suddenly filled up with a big ball of debris. From a scientific perspective, it’s very interesting because one of the great limitations we have in meteorology is that we’re not a laboratory science. But in this case, we had a tornado that was experiencing pure, simple conditions: lake for a few minutes and then pretty simply, woods. The structure of the tornado changes dramatically right when it crosses from the lake to the forest.
You have warned that the risk of a tornado-caused catastrophe in this
country is underestimated, or even overlooked altogether.
My colleagues and I wrote a paper in 2007 (pdf) that asked, what if one of those large tornadoes that we’ve observed with the Doppler on Wheels went through the suburbs of Chicago or St. Louis? This is a worst-case scenario, but I’d say it was a plausible worst-case scenario. Tens of thousands, even 100,000 homes could be destroyed. I think we should have at least some degree of preparation.