The chase
From mid-May through the end of June, ICECHIP storm chasers traveled across the Front Range of the Rockies and the central Plains, sometimes riding in vehicles armored against falling ice. They launched drones, released weather balloons and set up mobile doppler radars — all techniques honed by tornado chasers.
As one group positioned mobile doppler radars to intercept the storm at close range, other researchers were responsible for releasing weather balloons nearby or setting out sensors to measure the size and velocity of a hail strike.
During some storms, researchers released hundreds of pingpong ball-like devices called hailsondes into the tempests’ path to track the life cycle of a hail stone — when it is melting and freezing, and how wind dynamics that lift and drop these chunks of ice affect their growth.
Convective thunderstorms, with big internal updrafts, generate hail by circulating a mix of water and ice crystals into the freezing layers of the upper atmosphere. Hail typically forms at altitudes of 20,000 to 50,000 feet, where temperatures are between minus 22 degrees and 14 degrees Fahrenheit. Those same updrafts sweep hailsondes into the hail-generating parts of each storm.
“If we can track that sensor with time, we’re going to, at least for a couple of these storms, understand the exact path, the exact trajectory that a hailstone takes,” said Victor Gensini, a professor of meteorology at Northern Illinois University and an ICECHIP principal investigator.
In an atmosphere warmed by climate change, “we get a lot more instability,” Gensini said, which researchers think creates stronger updrafts.
Those stronger updrafts can support larger hailstones for more time, which allows balls or discs of ice to gain mass, before gravity sends them racing to the ground.
“It’s kind of like if you take a hair dryer and turn it on its end, it’s pretty easy to balance a pingpong ball, right, in that airstream,” Gensini explained. “But what would you need to balance a softball? You would need a much stronger updraft stream.”
Storm modeling suggests stronger updrafts will increase the frequency of large hail in the future, even as it decreases the likelihood of hail overall. Researchers suspect small hail will decrease because its lower mass means that it will take longer to fall. By the time it’s close to the surface, it has often melted down to water.
“There’s this kind of dichotomy, right, where you get less small hail but more large hail in these warmer atmospheres that have very strong updrafts,” Gensini said.
During their field campaign, the researchers amassed a collection of more than 10,000 hailstones in chests of dry ice to try to determine if their computer models are getting the dynamics of hail growth right.
“The hail record is kind of messy,” Gensini said of previous data, adding that observers have recorded more 2-, 3- and 4-inch hailstones, but it’s not clear if that’s because more people are chasing and finding big hail or because the atmosphere is producing more of it.
Gensini said the new measurements will help researchers compare what is happening in the air to what they’re finding on the ground, which should improve hail forecasts and mitigate economic losses.
In many of the areas where ICECHIP is working, there’s a lot of agriculture, according to Karen Kosiba, an atmospheric scientist with the University of Illinois Flexible Array of Radars and Mesonets team who is also working with ICECHIP.
“It affects their crops, their machinery, getting stuff into shelter,” she said. “There’s a lot of economic ties to the weather.”
